William Paley Institute
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Science and Creationism: A View from the National Academy of Sciences

Second Edition


National Academy of Sciences•National Academy of
Engineering•Institute of Medicine•National Research Council
NATIONAL ACADEMY PRESS

Washington, DC 1999


The National Academy of Sciences is a private, nonprofit,
self-perpetuating society of distinguished scholars engaged in scientific
and engineering research, dedicated to the furtherance of science and
technology and to their use for the general welfare. Upon the authority of
the charter granted to it by the Congress in 1863, the Academy has a
mandate that requires it to advise the federal government on scientific
and technical matters.


Part I

Preface


In his preface to the original 1984 version of this document, Frank Press,
my predecessor as president of the National Academy of Sciences, called
attention to a pair of illustrations similar to the ones on the front and
back of this booklet. The first is a photograph of Earth from space--the
one on this booklet was taken by the GOES-7 satellite in 1992 as it passed
over Earth and captured in graphic detail Hurricane Andrew. The second
shows a map of the world prepared during the 7th century by the scholar
Isidore of Seville. As Press pointed out, both illustrations reflect the
efforts of humans to understand the natural world. "How then," he wrote,
"can the two views be so different? The answer lies at the very heart of
the nature of this system of study we call science."

Since those words were written, the mapping of Earth has provided
further powerful examples of how science and science-based technologies
progress. Beginning in the early 1990s, a network of satellites has
allowed anyone with a hand-held receiver to know his or her position on
Earth to within a few feet. This Global Positioning System* (GPS) now is
being used to locate vessels lost at sea, study plate tectonics, trace
open routes through crowded city streets, and survey Earth's surface. Yet
the technology originated with a purely scientific objective--the desire
to build extremely accurate clocks to test Einstein's theory of
relativity.

The tremendous success of science in explaining natural phenomena and
fostering technological innovation arises from its focus on explanations
that can be inferred from confirmable data. Scientists seek to relate one
natural phenomenon to another and to recognize the causes and effects of
phenomena. In this way, they have developed explanations for the changing
of the seasons, the movements of the sun and stars, the structure of
matter, the shaping of mountains and valleys, the changes in the positions
of continents over time, the history of life on Earth, and many other
natural occurrences. By the same means, scientists have also deciphered
which substances in our environment are harmful to humans and which are
not, developed cures for diseases, and generated the knowledge needed to
produce innumerable labor-saving devices.

The concept of biological evolution is one of the most important
ideas ever generated by the application of scientific methods to the
natural world. The evolution of all the organisms that live on Earth today
from ancestors that lived in the past is at the core of genetics,
biochemistry, neurobiology, physiology, ecology, and other biological
disciplines. It helps to explain the emergence of new infectious diseases,
the development of antibiotic resistance in bacteria, the agricultural
relationships among wild and domestic plants and animals, the composition
of Earth's atmosphere, the molecular machinery of the cell, the
similarities between human beings and other primates, and countless other
features of the biological and physical world. As the great geneticist and
evolutionist Theodosius Dobzhansky wrote in 1973, "Nothing in biology
makes sense except in the light of evolution."

Nevertheless, the teaching of evolution in our schools remains
controversial. Some object to it on the grounds that evolution contradicts
the accounts of origins given in the first two chapters of Genesis. Some
wish to see "creation science"--which posits that scientific evidence
exists to prove that the universe and living things were specially created
in their present form--taught together with evolution as two alternative
scientific theories.

Scientists have considered the hypotheses proposed by creation
science and have rejected them because of a lack of evidence. Furthermore,
the claims of creation science do not refer to natural causes and cannot
be subject to meaningful tests, so they do not qualify as scientific
hypotheses. In 1987 the U.S. Supreme Court ruled that creationism is
religion, not science, and cannot be advocated in public school
classrooms. And most major religious groups have concluded that the
concept of evolution is not at odds with their descriptions of creation
and human origins.

This new edition of Science and Creationism: A View from the National
Academy of Sciences is a companion volume to a publication released in
1998 by the Academy, Teaching About Evolution and the Nature of Science.
That longer document is addressed to the teachers, educators, and
policymakers who design, deliver, and oversee classroom instruction in
biology. It summarizes the overwhelming observational evidence for
evolution and explains how science differs from other human endeavors. It
also suggests effective ways of teaching the subject and offers sample
teaching exercises, curriculum guides, and "dialogues" among fictional
teachers discussing the difficulties of presenting evolution in the
classroom.

This new edition of Science and Creationism has a somewhat different
purpose. It, too, summarizes key aspects of several of the most important
lines of the evidence supporting evolution. But it also describes some of
the positions taken by advocates of creation science and presents an
analysis of these claims. As such, this document lays out for a broader
audience the case against presenting religious concepts in science
classes. Both this document, and the earlier Teaching About Evolution and
the Nature of Science, are freely available online at the Academy website
(www.nap.edu).

Scientists, like many others, are touched with awe at the order and
complexity of nature. Indeed, many scientists are deeply religious. But
science and religion occupy two separate realms of human experience.
Demanding that they be combined detracts from the glory of each.

Bruce Alberts
President
National Academy of Sciences


*"The Global Positioning System: The Role of Atomic Clocks." Part of the
series Beyond Discovery: The Path from Research to Human Benefit by the
National Academy of Sciences (Washington, D.C.: National Academy Press,
1997).


Part II

Introduction

Science is a particular way of knowing about the world. In science,
explanations are limited to those based on observations and experiments
that can be substantiated by other scientists. Explanations that cannot be
based on empirical evidence are not a part of science.

In the quest for understanding, science involves a great deal of
careful observation that eventually produces an elaborate written
description of the natural world. Scientists communicate their findings
and conclusions to other scientists through publications, talks at
conferences, hallway conversations, and many other means. Other scientists
then test those ideas and build on preexisting work. In this way, the
accuracy and sophistication of descriptions of the natural world tend to
increase with time, as subsequent generations of scientists correct and
extend the work done by their predecessors.

Progress in science consists of the development of better
explanations for the causes of natural phenomena. Scientists never can be
sure that a given explanation is complete and final. Some of the
hypotheses advanced by scientists turn out to be incorrect when tested by
further observations or experiments. Yet many scientific explanations have
been so thoroughly tested and confirmed that they are held with great
confidence.

The theory of evolution is one of these well-established
explanations. An enormous amount of scientific investigation since the
mid-19th century has converted early ideas about evolution proposed by
Darwin and others into a strong and well-supported theory. Today,
evolution is an extremely active field of research, with an abundance of
new discoveries that are continually increasing our understanding of how
evolution occurs.

This booklet considers the science that supports the theory of
evolution, focusing on three categories of scientific evidence:
Evidence for the origins of the universe, Earth, and life
Evidence for biological evolution, including findings from paleontology,
comparative anatomy, biogeography, embryology, and molecular biology
Evidence for human evolution

At the end of each of these sections, the positions held by advocates
of "creation science" are briefly presented and analyzed as well.

The theory of evolution has become the central unifying concept of
biology and is a critical component of many related scientific
disciplines. In contrast, the claims of creation science lack empirical
support and cannot be meaningfully tested. These observations lead to two
fundamental conclusions: the teaching of evolution should be an integral
part of science instruction, and creation science is in fact not science
and should not be presented as such in science classes.


Terms Used in Describing the Nature of Science*

Fact: In science, an observation that has been repeatedly confirmed
and for all practical purposes is accepted as "true." Truth in
science, however, is never final, and what is accepted as a fact
today may be modified or even discarded tomorrow.

Hypothesis: A tentative statement about the natural world leading to
deductions that can be tested. If the deductions are verified, the
hypothesis is provisionally corroborated. If the deductions are
incorrect, the original hypothesis is proved false and must be
abandoned or modified. Hypotheses can be used to build more complex
inferences and explanations.

Law: A descriptive generalization about how some aspect of the
natural world behaves under stated circumstances. Theory: In
science, a well-substantiated explanation of some aspect of the
natural world that can incorporate facts, laws, inferences, and
tested hypotheses.

The contention that evolution should be taught as a "theory, not as
a fact" confuses the common use of these words with the scientific
use. In science, theories do not turn into facts through the
accumulation of evidence. Rather, theories are the end points of
science. They are understandings that develop from extensive
observation, experimentation, and creative reflection. They
incorporate a large body of scientific facts, laws, tested
hypotheses, and logical inferences. In this sense, evolution is one
of the strongest and most useful scientific theories we have.

