Evidence
of Evolution
The Nobel Prize
winning scientist Linus Pauling aptly described science as the search for
truth. Science does this by continuously comparing its theories
objectively with evidence in the natural world. When theories no longer
conform to the evidence, they are modified or rejected in favor of new
theories that do conform. In other words, science constantly tries to
prove its assumptions to be false and rejects implausible explanations.
In this way, scientific knowledge and understanding grow over time.
Religious explanations for the order of things are not science because they
are based primarily on faith and do not subject themselves to be objectively falsified.
Because of this fundamental difference in the approach to understanding our
natural world, the U.S. Supreme Court in effect decided in 1987 that the Biblically
based "creation science" is not a science and cannot be taught as such in
public schools as an alternative or in addition to the mainstream evolutionary
theory of the biological sciences. However, religious creation stories
and the idea of "intelligent design"
can be taught in philosophy, religion, or history courses. Religion and
Science provide different approaches to knowledge. It is important to
understand both.
What Is
Evolution?
Biological evolution is
genetic change in a population from one generation to another. The speed and
direction of change is variable with different species lines and at different times.
Continuous evolution over many generations can result in the development of new
varieties and species. Likewise, failure to evolve in response to environmental
changes can, and often does, lead to extinction.
When scientists
speak of evolution as a theory they do not mean that it is a mere speculation.
It is a theory in the same sense as the propositions that the earth is round
rather than flat or that our bodies are made of atoms are theories. Most
people would consider such fundamental theories to be sufficiently tested by
empirical evidence to conclude that they are indeed facts. As a result
of the massive amount of evidence for biological evolution accumulated over the last two
centuries, we can safely conclude that evolution has occurred and continues to
occur. All life forms, including humans, evolved from earlier species,
and all still living species of organisms continue to evolve
today. They are not unchanging end-products.
For those who
have difficulty in accepting evolution because of what they perceive as
contradictions with their fundamental religious beliefs, it may be useful to
distinguish the ultimate origin of life from its later evolution. Many,
if not most, biological scientists accept that primordial life on earth began
as a result of chance natural occurrences 3.5-4 billion years ago.
However, it is not necessary to believe in that view in order to accept that
living creatures evolved by natural means after the origin of the first life.
Charles Darwin modified his religious beliefs, as did many
others, as a result of the discovery of convincing proof of evolution.
Darwin's religious faith was also severely challenged by the death of his 10
year old daughter Annie in 1851. Apparently, he came to believe that his
God created the order of the universe including the rules of nature that
result in biological evolution. His famous book, On the Origin of
Species, was not a denial of his God's existence. However, he did reject a literal
interpretation of the Judeo-Christian Bible. His religious beliefs were
probably very similar to those who advocate "theistic evolution" today.
How
Do We Know
That Evolution Has Occurred?
The evidence for
evolution has primarily
come from four sources:
| 1.
|
the fossil record of change in
earlier species |
| 2. |
the chemical and anatomical similarities
of related life forms |
| 3. |
the geographic distribution of related
species |
| 4. |
the recorded genetic changes in living
organisms over many generations |
The Fossil
Record
| |
 |
Geological strata
containing an
evolutionary sequence of fossils |
Remains of animals and
plants found in sedimentary
rock deposits give us
an indisputable record of past changes through vast periods of time. This evidence attests to the
fact that there has been a tremendous variety of living things. Some extinct
species had traits that were transitional between major groups of organisms. Their
existence confirms that species are not fixed but can evolve into other species over time.
The evidence also shows that
what have appeared to be gaps in the fossil record are due to incomplete data
collection. The more that we learn about the evolution of specific species lines,
the more that these so-called gaps or "missing links in the chain of evolution"
are filled with transitional fossil specimens. One of the first of these
gaps to be filled was between small bipedal dinosaurs and birds. Just
two years after Darwin published On the Origin of Species, a 150-145
million year old fossil of Archaeopteryx
was found in southern Germany. It had jaws with teeth and a long bony
tail like dinosaurs, broad wings and feathers like birds, and skeletal
features of both. This discovery verified the assumption that birds
had reptilian ancestors.
 |
|
 |
|
 |
| Archaeopteryx
fossil |
|
Archaeopteryx
recreation |
|
Archaeopteryx
tail feathers |
Since the discovery of
Archaeopteryx, there have been many other crucial evolutionary gaps
filled in the fossil record. Perhaps, the most important one, from
our human perspective, was that between apes and our own species.
Since the 1920's, there have been literally hundreds of well-dated
intermediate fossils found in Africa that were transitional species
leading from apes to humans over the last 6-7 million years. This
evidence is presented in the last 3 tutorials of this series.
Transitional Tetrapod Fossil--another
example of filling what had been an evolutionary gap.
This link takes you to a QuickTime video. To return
here, you must click the "back" button
on your browser program. (length = 3 mins, 53 secs) |
The fossil record also provides
abundant evidence that the complex animals and plants of today were
preceded by earlier simple ones. In addition, it shows that multicelled organisms evolved only after the first single-celled ones.
