Evidence for Evolution
The process of evolution implies that organisms have an evolutionary history. Organisms were not
created as they are today, but instead developed and changed over time. This is what Darwin called
“descent with modification.” Also, implicit in evolutionary theory is the idea of common ancestry i.e.
that different species share common ancestors and today these different species have traits in common
because they inherited them from their common ancestor.
Evidence for an evolutionary history –summary
There is a lot of anatomical evidence that indicates clearly that organisms have an evolutionary history.
This anatomical evidence includes:
vestigial structures–structures that have no function today, but were functional in ancestors.
E.g. appendix
atavistic structures – (“throwback” structures) reappearance of ancestral traits in modern
individuals. E.g. tails in humans
jerry-built structures—poorly built generally anatomically inefficient structures that imply the
structure was modified from a different structure in the past or is constrained in its structure as a result
of previous evolutionary development. E.g. Panda’s thumb
Fossils also provide evidence of evolutionary history.
Evidence for common ancestry –summary
Homologous structures are anatomical features (e.g. the forelimbs of a dog, a bat, a human and a
whale) that are structures that are similar in position and structure, but not necessarily in function. The
forelimbs of these animals are all built from the same set of bones, but they are used for very different
things in each organism. The underlying similarity in bone structure implies that these very different
forelimbs were all modified from a basic set of bones inherited from the animals’ common ancestor.
Other things can be homologous too e.g. chromosomes and gene sequences, but the idea originated
with anatomical structures.
Evidence for an evolutionary history – detailed discussion
Vestigial Structures Many organisms possess rudimentary or functionless versions of body parts that
would have functioned in their ancestors.
Cave populations of Mexican tetra fish, which live entirely in the dark, have eye sockets but no eyes.
Kiwis (a medium sized flightless New Zealand bird) has tiny, stubby wings
Some snakes e.g. pythons and whales have tiny remnant hind limbs.
Human vestigial structures include the Coccyx, which is the vestigial tailbone at base of spine. It
represents the remains of our ancestor’s tail.
Arrector pili muscle at base of hair follicles. These are the muscles that make your hair stand up when
you are scared or cold. In animals with a dense coat of fur this would help to make the animal look
bigger and more intimidating or keep the animal warm by trapping a layer of insulating air. In use it
achieves nothing useful.
Appendix: reduced in size. It is functional in many animals and is used in the digestion of cellulose in
herbivores e.g. rabbits. In humans it is greatly reduced in size.
Vestigial Developmental traits—in many organisms features appear during development and then
disappear. These are often traits that were functional in ancestors, but are not used today in the adult
organism.
For example : in adult chickens: three bones in forefoot (wing), four in hindfoot. However, digit 5
appears briefly during embryonic development before disappearing.
In humans during embryonic development a coat of hair (called lanugo) appears and then usually
disappears before birth. Some babies however are born with this coating of hair (which drops off shortly
after birth).
Molecular vestigial traits
Human genome contains large numbers of pseudogenes that do not code for functional RNA or
proteins. These are pieces of DNA that used to be functional, but appear to have been turned off in the
evolutionary past. There are, for example, several pseudogenes of hemoglobin. In all, there may be as
many as 6,000 pseudogenes in human genome. Obviously, the presence of non-functional DNA implies
an evolutionary history.
Atavistic Structures
We contain DNA instructions for making many structures our ancestors possessed, but these genes are
usually turned off (see molecular vestigial traits above). Sometimes these genes are accidentally turned
on and atavistic structures occur, which are “throwbacks” to our ancestral state.
Examples include instances of extra toes in horses. The modern horse has only one toe (the hoof), but
sometimes horses are born with non functional extra toes either side of the main toe.
Occasionally humans are born with short non functional tails.
Whales and dolphins with remnant non-functional hind limbs are occasionally seen.
Jerry-rigged (sloppily built) structures e.g. the Panda’s thumb.
Natural selection unlike an engineer or designer does not build from scratch. Instead it must use
whatever materials are available to it and modify them for new purposes. The result is that structures
are far from perfectly well engineered and often are comparatively crude solutions to a problem.