*Adapted from Teaching About Evolution and the Nature of Science by
the National Academy of Sciences (Washington, D.C.: National Academy
Press, 1998).


Part III

The Origin of the Universe, Earth, and Life


The term "evolution" usually refers to the biological evolution of living
things. But the processes by which planets, stars, galaxies, and the
universe form and change over time are also types of "evolution." In all
of these cases there is change over time, although the processes involved
are quite different.

In the late 1920s the American astronomer Edwin Hubble made a very
interesting and important discovery. Hubble made observations that he
interpreted as showing that distant stars and galaxies are receding from
Earth in every direction. Moreover, the velocities of recession increase
in proportion with distance, a discovery that has been confirmed by
numerous and repeated measurements since Hubble's time. The implication of
these findings is that the universe is expanding.

Hubble's hypothesis of an expanding universe leads to certain
deductions. One is that the universe was more condensed at a previous
time. From this deduction came the suggestion that all the currently
observed matter and energy in the universe were initially condensed in a
very small and infinitely hot mass. A huge explosion, known as the Big
Bang, then sent matter and energy expanding in all directions.

This Big Bang hypothesis led to more testable deductions. One such
deduction was that the temperature in deep space today should be several
degrees above absolute zero. Observations showed this deduction to be
correct. In fact, the Cosmic Microwave Background Explorer (COBE)
satellite launched in 1991 confirmed that the background radiation field
has exactly the spectrum predicted by a Big Bang origin for the universe.
As the universe expanded, according to current scientific
understanding, matter collected into clouds that began to condense and
rotate, forming the forerunners of galaxies. Within galaxies, including
our own Milky Way galaxy, changes in pressure caused gas and dust to form
distinct clouds. In some of these clouds, where there was sufficient mass
and the right forces, gravitational attraction caused the cloud to
collapse. If the mass of material in the cloud was sufficiently
compressed, nuclear reactions began and a star was born.

Some proportion of stars, including our sun, formed in the middle of
a flattened spinning disk of material. In the case of our sun, the gas and
dust within this disk collided and aggregated into small grains, and the
grains formed into larger bodies called planetesimals ("very small
planets"), some of which reached diameters of several hundred kilometers.
In successive stages these planetesimals coalesced into the nine planets
and their numerous satellites. The rocky planets, including Earth, were
near the sun, and the gaseous planets were in more distant orbits.
The ages of the universe, our galaxy, the solar system, and Earth can
be estimated using modern scientific methods. The age of the universe can
be derived from the observed relationship between the velocities of and
the distances separating the galaxies. The velocities of distant galaxies
can be measured very accurately, but the measurement of distances is more
uncertain. Over the past few decades, measurements of the Hubble expansion
have led to estimated ages for the universe of between 7 billion and 20
billion years, with the most recent and best measurements within the range
of 10 billion to 15 billion years.

The age of the Milky Way galaxy has been calculated in two ways. One
involves studying the observed stages of evolution of different-sized
stars in globular clusters. Globular clusters occur in a faint halo
surrounding the center of the Galaxy, with each cluster containing from a
hundred thousand to a million stars. The very low amounts of elements
heavier than hydrogen and helium in these stars indicate that they must
have formed early in the history of the Galaxy, before large amounts of
heavy elements were created inside the initial generations of stars and
later distributed into the interstellar medium through supernova
explosions (the Big Bang itself created primarily hydrogen and helium
atoms). Estimates of the ages of the stars in globular clusters fall
within the range of 11 billion to 16 billion years.

A second method for estimating the age of our galaxy is based on the
present abundances of several long-lived radioactive elements in the solar
system. Their abundances are set by their rates of production and
distribution through exploding supernovas. According to these
calculations, the age of our galaxy is between 9 billion and 16 billion
years. Thus, both ways of estimating the age of the Milky Way galaxy agree
with each other, and they also are consistent with the independently
derived estimate for the age of the universe.

Radioactive elements occurring naturally in rocks and minerals also
provide a means of estimating the age of the solar system and Earth.
Several of these elements decay with half lives between 700 million and
more than 100 billion years (the half life of an element is the time it
takes for half of the element to decay radioactively into another
element). Using these time-keepers, it is calculated that meteorites,
which are fragments of asteroids, formed between 4.53 billion and 4.58
billion years ago (asteroids are small "planetoids" that revolve around
the sun and are remnants of the solar nebula that gave rise to the sun and
planets). The same radioactive time-keepers applied to the three oldest
lunar samples returned to Earth by the Apollo astronauts yield ages
between 4.4 billion and 4.5 billion years, providing minimum estimates for
the time since the formation of the moon.

The oldest known rocks on Earth occur in northwestern Canada (3.96
billion years), but well-studied rocks nearly as old are also found in
other parts of the world. In Western Australia, zircon crystals encased
within younger rocks have ages as old as 4.3 billion years, making these
tiny crystals the oldest materials so far found on Earth.
The best estimates of Earth's age are obtained by calculating the
time required for development of the observed lead isotopes in Earth's
oldest lead ores. These estimates yield 4.54 billion years as the age of
Earth and of meteorites, and hence of the solar system.

The origins of life cannot be dated as precisely, but there is
evidence that bacteria-like organisms lived on Earth 3.5 billion years
ago, and they may have existed even earlier, when the first solid crust
formed, almost 4 billion years ago. These early organisms must have been
simpler than the organisms living today. Furthermore, before the earliest
organisms there must have been structures that one would not call "alive"
but that are now components of living things. Today, all living organisms
store and transmit hereditary information using two kinds of molecules:
DNA and RNA. Each of these molecules is in turn composed of four kinds of
subunits known as nucleotides. The sequences of nucleotides in particular
lengths of DNA or RNA, known as genes, direct the construction of
molecules known as proteins, which in turn catalyze biochemical reactions,
provide structural components for organisms, and perform many of the other
functions on which life depends. Proteins consist of chains of subunits
known as amino acids. The sequence of nucleotides in DNA and RNA therefore
determines the sequence of amino acids in proteins; this is a central
mechanism in all of biology.

Experiments conducted under conditions intended to resemble those
present on primitive Earth have resulted in the production of some of the
chemical components of proteins, DNA, and RNA. Some of these molecules
also have been detected in meteorites from outer space and in interstellar
space by astronomers using radiotelescopes. Scientists have concluded that
the "building blocks of life" could have been available early in Earth's
history.

An important new research avenue has opened with the discovery that
certain molecules made of RNA, called ribozymes, can act as catalysts in
modern cells. It previously had been thought that only proteins could
serve as the catalysts required to carry out specific biochemical
functions. Thus, in the early prebiotic world, RNA molecules could have
been "autocatalytic"--that is, they could have replicated themselves well
before there were any protein catalysts (called enzymes). Laboratory
experiments demonstrate that replicating autocatalytic RNA molecules
undergo spontaneous changes and that the variants of RNA molecules with
the greatest autocatalytic activity come to prevail in their environments.
Some scientists favor the hypothesis that there was an early "RNA world,"
and they are testing models that lead from RNA to the synthesis of simple
DNA and protein molecules. These assemblages of molecules eventually could
have become packaged within membranes, thus making up "protocells"--early
versions of very simple cells.

For those who are studying the origin of life, the question is no
longer whether life could have originated by chemical processes involving
nonbiological components. The question instead has become which of many
pathways might have been followed to produce the first cells.
Will we ever be able to identify the path of chemical evolution that
succeeded in initiating life on Earth? Scientists are designing
experiments and speculating about how early Earth could have provided a
hospitable site for the segregation of molecules in units that might have
been the first living systems. The recent speculation includes the
possibility that the first living cells might have arisen on Mars, seeding
Earth via the many meteorites that are known to travel from Mars to our
planet.

Of course, even if a living cell were to be made in the laboratory,
it would not prove that nature followed the same pathway billions of years
ago. But it is the job of science to provide plausible natural
explanations for natural phenomena. The study of the origin of life is a
very active research area in which important progress is being made,
although the consensus among scientists is that none of the current
hypotheses has thus far been confirmed. The history of science shows that
seemingly intractable problems like this one may become amenable to
solution later, as a result of advances in theory, instrumentation, or the
discovery of new facts.