This fits the predictions of evolutionary theory.
Chemical
and Anatomical Similarities
Living things on earth are
fundamentally similar in the way that their basic anatomical structures develop and in their chemical
compositions. No matter whether they are simple single-celled protozoa
or highly
complex organisms with billions of cells, they all begin as single cells that reproduce
themselves by similar division processes. After a limited life span, they also all
grow old and die.
All living things
on earth share the
ability to create complex molecules out of carbon and a few other elements. In fact,
99% of the proteins, carbohydrates, fats, and other molecules of living things are made
from only 6 of the 92 most common elements. This is not a mere coincidence.
All plants and animals
receive their specific characteristics from their parents by inheriting particular
combinations of genes. Molecular biologists have discovered that genes are, in fact,
segments of DNA
molecules in our cells.
 |
section of a DNA molecule |
These segments of DNA contain
chemically coded recipes for creating proteins by linking together particular amino acids
in specific sequences.
 |
simple protein molecule |
|
|
 |
|
Human arm bones
(typical vertebrate pattern) |
All of the tens
of thousands of types of proteins in living things are mostly made of only 20 kinds of
amino acids. Despite the great diversity of life on our planet, the simple language
of the DNA code is the same for all living things. This is evidence of the
fundamental molecular unity of life.
In addition to molecular
similarities, most living things are alike in that they either get the energy needed for
growth, repair, and reproduction directly from sunlight, by photosynthesis
,
or they get it indirectly by consuming green plants and other organisms that
eat plants.
Many groups of
species share the same types of body structures because they inherited them
from a common ancestor that had them. This is the case with the
vertebrates
,
which are the
animals that have internal skeletons. The arms of humans, the forelegs of dogs and cats, the wings
of birds, and the flippers of whales and seals all have the same types of bones (humerus, radius, and ulna) because they have retained these
traits of their shared common ancient vertebrate ancestor.
All of these major
chemical and anatomical
similarities between living things can be most logically accounted for by assuming that
they either share a common ancestry or they came into existence as a result of
similar natural processes. These facts make it difficult to accept a theory of
special and independent creation of different species.
Geographic
Distribution
of Related Species
Another clue to patterns of past evolution is found in the natural geographic
distribution of related species. It is clear that major isolated land areas and
island groups often evolved their own distinct plant and animal communities. For
instance, before humans arrived 60-40,000 years ago, Australia had more than 100 species of
kangaroos, koalas, and other marsupials
but none of the more
advanced terrestrial placental mammals
such as dogs, cats, bears, horses. Land mammals were entirely
absent from the even more isolated islands that make up Hawaii and New Zealand. Each
of these places had a great number of plant, insect, and bird species that were found
nowhere else in the world. The most likely explanation for the existence of
Australia's, New
Zealand's, and Hawaii's mostly unique biotic environments is that the life forms in these areas
have been evolving in isolation from the rest of the world for millions of years.
Genetic
Changes Over Generations
The earth's environments are
constantly changing, usually in subtle and complex ways. When the changes are so
great as to go beyond what most members of a population of organisms can tolerate,
widespread death
occurs. As Charles Darwin observed, however, not all individuals always
perish. Fortunately, natural populations have genetic diversity. Those
individuals whose characteristics allow them to survive an environmental crisis likely
will be the only ones able to reproduce. Subsequently, their traits will be more
common in the next generation--evolution of the population will have occurred.
This process of
natural selection resulting in evolution can be easily demonstrated over a 24
hour period in a laboratory Petri dish of bacteria living in a nutrient medium.
When a lethal dose of antibiotic is added, there will be a mass die-off.
However, a few of the bacteria usually are immune and survive. The next
generation is mostly immune because they have inherited immunity from the
survivors. That is the case with the purple bacteria in the Petri dishes
shown below--the bacteria population has evolved.
Evolution
of antibiotic resistant bacteria
|
 |
This same
phenomenon of bacteria evolution speeded up by human actions occurs in our own
bodies at times when an antibiotic drug is unable to completely eliminate a
bacterial infection. That is the reason that medical doctors are
sometimes hesitant to recommend an antibiotic for their patients and insist
that the full dosage be used even if the symptoms of illness go away.
They do not want to allow any potentially antibiotic resistant bacteria to
survive.
Antibiotic
resistance--how mutation and fast reproductive rates of
microorganisms can outpace modern medical
breakthroughs. This
link takes you to an external website. To return here, you must click
the "back" button on your browser
program. |
| |
|
 |
Dog variety resulting
from
selective breeding
over
many generations |
People have developed many
new varieties of plants and animals by selective breeding. This process is similar
to the bacteria experiment described above. Selection of specimens to breed based on particular
traits is, in effect, changing the environment for the population. Those individuals
lacking the desirable characteristics are not allowed to breed. Therefore, the
following generations more commonly have the desired traits.
| |
 |
| |
Insect with a high
reproductive potential |
Species that mature and
reproduce large numbers in a short amount of time have a potential for very fast
evolutionary changes. Insects and microorganisms often evolve at
such rapid rates that our actions to combat them quickly lose their effectiveness.