Examples include the Panda’s “thumb.” Panda’s have five digits, but also have an apparent sixth digit a
”thumb.” Pandas use this “thumb” to hold bamboo when they are eating. The thumb is actually a wrist
bone (the radial sesamoid) that is found in all bears, but has become enlarged in pandas. The panda’s
thumb is not a very efficient solution to the problem of holding bamboo, but clearly suggests evolution
rather than design.
Jerry-rigged (sloppily built) structures: The human eye
The retina of the human eye is wired backwards. The photoreceptors face away from the light. Its blood
and nerve supply lie between the light source and the retina. This results in blood vessels interfering
with the passage of light and a blind spot where the blood vessels and nerves exit the eye. The basic
structure of the human eye is a consequence of developmental pathways laid down in (presumably
transparent) ancestors more than 500 million years ago. In those ancestors the direction from which
light was sensed and the position of the nerves and blood vessels presumably did not matter. As our
eye design is based on that original pathways we are stuck with an eye that is far from optimally
designed.
Jerry-rigged (sloppily built) structures : mammalian recurrent laryngeal nerve
The left recurrent laryngeal nerve innervates the larynx from the vagus nerve (10th cranial nerve) . It
takes a very roundabout path to the larynx as it loops under the aorta and then back up to the larynx.
Why does it take this long indirect route (which is about 15 feet in giraffes)?
The length of this nerve is a consequences of developmental pathways inherited from our fish ancestors.
Early in human embryological development brachial arches form in the neck. In our fish ancestors these
would have formed the gill arches of the fish. In mammals arches 4 and 6 give rise to some of the major
blood vessels. The 4th branch of the vagus nerve lies behind the 6th brachial arch and as this arch moves
back into the thorax during development the nerve has to move and grow with it.
Fossil evidence of evolutionary history
Law of Succession: Fossil and living organisms in same area are related to each other and differ from
organisms in other areas. The fact that recent fossils found in continents resemble contemporary
organisms is of course consistent with the current fauna having evolved in place.
E.g. Australia is filled with marsupials and Australian fossils are of similar marsupial forms.
South America contains both fossil and living armadillos. Extinct glyptodont (2,000 kg) resembles
modern-day armadillo (2 kg).
Transitional forms
If fossil organisms are ancestors of modern organisms then there should be transitional fossils that show
characteristics intermediate between older and more recent groups.
Examples of transitional forms include Archaeopteryx the oldest known fossil bird (name means
“ancient wing) has mix of reptilian and avian features. Reptilian: long tail, teeth, long clawed fingers.
Avian: feathers, ribs with uncinate processes, avian shoulder girdle.
Jaw evolution in mammals
In the distant ancestor of mammals known as the synapsids the jaw was made up of an anterior toothbearing dentary with a series of bones (the post-dentary bones) forming the posterior half. In this
condition the articular bone of the lower jaw articulated (hinged) with the quadrate bone of the skull.
In modern mammals however the dentary articulates with a skull bone called the squamosal.
How did we transition from one articulation articular-quadrate to the other dentary-squamosal?
Our ancestors could not have gone through a “no articulation” phase. Organisms cannot get less well
adapted to later become better adapted.
We can predict that in the proto-mammal jaw there had to have been two jaw joints at one point.
We have clear fossil evidence of this transition In a group of synapsids called the cynodonts a process of
the dentary grew back and eventually made contact with the squamosal bone of the skull. This contact
eventually formed the dentary-squamosal joint. These animals also had an A-Q joint.
Later the original (A-Q) joint was lost, and the jaw was reduced to a single bone the dentary articulating
with the squamosal bone of the skull. Some of the post-dentary bones (incus, malleus and stapes) came
to form part of the inner ear .
Transitional fossils in Whale evolution
Whales are aquatic mammals that evolved from terrestrial ancestors. The evolution of whales is well
documented by fossil discoveries. Modern whales have peg-like teeth or baleen for feeding. Early fossil
whales such as Dorudon (40 mya) however had more complex teeth that were similar to those of
contemporary terrestrial mammals.