Creationist Views of the Origin of the Universe, Earth, and Life
Many religious persons, including many scientists, hold that God
created the universe and the various processes driving physical and
biological evolution and that these processes then resulted in the
creation of galaxies, our solar system, and life on Earth. This belief,
which sometimes is termed "theistic evolution," is not in disagreement
with scientific explanations of evolution. Indeed, it reflects the
remarkable and inspiring character of the physical universe revealed by
cosmology, paleontology, molecular biology, and many other scientific
disciplines.

The advocates of "creation science" hold a variety of viewpoints.
Some claim that Earth and the universe are relatively young, perhaps only
6,000 to 10,000 years old. These individuals often believe that the
present physical form of Earth can be explained by "catastrophism,"
including a worldwide flood, and that all living things (including humans)
were created miraculously, essentially in the forms we now find them.
Other advocates of creation science are willing to accept that Earth,
the planets, and the stars may have existed for millions of years. But
they argue that the various types of organisms, and especially humans,
could only have come about with supernatural intervention, because they
show "intelligent design."

In this booklet, both these "Young Earth" and "Old Earth" views are
referred to as "creationism" or "special creation."

There are no valid scientific data or calculations to substantiate
the belief that Earth was created just a few thousand years ago. This
document has summarized the vast amount of evidence for the great age of
the universe, our galaxy, the solar system, and Earth from astronomy,
astrophysics, nuclear physics, geology, geochemistry, and geophysics.
Independent scientific methods consistently give an age for Earth and the
solar system of about 5 billion years, and an age for our galaxy and the
universe that is two to three times greater. These conclusions make the
origin of the universe as a whole intelligible, lend coherence to many
different branches of science, and form the core conclusions of a
remarkable body of knowledge about the origins and behavior of the
physical world.

Nor is there any evidence that the entire geological record, with its
orderly succession of fossils, is the product of a single universal flood
that occurred a few thousand years ago, lasted a little longer than a
year, and covered the highest mountains to a depth of several meters. On
the contrary, intertidal and terrestrial deposits demonstrate that at no
recorded time in the past has the entire planet been under water.
Moreover, a universal flood of sufficient magnitude to form the
sedimentary rocks seen today, which together are many kilometers thick,
would require a volume of water far greater than has ever existed on and
in Earth, at least since the formation of the first known solid crust
about 4 billion years ago. The belief that Earth's sediments, with their
fossils, were deposited in an orderly sequence in a year's time defies all
geological observations and physical principles concerning sedimentation
rates and possible quantities of suspended solid matter.

Geologists have constructed a detailed history of sediment deposition
that links particular bodies of rock in the crust of Earth to particular
environments and processes. If petroleum geologists could find more oil
and gas by interpreting the record of sedimentary rocks as having resulted
from a single flood, they would certainly favor the idea of such a flood,
but they do not. Instead, these practical workers agree with academic
geologists about the nature of depositional environments and geological
time. Petroleum geologists have been pioneers in the recognition of fossil
deposits that were formed over millions of years in such environments as
meandering rivers, deltas, sandy barrier beaches, and coral reefs.
The example of petroleum geology demonstrates one of the great
strengths of science. By using knowledge of the natural world to predict
the consequences of our actions, science makes it possible to solve
problems and create opportunities using technology. The detailed knowledge
required to sustain our civilization could only have been derived through
scientific investigation.

The arguments of creationists are not driven by evidence that can be
observed in the natural world. Special creation or supernatural
intervention is not subjectable to meaningful tests, which require
predicting plausible results and then checking these results through
observation and experimentation. Indeed, claims of "special creation"
reverse the scientific process. The explanation is seen as unalterable,
and evidence is sought only to support a particular conclusion by whatever
means possible.


Part IV

Evidence Supporting Biological Evolution

A long path leads from the origins of primitive "life," which existed at
least 3.5 billion years ago, to the profusion and diversity of life that
exists today. This path is best understood as a product of evolution.
Contrary to popular opinion, neither the term nor the idea of
biological evolution began with Charles Darwin and his foremost work, On
the Origin of Species by Means of Natural Selection (1859). Many scholars
from the ancient Greek philosophers on had inferred that similar species
were descended from a common ancestor. The word "evolution" first appeared
in the English language in 1647 in a nonbiological connection, and it
became widely used in English for all sorts of progressions from simpler
beginnings. The term Darwin most often used to refer to biological
evolution was "descent with modification," which remains a good brief
definition of the process today.

Darwin proposed that evolution could be explained by the
differential survival of organisms following their naturally occurring
variation--a process he termed "natural selection." According to this
view, the offspring of organisms differ from one another and from their
parents in ways that are heritable--that is, they can pass on the
differences genetically to their own offspring. Furthermore, organisms in
nature typically produce more offspring than can survive and reproduce
given the constraints of food, space, and other environmental resources.
If a particular off spring has traits that give it an advantage in a
particular environment, that organism will be more likely to survive and
pass on those traits. As differences accumulate over generations,
populations of organisms diverge from their ancestors.

Darwin's original hypothesis has undergone extensive modification and
expansion, but the central concepts stand firm. Studies in genetics and
molecular biology--fields unknown in Darwin's time--have explained the
occurrence of the hereditary variations that are essential to natural
selection. Genetic variations result from changes, or mutations, in the
nucleotide sequence of DNA, the molecule that genes are made from. Such
changes in DNA now can be detected and described with great precision.
Genetic mutations arise by chance. They may or may not equip the
organism with better means for surviving in its environment. But if a gene
variant improves adaptation to the environment (for example, by allowing
an organism to make better use of an available nutrient, or to escape
predators more effectively--such as through stronger legs or disguising
coloration), the organisms carrying that gene are more likely to survive
and reproduce than those without it. Over time, their descendants will
tend to increase, changing the average characteristics of the population.
Although the genetic variation on which natural selection works is based
on random or chance elements, natural selection itself produces "adaptive"
change--the very opposite of chance.

Scientists also have gained an understanding of the processes by
which new species originate. A new species is one in which the individuals
cannot mate and produce viable descendants with individuals of a
preexisting species. The split of one species into two often starts
because a group of individuals becomes geographically separated from the
rest. This is particularly apparent in distant remote islands, such as the
Galápagos and the Hawaiian archipelago, whose great distance from the
Americas and Asia means that arriving colonizers will have little or no
opportunity to mate with individuals remaining on those continents.
Mountains, rivers, lakes, and other natural barriers also account for
geographic separation between populations that once belonged to the same
species.

Once isolated, geographically separated groups of individuals become
genetically differentiated as a consequence of mutation and other
processes, including natural selection. The origin of a species is often a
gradual process, so that at first the reproductive isolation between
separated groups of organisms is only partial, but it eventually becomes
complete. Scientists pay special attention to these intermediate
situations, because they help to reconstruct the details of the process
and to identify particular genes or sets of genes that account for the
reproductive isolation between species.

A particularly compelling example of speciation involves the 13
species of finches studied by Darwin on the Galápagos Islands, now known
as Darwin's finches. The ancestors of these finches appear to have
emigrated from the South American mainland to the Galápagos. Today the
different species of finches on the island have distinct habitats, diets,
and behaviors, but the mechanisms involved in speciation continue to
operate. A research group led by Peter and Rosemary Grant of Princeton
University has shown that a single year of drought on the islands can
drive evolutionary changes in the finches. Drought diminishes supplies of
easily cracked nuts but permits the survival of plants that produce
larger, tougher nuts. Droughts thus favor birds with strong, wide beaks
that can break these tougher seeds, producing populations of birds with
these traits. The Grants have estimated that if droughts occur about once
every 10 years on the islands, a new species of finch might arise in only
about 200 years.

The following sections consider several aspects of biological
evolution in greater detail, looking at paleontology, comparative anatomy,
biogeography, embryology, and molecular biology for further evidence
supporting evolution.

The Fossil Record

Although it was Darwin, above all others, who first marshaled
convincing evidence for biological evolution, earlier scholars had
recognized that organisms on Earth had changed systematically over long
periods of time. For example, in 1799 an engineer named William Smith
reported that, in undisrupted layers of rock, fossils occurred in a
definite sequential order, with more modern-appearing ones closer to the
top. Because bottom layers of rock logically were laid down earlier and
thus are older than top layers, the sequence of fossils also could be
given a chronology from oldest to youngest. His findings were confirmed
and extended in the 1830s by the paleontologist William Lonsdale, who
recognized that fossil remains of organisms from lower strata were more
primitive than the ones above. Today, many thousands of ancient rock
deposits have been identified that show corresponding successions of
fossil organisms.