We must constantly develop new pesticides, antibiotics, and other measures in an ever escalating biological
arms race with these creatures. Unfortunately, there are a few
kinds of insects and microbes that are now significantly or completely
resistant to our counter measures, and some of these species are responsible
for devastating crop losses and deadly diseases.
If evolution has occurred,
there should be many anatomical similarities among varieties and species that have
diverged from a common ancestor. Those species with the most recent common ancestor
should share the most traits. For instance, the many anatomical
similarities of wolves, dogs, and other members of the
genus Canis are due to the fact that
they are descended from the same ancient canine species and still share 99.8%
of their genes. Wolves and dogs also share
similarities with foxes, indicating a slightly more distant ancestor with them.
Genetic Tool Kit--evidence
of a common set of genes for body parts shared by many, if not
most, creatures. This link takes you to a
video at an external website. To return, here, you
must click the "back" button on your browser
program.
(length = 4 mins, 47 secs) |
Given
the abundant evidence supporting the theory of biological evolution, it is
highly probable that evolution has occurred and is still occurring today.
However, there remains speculation in regards to the specific evolutionary path
of some species lines and the relative importance of the different natural
processes responsible for their evolution.
Much
has been added to our understanding of the nature of evolution since the 19th
century. It is now known that there are six different processes that can
operate independently or in consort to bring about evolution. The
understanding of these processes has become the basis for an overall
synthetic theory of
evolution
. This theory encompasses multiple
causes, including Charles Darwin's concept of natural selection, Gregor Mendel's
experimental results concerning genetic
inheritance, as well as a number of crucial 20th century discoveries. The
synthetic theory of evolution will be revisited with more detail in the
6th tutorial of this biological
anthropology series.
The Public Perception of
Evolution in the United States
Biological evolution is far from being
universally accepted by Americans. Annual national polls carried out since
the mid 1980's by the Center for Biomedical Communication at
Northwestern University School of Medicine indicate that the percentage of Americans
who accept evolution has dropped from 45% to 40%. Curiously, the number
who reject evolution have also dropped from 48% to 39% over the same time
period. Those who are uncertain about whether evolution occurs or not have
increased from 7% to 21%. While it is encouraging that fewer people are
now
hostile to the idea of biological evolution, the U.S. still has a higher
percentage of its population who hold this view than 33 of the 34 European nations and
Japan. This is very likely a consequence of the relative emphasis placed
on teaching science in public schools in the different countries. In addition, anti-evolution
sentiment is far stronger in American national politics.
For the vast majority of biologists, the
debate over whether evolution occurs took place in the 19th century and has long
been settled--evolution won. The noted environmental biologist Theodosius
Dobzhansky summed it up in 1973 by saying "nothing in biology makes sense except
in the light of evolution".
NOTE:
Some critics have said that the kinds of rapid evolutionary
changes in insects and bacteria referred to above are not good evidence of the
process of natural evolution because they occur as a result of human
interference. However, there is abundant evidence of rapid evolution
occurring today independent of people. An example was described by
Cristina Sandoval in the May 23, 2002 issue of Nature. A species of
insect called the "walking stick" (Timena cristinae) found in the Santa Ynez Mountains of California
now exists in two distinct varieties or forms that are in the process of evolving into
two separate species by adapting to different environments. The insect
forms differ in terms of genetically determined color patterns--one is
striped and the other is not. The striped ones hide from predators on
the striped chamise plant, while the unstriped ones hide on the unstriped blue
lilac plant. Those that have inherited the appropriate
camouflaging color pattern for their chosen environment survive the onslaught of
lizards and birds. In this case, the natural predators, rather than
humans, are the driving forces of natural selection. Mating experiments
show that each variety of "walking stick" prefers to mate only with others
having the same color pattern. This breeding isolation is leading to the
evolution of two distinct species.
NEWS:
On July 17, 2005, The Pew Research Center for the People and the
Press conducted a national poll in the United States concerning the teaching of
creationism and evolution. In regards to beliefs about how life developed,
42% of the respondents said that "living things have existed in their present
form since the beginning of time." Only 26% said that they had evolved
through time as a result of "natural process such as natural selection," while
18% said that evolution occurred but was guided by a supreme being. In
response to the question of whether creationism should be taught in public
schools instead of evolution, 38% said yes and 49% said no. When asked
whether creationism should be taught along with evolution, 64% said yes and 26%
said not. The older the respondent, the more likely he/she was to reject
evolution and its teaching in favor of creationism. The sample consisted
of 2,000 people and the margin of error was ±3.5%.
POSTSCRIPT:
For additional information
regarding "creation science" and "intelligent design", look at the 2007 PBS Nova
documentary and the entertaining 2008 follow-up lecture by Dr. Kenneth Miller linked below.
Copyright © 1998-2013 by Dennis
O'Neil.
All rights reserved.
illustration credits