Dorudon and modern whales share numerous features of the skull in common, including a distinctive
thick-walled ectotympanic bone.
The same distinctive ectotympanic bone is found in Pakicetus a terrestrial carnivorous animal from 50
mya. Pakicetus also possesses another distinctive bone its ankle bone or astragalus. In Pakicetus the
astragalus has a double-pulley like morphology and this structure is found only in artiodactyls (hoofed
mammals such as cows, pigs and deer). This implies whales share a most recent common ancestor with
artiodactyls and so are most closely related to this group of mammals.
Evidence of common ancestry. Homologous structures
Homologous structures are constructed from the same basic components. For example, the forelimbs of
human, mole, horse, dolphin and bat contain the same bones, but used in radically different ways. Even
though the forelimbs have evolved to carry out very different tasks they are all constructed from the
same bones. This makes no sense if organisms were created, but does if organisms share a common
ancestor.
Developmental Homology
Embryos of diverse array of vertebrates are very similar in early development. For example all display
gill slits (that disappear later) even though adults have no gills in many of these organisms. This
similarity Is due to the temporary expression of old developmental pathways inherited from ancient
ancestors.
Remember that not all similarities in appearance are due to homology. For example, the streamlined
shapes of fish and whales are not a result of common ancestry but of convergent evolution. Convergent
evolution means the same evolutionary solution to a problem has developed independently in unrelated
lineages. E.g. Flying squirrels in Africa, U.S. and Australia have all evolved a flap of skin between their
limbs that they use for gliding.
Molecular Homologies
In many cases there are similarities in subcellular elements and genes that are also homologous.
Genetic code: For example, with few exceptions all organisms use same genetic code to code for the
construction of proteins from DNA.
A Molecular homology. Chromosome 17 in humans PMP22 gene has duplicate sequence of DNA
(CMT1A repeat) on either side of it.
This is a result of the duplication and insertion of this piece of DNA. Occasionally this causes inaccurate
crossing over during meiosis. Interestingly, humans share the CMT1A repeat with bonobos and
chimpanzees, but not with gorillas, orang-utans or other primates.
What this suggests is the CMT1A duplication occurred in the past before the bonobo, chimps and human
lineages split and is derived from common ancestor of bonobos, chimps and humans.
Chromosomal Homology
Humans have 23 pairs of chromosomes, but chimps, and other great apes have 24. If humans and the
other great apes share a common ancestor how is this possible?
[Terminology: Centromere: portion of chromosome that link sister chromatids during cell division.
Telomere: protective cap at the end of a chromosome. ]
It turns out chromosome 2 in humans resulted from the fusion of two chromsomes. Evidence for this is
that chromosome 2 contains two centromeres instead of the usual one as well as telomeres in the
interior of the chromosome. It also contains the same genes as in the equivalent two ape
chromosomes.
Some biogeographic evidence for evolution: adaptive radiation
Many remote islands have unique floras and faunas. They are populated by arrays of diverse but closely
related organisms. This is hard to explain from a creationist perspective, but makes sense if the
organisms found there today are derived from an ancient colonist ancestor that speciated to produce
This process is called adaptive radiation.
Process: Ancestral colonist arrives on island. A shortage of resident species means many niches are
unfilled so there is no competition. As a result, the ancestral species gives rise to new species that
occupy various unfilled niches.
Examples include Darwin’s finches on the Galapagos Island and Drosophila fruit flies on the Hawaiian
Islands.
On Galapagos Islands there are 13 species of anatomically very different, but closely related species of
finch.
 They differ greatly in beak size and diet having evolved very different lifestyles.
Hawaiian Drosophila are another excellent example of adaptive radiation
More than 25% of the world’s Drosophila fruit flies occur only in the Hawaiian Islands. There are
few insect competitors so the original colonizers diversified to fill a large number of niches that
are filled elsewhere by other types of insect.