Thus, the general sequence of fossils had already been recognized
before Darwin conceived of descent with modification. But the
paleontologists and geologists before Darwin used the sequence of fossils
in rocks not as proof of biological evolution, but as a basis for working
out the original sequence of rock strata that had been structurally
disturbed by earthquakes and other forces.

In Darwin's time, paleontology was still a rudimentary science. Large
parts of the geological succession of stratified rocks were unknown or
inadequately studied.

Darwin, therefore, worried about the rarity of intermediate forms
between some major groups of organisms.

Today, many of the gaps in the paleontological record have been
filled by the research of paleontologists. Hundreds of thousands of fossil
organisms, found in well-dated rock sequences, represent successions of
forms through time and manifest many evolutionary transitions. As
mentioned earlier, microbial life of the simplest type was already in
existence 3.5 billion years ago. The oldest evidence of more complex
organisms (that is, eucaryotic cells, which are more complex than
bacteria) has been discovered in fossils sealed in rocks approximately 2
billion years old. Multicellular organisms, which are the familiar fungi,
plants, and animals, have been found only in younger geological strata.
The following list presents the order in which increasingly complex forms
of life appeared:

Life Form

Millions of Years Since
First Known Appearance
(Approximate)

Microbial (procaryotic cells)3,500
Complex (eucaryotic cells)2,000
First multicellular animals670
Shell-bearing animals540
Vertebrates (simple fishes)490
Amphibians350
Reptiles310
Mammals200
Nonhuman primates60
Earliest apes25
Australopithecine ancestors of humans4
Modern humans0.15 (150,000 years)

So many intermediate forms have been discovered between fish and
amphibians, between amphibians and reptiles, between reptiles and mammals,
and along the primate lines of descent that it often is difficult to
identify categorically when the transition occurs from one to another
particular species. Actually, nearly all fossils can be regarded as
intermediates in some sense; they are life forms that come between the
forms that preceded them and those that followed.

The fossil record thus provides consistent evidence of systematic
change through time--of descent with modification. From this huge body of
evidence, it can be predicted that no reversals will be found in future
paleontological studies. That is, amphibians will not appear before
fishes, nor mammals before reptiles, and no complex life will occur in the
geological record before the oldest eucaryotic cells. This prediction has
been upheld by the evidence that has accumulated until now: no reversals
have been found.

Common Structures

Inferences about common descent derived from paleontology are
reinforced by comparative anatomy. For example, the skeletons of humans,
mice, and bats are strikingly similar, despite the different ways of life
of these animals and the diversity of environments in which they flourish.
The correspondence of these animals, bone by bone, can be observed in
every part of the body, including the limbs; yet a person writes, a mouse
runs, and a bat flies with structures built of bones that are different in
detail but similar in general structure and relation to each other.

Scientists call such structures homologies and have concluded that
they are best explained by common descent. Comparative anatomists
investigate such homologies, not only in bone structure but also in other
parts of the body, working out relationships from degrees of similarity.
Their conclusions provide important inferences about the details of
evolutionary history, inferences that can be tested by comparisons with
the sequence of ancestral forms in the paleontological record.

The mammalian ear and jaw are instances in which paleontology and
comparative anatomy combine to show common ancestry through transitional
stages. The lower jaws of mammals contain only one bone, whereas those of
reptiles have several. The other bones in the reptile jaw are homologous
with bones now found in the mammalian ear. Paleontologists have discovered
intermediate forms of mammal-like reptiles (Therapsida) with a double jaw
joint--one composed of the bones that persist in mammalian jaws, the other
consisting of bones that eventually became the hammer and anvil of the
mammalian ear.

The Distribution of Species

Biogeography also has contributed evidence for descent from common
ancestors. The diversity of life is stupendous. Approximately 250,000
species of living plants, 100,000 species of fungi, and one million
species of animals have been described and named, each occupying its own
peculiar ecological setting or niche; and the census is far from complete.
Some species, such as human beings and our companion the dog, can live
under a wide range of environments. Others are amazingly specialized. One
species of a fungus (Laboulbenia) grows exclusively on the rear portion of
the covering wings of a single species of beetle (Aphaenops cronei) found
only in some caves of southern France. The larvae of the fly Drosophila
carcinophila can develop only in specialized grooves beneath the flaps of
the third pair of oral appendages of a land crab that is found only on
certain Caribbean islands.

How can we make intelligible the colossal diversity of living beings
and the existence of such extraordinary, seemingly whimsical creatures as
the fungus, beetle, and fly described above? And why are island groups
like the Galápagos so often inhabited by forms similar to those on the
nearest mainland but belonging to different species? Evolutionary theory
explains that biological diversity results from the descendants of local
or migrant predecessors becoming adapted to their diverse environments.
This explanation can be tested by examining present species and local
fossils to see whether they have similar structures, which would indicate
how one is derived from the other. Also, there should be evidence that
species without an established local ancestry had migrated into the
locality.

Wherever such tests have been carried out, these conditions have been
confirmed. A good example is provided by the mammalian populations of
North and South America, where strikingly different native organisms
evolved in isolation until the emergence of the isthmus of Panama
approximately 3 million years ago. Thereafter, the armadillo, porcupine,
and opossum--mammals of South American origin--migrated north, along with
many other species of plants and animals, while the mountain lion and
other North American species made their way across the isthmus to the
south.

The evidence that Darwin found for the influence of geographical
distribution on the evolution of organisms has become stronger with
advancing knowledge. For example, approximately 2,000 species of flies
belonging to the genus Drosophila are now found throughout the world.
About one-quarter of them live only in Hawaii. More than a thousand
species of snails and other land mollusks also are found only in Hawaii.
The biological explanation for the multiplicity of related species in
remote localities is that such great diversity is a consequence of their
evolution from a few common ancestors that colonized an isolated
environment. The Hawaiian Islands are far from any mainland or other
islands, and on the basis of geological evidence they never have been
attached to other lands. Thus, the few colonizers that reached the
Hawaiian Islands found many available ecological niches, where they could,
over numerous generations, undergo evolutionary change and
diversification. No mammals other than one bat species lived in the
Hawaiian Islands when the first human settlers arrived; similarly, many
other kinds of plants and animals were absent.

The Hawaiian Islands are not less hospitable than other parts of the
world for the absent species. For example, pigs and goats have multiplied
in the wild in Hawaii, and other domestic animals also thrive there. The
scientific explanation for the absence of many kinds of organisms, and the
great multiplication of a few kinds, is that many sorts of organisms never
reached the islands, because of their geographic isolation. Those that did
reach the islands diversified over time because of the absence of related
organisms that would compete for resources.

Similarities During Development

Embryology, the study of biological development from the time of
conception, is another source of independent evidence for common descent.
Barnacles, for instance, are sedentary crustaceans with little apparent
similarity to such other
crustaceans as lobsters, shrimps, or copepods. Yet barnacles pass through
a free-swimming larval stage in which they look like other crustacean
larvae. The similarity of larval stages supports the conclusion that all
crustaceans have homologous parts and a common ancestry.

Similarly, a wide variety of organisms from fruit flies to worms to
mice to humans have very similar sequences of genes that are active early
in development. These genes influence body segmentation or orientation in
all these diverse groups. The presence of such similar genes doing similar
things across such a wide range of organisms is best explained by their
having been present in a very early common ancestor of all of these
groups.

New Evidence from Molecular Biology

The unifying principle of common descent that emerges from all the
foregoing lines of evidence is being reinforced by the discoveries of
modern biochemistry and molecular biology.

The code used to translate nucleotide sequences into amino acid
sequences is essentially the same in all organisms. Moreover, proteins in
all organisms are invariably composed of the same set of 20 amino acids.
This unity of composition and function is a powerful argument in favor of
the common descent of the most diverse organisms.

In 1959, scientists at Cambridge University in the United Kingdom
determined the three-dimensional structures of two proteins that are found
in almost every multicelled animal: hemoglobin and myoglobin. Hemoglobin
is the protein that carries oxygen in the blood. Myoglobin receives oxygen
from hemoglobin and stores it in the tissues until needed. These were the
first three-dimensional protein structures to be solved, and they yielded
some key insights. Myoglobin has a single chain of 153 amino acids wrapped
around a group of iron and other atoms (called "heme") to which oxygen
binds. Hemoglobin, in contrast, is made of up four chains: two identical
chains consisting of 141 amino acids, and two other identical chains
consisting of 146 amino acids. However, each chain has a heme exactly like
that of myoglobin, and each of the four chains in the hemoglobin molecule
is folded exactly like myoglobin. It was immediately obvious in 1959 that
the two molecules are very closely related.

During the next two decades, myoglobin and hemoglobin sequences were
determined for dozens of mammals, birds, reptiles, amphibians, fish,
worms, and molluscs. All of these sequences were so obviously related that
they could be compared with confidence with the three-dimensional
structures of two selected standards--whale myoglobin and horse
hemoglobin. Even more significantly, the differences between sequences
from different organisms could be used to construct a family tree of
hemoglobin and myoglobin variation among organisms. This tree agreed
completely with observations derived from paleontology and anatomy about
the common descent of the corresponding organisms.

Similar family histories have been obtained from the
three-dimensional structures and amino acid sequences of other proteins,
such as cytochrome c (a protein engaged in energy transfer) and the
digestive proteins trypsin and chymotrypsin. The examination of molecular
structure offers a new and extremely powerful tool for studying
evolutionary relationships. The quantity of information is potentially
huge--as large as the thousands of different proteins contained in living
organisms, and limited only by the time and resources of molecular
biologists.

As the ability to sequence the nucleotides making up DNA has
improved, it also has become possible to use genes to reconstruct the
evolutionary history of organisms. Because of mutations, the sequence of
nucleotides in a gene gradually changes over time. The more closely
related two organisms are, the less different their DNA will be. Because
there are tens of thousands of genes in humans and other organisms, DNA
contains a tremendous amount of information about the evolutionary history
of each organism.
Genes evolve at different rates because, although mutation is a
random event, some proteins are much more tolerant of changes in their
amino acid sequence than are other proteins. For this reason, the genes
that encode these more tolerant, less constrained proteins evolve faster.
The average rate at which a particular kind of gene or protein evolves
gives rise to the concept of a "molecular clock." Molecular clocks run
rapidly for less constrained proteins and slowly for more constrained
proteins, though they all time the same evolutionary events.

The figure on this page compares three molecular clocks: for
cytochrome c proteins, which interact intimately with other macromolecules
and are quite constrained in their amino acid sequences; for the less
rigidly constrained hemoglobins, which interact mainly with oxygen and
other small molecules; and for fibrinopeptides, which are protein
fragments that are cut from larger proteins (fibrinogens) when blood
clots. The clock for fibrinopeptides runs rapidly; 1 percent of the amino
acids change in a little longer than 1 million years. At the other
extreme, the molecular clock runs slowly for cytochrome c; a 1 percent
change in amino acid sequence requires 20 million years. The hemoglobin
clock is intermediate.

The concept of a molecular clock is useful for two purposes. It
determines evolutionary relationships among organisms, and it indicates
the time in the past when species started to diverge from one another.
Once the clock for a particular gene or protein has been calibrated by
reference to some event whose time is known, the actual chronological time
when all other events occurred can be determined by examining the protein
or gene tree.

An interesting additional line of evidence supporting evolution
involves sequences of DNA known as "pseudogenes." Pseudogenes are remnants
of genes that no longer function but continue to be carried along in DNA
as excess baggage. Pseudogenes also change through time, as they are
passed on from ancestors to descendants, and they offer an especially
useful way of reconstructing evolutionary relationships.
With functioning genes, one possible explanation for the relative
similarity between genes from different organisms is that their ways of
life are similar--for example, the genes from a horse and a zebra could be
more similar because of their similar habitats and behaviors than the
genes from a horse and a tiger. But this possible explanation does not
work for pseudogenes, since they perform no function. Rather, the degree
of similarity between pseudogenes must simply reflect their evolutionary
relatedness. The more remote the last common ancestor of two organisms,
the more dissimilar their pseudogenes will be.

The evidence for evolution from molecular biology is overwhelming
and is growing quickly. In some cases, this molecular evidence makes it
possible to go beyond the paleontological evidence. For example, it has
long been postulated that whales descended from land mammals that had
returned to the sea. From anatomical and paleontological evidence, the
whales' closest living land relatives seemed to be the even-toed hoofed
mammals (modern cattle, sheep, camels, goats, etc.). Recent comparisons of
some milk protein genes (beta-casein and kappa-casein) have confirmed this
relationship and have suggested that the closest land-bound living
relative of whales may be the hippopotamus. In this case, molecular
biology has augmented the fossil record.

Creationism and the Evidence for Evolution

Some creationists cite what they say is an incomplete fossil record
as evidence for the failure of evolutionary theory. The fossil record was
incomplete in Darwin's time, but many of the important gaps that existed
then have been filled by subsequent paleontological research. Perhaps the
most persuasive fossil evidence for evolution is the consistency of the
sequence of fossils from early to recent. Nowhere on
Earth do we find, for example, mammals in Devonian (the age of
fishes) strata, or human fossils coexisting with dinosaur remains.
Undisturbed strata with simple unicellular organisms predate those with
multicellular organisms, and invertebrates precede vertebrates; nowhere
has this sequence been found inverted. Fossils from adjacent strata are
more similar than fossils from temporally distant strata. The most
reasonable scientific conclusion that can be drawn from the fossil record
is that descent with modification has taken place as stated in
evolutionary theory.

Special creationists argue that "no one has seen evolution occur."
This misses the point about how science tests hypotheses. We don't see
Earth going around the sun or the atoms that make up matter. We "see"
their consequences. Scientists infer that atoms exist and Earth revolves
because they have tested predictions derived from these concepts by
extensive observation and experimentation.

Furthermore, on a minor scale, we "experience" evolution occurring
every day. The annual changes in influenza viruses and the emergence of
antibiotic-resistant bacteria are both products of evolutionary forces.
Indeed, the rapidity with which organisms with short generation times,
such as bacteria and viruses, can evolve under the influence of their
environments is of great medical significance. Many laboratory experiments
have shown that, because of mutation and natural selection, such
microorganisms can change in specific ways from those of immediately
preceding generations.

On a larger scale, the evolution of mosquitoes resistant to
insecticides is another example of the tenacity and adaptability of
organisms under environmental stress. Similarly, malaria parasites have
become resistant to the drugs that were used extensively to combat them
for many years. As a consequence, malaria is on the increase, with more
than 300 million clinical cases of malaria occurring every year.
Molecular evolutionary data counter a recent proposition called
"intelligent design theory." Proponents of this idea argue that structural
complexity is proof of the direct hand of God in specially creating
organisms as they are today. These arguments echo those of the 18th
century cleric William Paley who held that the vertebrate eye, because of
its intricate organization, had been specially designed in its present
form by an omnipotent Creator. Modern-day intelligent design proponents
argue that molecular structures such as DNA, or molecular processes such
as the many steps that blood goes through when it clots, are so
irreducibly complex that they can function only if all the components are
operative at once. Thus, proponents of intelligent design say that these
structures and processes could not have evolved in the stepwise mode
characteristic of natural selection.

However, structures and processes that are claimed to be
"irreducibly" complex typically are not on closer inspection. For example,
it is incorrect to assume that a complex structure or biochemical process
can function only if all its components are present and functioning as we
see them today. Complex biochemical systems can be built up from simpler
systems through natural selection. Thus, the "history" of a protein can be
traced through simpler organisms. Jawless fish have a simpler hemoglobin
than do jawed fish, which in turn have a simpler hemoglobin than mammals.

The evolution of complex molecular systems can occur in several
ways. Natural selection can bring together parts of a system for one
function at one time and then, at a later time, recombine those parts with
other systems of components to produce a system that has a different
function. Genes can be duplicated, altered, and then amplified through
natural selection. The complex biochemical cascade resulting in blood
clotting has been explained in this fashion.

Similarly, evolutionary mechanisms are capable of explaining the
origin of highly complex anatomical structures. For example, eyes may have
evolved independently many times during the history of life on Earth. The
steps proceed from a simple eye spot made up of light-sensitive retinula
cells (as is now found in the flatworm), to formation of individual
photosensitive units (ommatidia) in insects with light focusing lenses, to
the eventual formation of an eye with a single lens focusing images onto a
retina. In humans and other vertebrates, the retina consists not only of
photoreceptor cells but also of several types of neurons that begin to
analyze the visual image. Through such gradual steps, very different kinds
of eyes have evolved, from simple light-sensing organs to highly complex
systems for vision.








Part V

Human Evolution

Studies in evolutionary biology have led to the conclusion that human
beings arose from ancestral primates. This association was hotly debated
among scientists in Darwin's day. But today there is no significant
scientific doubt about the close evolutionary relationships among all
primates, including humans.

Many of the most important advances in paleontology over the past
century relate to the evolutionary history of humans. Not one but many
connecting links--intermediate between and along various branches of the
human family tree--have been found as fossils. These linking fossils occur
in geological deposits of intermediate age. They document the time and
rate at which primate and human evolution occurred.

Scientists have unearthed thousands of fossil specimens representing
members of the human family. A great number of these cannot be assigned to
the modern human species, Homo sapiens. Most of these specimens have been
well dated, often by means of radiometric techniques. They reveal a
well-branched tree, parts of which trace a general evolutionary sequence
leading from ape-like forms to modern humans.

Paleontologists have discovered numerous species of extinct apes in
rock strata that are older than four million years, but never a member of
the human family at that great age. Australopithecus, whose earliest known
fossils are about four million years old, is a genus with some features
closer to apes and some closer to modern humans. In brain size,
Australopithecus was barely more advanced than apes. A number of features,
including long arms, short legs, intermediate toe structure, and features
of the upper limb, indicate that the members of this species spent part of
the time in trees. But they also walked upright on the ground, like
humans. Bipedal tracks of Australopithecus have been discovered,
beautifully preserved with those of other extinct animals, in hardened
volcanic ash. Most of our Australopithecus ancestors died out close to
two-and-a-half million years ago, while other Australopithecus species,
which were on side branches of the human tree, survived alongside more
advanced hominids for another million years.

Distinctive bones of the oldest species of the human genus, Homo,
date back to rock strata about 2.4 million years old. Physical
anthropologists agree that Homo evolved from one of the species of
Australopithecus. By two million years ago, early members of Homo had an
average brain size one-and-a-half times larger than that of
Australopithecus, though still substantially smaller than that of modern
humans. The shapes of the pelvic and leg bones suggest that these early
Homo were not part-time climbers like Australopithecus but walked and ran
on long legs, as modern humans do. Just as Australopithecus showed a
complex of ape-like, human-like, and intermediate features, so was early
Homo intermediate between Australopithecus and modern humans in some
features, and close to modern humans in other respects. The earliest stone
tools are of virtually the same age as the earliest fossils of Homo. Early
Homo, with its larger brain than Australopithecus, was a maker of stone
tools.

The fossil record for the interval between 2.4 million years ago and
the present includes the skeletal remains of several species assigned to
the genus Homo. The more recent species had larger brains than the older
ones. This fossil record is complete enough to show that the human genus
first spread from its place of origin in Africa to Europe and Asia a
little less than two million years ago. Distinctive types of stone tools
are associated with various populations. More recent species with larger
brains generally used more sophisticated tools than more ancient species.
Molecular biology also has provided strong evidence of the close
relationship between humans and apes. Analysis of many proteins and genes
has shown that humans are genetically similar to chimpanzees and gorillas
and less similar to orangutans and other primates.

DNA has even been extracted from a well-preserved skeleton of the
extinct human creature known as Neanderthal, a member of the genus Homo
and often considered either as a subspecies of Homo sapiens or as a
separate species. Application of the molecular clock, which makes use of
known rates of genetic mutation, suggests that Neanderthal's lineage
diverged from that of modern Homo sapiens less than half a million years
ago, which is entirely compatible with evidence from the fossil record.
Based on molecular and genetic data, evolutionists favor the
hypothesis that modern Homo sapiens, individuals very much like us,
evolved from more archaic humans about 100,000 to 150,000 years ago. They
also believe that this transition occurred in Africa, with modern humans
then dispersing to Asia, Europe, and eventually Australasia and the
Americas.

Discoveries of hominid remains during the past three decades in East
and South Africa, the Middle East, and elsewhere have combined with
advances in molecular biology to initiate a new discipline--molecular
paleoanthropology. This field of inquiry is providing an ever-growing
inventory of evidence for a genetic affinity between human beings and the
African apes.

Opinion polls show that many people believe that divine intervention
actively guided the evolution of human beings. Science cannot comment on
the role that supernatural forces might play in human affairs. But
scientific investigations have concluded that the same forces responsible
for the evolution of all other life forms on Earth can account for the
evolution of human beings.








Part VI

Conclusion

Science is not the only way of acquiring knowledge about ourselves and the
world around us. Humans gain understanding in many other ways, such as
through literature, the arts, philosophical reflection, and religious
experience. Scientific knowledge may enrich aesthetic and moral
perceptions, but these subjects extend beyond science's realm, which is to
obtain a better understanding of the natural world.

The claim that equity demands balanced treatment of evolutionary
theory and special creation in science classrooms reflects a
misunderstanding of what science is and how it is conducted. Scientific
investigators seek to understand natural phenomena by observation and
experimentation. Scientific interpretations of facts and the explanations
that account for them therefore must be testable by observation and
experimentation.

Creationism, intelligent design, and other claims of supernatural
intervention in the origin of life or of species are not science because
they are not testable by the methods of science. These claims subordinate
observed data to statements based on authority, revelation, or religious
belief. Documentation offered in support of these claims is typically
limited to the special publications of their advocates. These publications
do not offer hypotheses subject to change in light of new data, new
interpretations, or demonstration of error. This contrasts with science,
where any hypothesis or theory always remains subject to the possibility
of rejection or modification in the light of new knowledge.

No body of beliefs that has its origin in doctrinal material rather
than scientific observation, interpretation, and experimentation should be
admissible as science in any science course. Incorporating the teaching of
such doctrines into a science curriculum compromises the objectives of
public education. Science has been greatly successful at explaining
natural processes, and this has led not only to increased understanding of
the universe but also to major improvements in technology and public
health and welfare. The growing role that science plays in modern life
requires that science, and not religion, be taught in science classes.








Part VII

Appendix -- Frequently Asked Questions*

What is evolution?

Evolution in the broadest sense explains that what we see today is
different from what existed in the past. Galaxies, stars, the solar
system, and Earth have changed through time, and so has life on Earth.
Biological evolution concerns changes in living things during the
history of life on Earth. It explains that living things share common
ancestors. Over time, biological processes such as natural selection give
rise to new species. Darwin called this process "descent with
modification," which remains a good definition of biological evolution
today.

Isn't evolution just an inference?

No one saw the evolution of one-toed horses from three-toed horses,
but that does not mean that we cannot be confident that horses evolved.
Science is practiced in many ways besides direct observation and
experimentation. Much scientific discovery is done through indirect
experimentation and observation in which inferences are made, and
hypotheses generated from those inferences are tested.

For instance, particle physicists cannot directly observe subatomic
particles because the particles are too small. They make inferences about
the weight, speed, and other properties of the particles based on other
observations. A logical hypothesis might be something like this: If the
weight of this particle is Y, when I bombard it, X will happen. If X does
not happen, then the hypothesis is disproved. Thus, we can learn about the
natural world even if we cannot directly observe a phenomenon--and that is
true about the past, too.

In historical sciences like astronomy, geology, evolutionary biology,
and archaeology, logical inferences are made and then tested against data.
Sometimes the test cannot be made until new data are available, but a
great deal has been done to help us understand the past. For example,
scorpionflies (Mecoptera) and true flies (Diptera) have enough
similarities that entomologists consider them to be closely related.
Scorpionflies have four wings of about the same size, and true flies have
a large front pair of wings but the back pair is replaced by small
club-shaped structures. If two-winged flies evolved from scorpionfly-like
ancestors, as comparative anatomy suggests, then an intermediate true fly
with four wings should have existed--and in 1976 fossils of such a fly
were discovered. Furthermore, geneticists have found that the number of
wings in flies can be changed through mutations in a single gene.
Something that happened in the past is thus not "off limits" for
scientific study. Hypotheses can be made about such phenomena, and these
hypotheses can be tested and can lead to solid conclusions. Furthermore,
many key mechanisms of evolution occur over relatively short periods and
can be observed directly--such as the evolution of bacteria resistant to
antibiotics.

Evolution is a well-supported theory drawn from a variety of sources
of data, including observations about the fossil record, genetic
information, the distribution of plants and animals, and the similarities
across species of anatomy and development. Scientists have inferred that
descent with modification offers the best scientific explanation for these
observations.

Is evolution a fact or a theory?

The theory of evolution explains how life on Earth has changed. In
scientific terms, "theory" does not mean "guess" or "hunch" as it does in
everyday usage. Scientific theories are explanations of natural phenomena
built up logically from testable observations and hypotheses. Biological
evolution is the best scientific explanation we have for the enormous
range of observations about the living world.

Scientists most often use the word "fact" to describe an observation.
But scientists can also use fact to mean something that has been tested or
observed so many times that there is no longer a compelling reason to keep
testing or looking for examples. The occurrence of evolution in this sense
is a fact. Scientists no longer question whether descent with modification
occurred because the evidence supporting the idea is so strong.

Don't many famous scientists reject evolution?

No. The scientific consensus around evolution is overwhelming. Those
opposed to the teaching of evolution sometimes use quotations from
prominent scientists out of context to claim that scientists do not
support evolution. However, examination of the quotations reveals that the
scientists are actually disputing some aspect of how evolution occurs, not
whether evolution occurred. For example, the biologist Stephen Jay Gould
once wrote that "the extreme rarity of transitional forms in the fossil
record persists as the trade secret of paleontology." But Gould, an
accomplished paleontologist and eloquent educator about evolution, was
arguing about how evolution takes place. He was discussing whether the
rate of change of species is slow and gradual or whether it takes place in
bursts after long periods when little change occurs--an idea known as
punctuated equilibrium. As Gould writes in response, "This quotation,
although accurate as a partial citation, is dishonest in leaving out the
following explanatory material showing my true purpose--to discuss rates
of evolutionary change, not to deny the fact of evolution itself." Gould
defines punctuated equilibrium as follows:

Punctuated equilibrium is neither a creationist idea nor even a
non-Darwinian evolutionary theory about sudden change that produces a new
species all at once in a single generation. Punctuated equilibrium accepts
the conventional idea that new species form over hundreds or thousands of
generations and through an extensive series of intermediate stages. But
geological time is so long that even a few thousand years may appear as a
mere "moment" relative to the several million years of existence for most
species. Thus, rates of evolution vary enormously and new species may
appear to arise "suddenly" in geological time, even though the time
involved would seem long, and the change very slow, when compared to a
human lifetime.

If humans evolved from apes, why are there still apes?

Humans did not evolve from modern apes, but humans and modern apes
shared a common ancestor, a species that no longer exists. Because we
share a recent common ancestor with chimpanzees and gorillas, we have many
anatomical, genetic, biochemical, and even behavioral similarities with
these African great apes. We are less similar to the Asian
apes--orangutans and gibbons--and even less similar to monkeys, because we
share common ancestors with these groups in the more distant past.
Evolution is a branching or splitting process in which populations
split off from one another and gradually become different. As the two
groups become isolated from each other, they stop sharing genes, and
eventually genetic differences increase until members of the groups can no
longer interbreed. At this point, they have become separate species.
Through time, these two species might give rise to new species, and so on
through millennia.

Why can't we teach creation science in my school?

The courts have ruled that "creation science" is actually a religious
view. Because public schools must be religiously neutral under the U.S.
Constitution, the courts have held that it is unconstitutional to present
creation science as legitimate scholarship.

In particular, in a trial in which supporters of creation science
testified in support of their view, a district court declared that
creation science does not meet the tenets of science as scientists use the
term (McLean v. Arkansas Board of Education). The Supreme Court has held
that it is illegal to require that creation science be taught when
evolution is taught (Edwards v. Aguillard). In addition, district courts
have decided that individual teachers cannot advocate creation science on
their own (Peloza v. San Juan Capistrano School District and Webster v.
New Lennox School District). (See Teaching About Evolution and the Nature
of Science, Appendix A. National Academy of Sciences, Washington, D.C.
1998.)

Teachers' organizations such as the National Science Teachers
Association, the National Association of Biology Teachers, the National
Science Education Leadership Association, and many others also have
rejected the science and pedagogy of creation science and have strongly
discouraged its presentation in the public schools. In addition, a
coalition of religious and other organizations has noted in "A Joint
Statement of Current Law" that "in science class, [schools] may present
only genuinely scientific critiques of, or evidence for, any explanation
of life on Earth, but not religious critiques (beliefs unverifiable by
scientific methodology)." (See Teaching About Evolution and the Nature of
Science, Appendices B and C, National Academy of Sciences, Washington,
D.C., 1998.)

Some argue that "fairness" demands the teaching of creationism along
with evolution. But a science curriculum should cover science, not the
religious views of particular groups or individuals.

If evolution is taught in schools, shouldn't creationism be given equal
time?

Some religious groups deny that microorganisms cause disease, but the
science curriculum should not therefore be altered to reflect this belief.
Most people agree that students should be exposed to the best possible
scholarship in each field. That scholarship is evaluated by professionals
and educators in those fields. Scientists as well as educators have
concluded that evolution--and only evolution--should be taught in science
classes because it is the only scientific explanation for why the universe
is the way it is today.

Many people say that they want their children to be exposed to
creationism in school, but there are thousands of different ideas about
creation among the world's people. Comparative religions might comprise a
worthwhile field of study, but not one appropriate for a science class.
Furthermore, the U.S. Constitution states that schools must be religiously
neutral, so legally a teacher cannot present any particular creationist
view as being more "true" than others.

*Adapted from Teaching About Evolution and the Nature of Science by the
National Academy of Sciences (Washington, D.C.: National Academy Press,
1998).








Part VIII

Recommended Readings

Evolution

Dawkins, Richard
1996 Climbing Mount Improbable, W.W. Norton: New York and London. An
authoritative and elegant account of the evolutionary explanation of the
"design" of organisms.

Fortey, Richard
1998 Life: A Natural History of the First Four Billion Years of Life
on Earth, Alfred P. Knopf: New York. A lively account of the history of
life on Earth.

Gould, Stephen J.
1992 The Panda's Thumb, W.W. Norton: New York. Gould's Natural History
columns have been collected into a series of books including Hen's Teeth
and Horse's Toes, An Urchin in the Storm, Eight Little Piggies, The
Flamingo's Smile, and Bully for Brontosaurus. All are good popular
introductions to the basic ideas behind evolution, and extremely
readable.

Horner, John R., and Edwin Dobb
1997 Dinosaur Lives: Unearthing an Evolutionary Saga, HarperCollins:
New York. What it's like to uncover fossilized bones, eggs, and more,
plus Horner's views on dinosaurs.

Howells, W.W.
1997 Getting Here: The Story of Human Evolution, Compass Press:
Washington, D.C. A very readable survey of human evolution by one of the
fathers of physical anthropology.

Johanson, Donald C., Lenora Johanson, and Blake Edgar
1994 Ancestors: In Search of Human Origins, Villard Books: New York.
The companion volume to Johanson's NOVA series, "In Search of Human
Origins."

Mayr, Ernst
1991 One Long Argument: Charles Darwin and the Genesis of Modern
Evolutionary Thought, Harvard University Press: Cambridge, MA. An easily
understandable distillation of Charles Darwin's scientific
contributions.

National Academy of Sciences
1998 Teaching About Evolution and the Nature of Science, National
Academy Press: Washington, DC. An engaging, conversational, and
well-structured framework for understanding and teaching evolution.

Nesse, Randolph, and George C. Williams
1996 Why We Get Sick: The New Science of Darwinian Medicine, Vintage
Books: New York. The principle of natural selection as applied to
modern-day health and disease. Helps to illustrate evolution as an
ongoing phenomenon.

Tattersall, Ian
1998 Becoming Human, Harcourt Brace: New York. A description of the
current state of understanding about the differences between
Neanderthals and Homo sapiens.

Weiner, Jonathan
1994 The Beak of the Finch: A Story of Evolution in Our Time, Alfred
P. Knopf: New York. Discussion of basic evolutionary principles and how
they are illustrated by ongoing evolution on the Galápagos Islands.

Whitfield, Philip
1993 From So Simple a Beginning, Macmillan: New York. A large-format,
beautifully illustrated book explaining evolution from genetic, fossil,
and geological perspectives. A good general introduction for
nonspecialists.

Zimmer, Carl
1999 At the Water's Edge: Macroevolution and the Transformation of
Life, Free Press: New York. Some creatures moved from water to land (the
evolution land vertebrates) and others from land to water (the evolution
of whales from land animals). Zimmer clearly explains these two events
in the history of vertebrates and what might have brought them about.

Evolution: Books for Children and Young Adults

Cole, Joanna, and Juan Carlos Barberis
1987 The Human Body: How We Evolved, Illustrated by Walter
Gaffney-Kessell, William Morrow and Company: New York. This book traces
the evolution of humans, from early prehistoric ancestors to modern
tool-users. Grades 4-7.

Lauber, Patricia, and Douglas Henderson
1994 How Dinosaurs Came to Be, Simon and Schuster: New York. A
description of the ancestors to the dinosaurs. Grades 4-7.

Matsen, Brad, and Ray Troll
1994 Planet Ocean: A Story of Life, the Sea, and Dancing to the Fossil
Record, 10 Speed Press: Berkeley, CA. Whimsically illustrated tour of
history for older kids and adults. Grades junior high to high school.

McNulty, Faith
1999 How Whales Walked into the Sea, Illustrated by Ted Lewin,
Scholastic Trade: New York. This wonderfully illustrated book describes
the evolution of whales from land mammals. Grades K-3.

Stein, Sara
1986 The Evolution Book, Workman Publishing Co., Inc.: New York. A
hands-on, project-oriented survey of evolution and its mechanisms.

Grades 4-8.

Troll, Ray, and Brad Matsen
1996 Raptors Fossils Fins and Fangs: A Prehistoric Creature Feature,
Tricycle Press: Berkeley, CA. A light-hearted trip through time ("Good
Gracious -- Cretaceous!") with similes kids will like ("Pterosaurs --
some as big as jet fighters."). Grades 4-6.

Origin of the Universe and Earth

Dalrymple, G. Brent
1991 The Age of the Earth, Stanford University Press: Stanford, CA. A
comprehensive discussion of the evidence for the ages of the Earth,
moon, meteorites, solar system, Galaxy, and universe.

Longair, Malcolm S.
1996 Our Evolving Universe, Cambridge University Press: New York. A
brief discussion of the origin and evolution of the universe.

Silk, Joseph
1994 A Short History of the Universe, Scientific American Library: New
York. Popular treatment of the evolution of the universe.

Weinberg, Steven
1993 The First Three Minutes: A Modern View of the Origin of the
Universe, Basic Books: New York. An explanation of what happened during
the Big Bang.

Evolution and Creationism Controversy

Godfrey, Laurie, ed.
1983 Scientists Confront Creationism, W.W. Norton: New York. A
collection of articles by scientists analyzing and refuting arguments of
creation science.

Kitcher, Philip
1982 Abusing Science: The Case Against Creationism, MIT Press:
Cambridge, MA. A philosophical as well as scientific analysis of
creation science.

Matsumura, Molleen
1995 Voices for Evolution, National Center for Science Education, Inc:
Berkeley, CA. A collection of statements supporting the teaching of
evolution from many different types of organizations: scientific, civil
liberties, religious, and educational.

Numbers, Ronald
1992 The Creationists: The Evolution of Scientific Creationism,
University of California Press: Berkeley, CA. A thorough history of the
American creationist movement.

Pennock, Robert T.
1999 Tower of Babel: The Evidence Against the New Creationism, MIT
Press: Cambridge, MA. A philosopher of science analyzes the newer
"intelligent design" theory and "theistic science" creationism.

Skehan, James W.
1986 Modern Science and the Book of Genesis, National Science Teachers
Association: Washington, DC. Written by a geologist (former Director of
the Weston Seismological Observatory) and bible scholar, trained as a
Jesuit priest.

Strahler, Arthur
1987 Science and Earth History: The Evolution/Creation Controversy,
Prometheus Press: Buffalo, NY. A comprehensive analysis of creationist
scientific claims.

Toumey, Christopher P.
1994 God's Own Scientists: Creationists in a Secular World, Rutgers
University Press: New Brunswick, NJ. An anthropologist's view of
creationism as a belief system upholding the moral authority of both
science and religion.








Part IX

Reviewers

This report has been reviewed by individuals chosen for their diverse
perspectives and technical expertise, in accordance with procedures
approved by the National Research Council's Report Review Committee. The
purpose of the independent review is to provide candid and critical
comments that will assist the authors and the National Academy of Sciences
in making their published report as sound as possible and to ensure that
the report meets institutional standards for objectivity, evidence, and
responsiveness to the study charge. The contents of the review comments
and draft manuscript remain confidential to protect the integrity of the
deliberative process. We wish to thank the following individuals for their
participation in the review of this report:

John Baldeschwieler
J. Stanley Johnson Professor and
Professor of Chemistry
Division of Chemistry and Chemical Engineering
California Institute of Technology
Pasadena, California
John E. Dowling
Maria Moors Cabot Professor
of Natural Science
The Biological Laboratories
Harvard University
Cambridge, Massachusetts
Marye Anne Fox
Chancellor
North Carolina State University
Raleigh, North Carolina
Wilford Gardner
Dean Emeritus
College of Natural Resources
University of California
Berkeley, California
Timothy Goldsmith
Professor of Biology
Department of Molecular, Cellular, and
Developmental Biology
Yale University
New Haven, Connecticut
Avram Goldstein
Professor of Pharmacology, Emeritus
Stanford University
Stanford, California
Ursula Goodenough
Professor
Department of Biology
Washington University
Saint Louis, Missouri Robert Griffiths
Professor of Physics
Carnegie Mellon University
Pittsburgh, Pennsylvania
Norman Horowitz
Professor Emeritus
Division of Biology
California Institute of Technology
Pasadena, California
Susan Kidwell
Professor
Department of Geophysical Sciences
University of Chicago
Chicago, Illinois
David Pilbeam
Henry Ford II Professor of Social Sciences
Peabody Museum
Harvard University
Cambridge, Massachusetts
Luis Sequeira
J.C. Walker Professor Emeritus
Department of Plant Pathology
University of Wisconsin
Madison, Wisconsin
Phillip Tobias
Professor Emeritus
Department of Anatomical Sciences
University of Witwatersrand
Medical School
Johannesburg, Republic of South Africa

And other anonymous reviews.

While the individuals listed above have provided many constructive
comments and suggestions, responsibility for the final content of this
report rests solely with the authoring committee and the National Academy
of Sciences.
 


Part X


Council of the National Academy of Sciences


Bruce Alberts
President
National Academy of Sciences
Washington, DC
Mary Ellen Avery
Professor of Pediatrics
Harvard Medical School
Boston, Massachusetts
Lewis M. Branscomb
Professor Emeritus
John F. Kennedy School of Government
Harvard University
Cambridge, Massachusetts
Ralph J. Cicerone
Chancellor
University of California
Irvine, California
Marye Anne Fox
Chancellor
North Carolina State University
Raleigh, North Carolina
Ralph E. Gomory
President
Alfred P. Sloan Foundation
New York, New York
Ronald L. Graham
Chief Scientist
AT&T Labs
Florham Park, New Jersey
Jack Halpern
Louis Block Distinguished Professor Emeritus
Department of Chemistry
University of Chicago
Chicago, Illinois
David M. Kipnis
Distinguished University Professor
Washington University School of Medicine
Saint Louis, Missouri
Daniel E. Koshland Jr.
Professor in the Graduate School
Department of Molecular and Cellular Biology
University of California
Berkeley, California Peter Raven
Director
Missouri Botanical Garden
Saint Louis, Missouri
Sherwood F. Rowland
Donald Bren Research Professor of
Chemistry and Earth System Science
Department of Chemistry
University of California
Irvine, California
William J. Rutter
Chairman
Chiron Corporation
Emeryville, California
Luis Sequeira
J.C. Walker Professor Emeritus
Department of Plant Pathology
University of Wisconsin
Madison, Wisconsin
Carla J. Shatz
Investigator
Howard Hughes Medical Institute Professor
Department of Molecular and
Cellular Biology
University of California
Berkeley, California
Jean D. Wilson
Charles Cameron Sprague Distinguished Chair
in Biomedical Science
University of Texas Southwestern
Medical Center
Dallas, Texas
Robert H. Wurtz
Chief
Laboratory of Sensorimotor Research
National Institutes of Health
Bethesda, Maryland
 

Promoting an Understanding of the Intelligent Design of the Universe