EVIDENCE FOR EVOLUTION
There is a wide range of evidence for evolution from both palaeontology and from
living organisms. The following is a brief summary of some of this evidence:
(1) Fossil evidence
The fossil record, although certainly not complete, does indicate several major
evolutionary themes over long periods of time. The trend from simple to complex
organisms and the trend from aquatic to terrestrial organisms is clearly evident from
the fossils, as is the development of sexually reproducing organisms from asexually
reproducing ones.
(2) Transitional forms
Transitional forms (or link organisms) are evident in the fossil record as well as
amongst living organisms. Fossil remains of a fish (Crossopterygian) that could
absorb oxygen directly from the air have been found in rocks formed 400 million
years ago. This fish, in addition to the usual scales, fins and gills also had bones
inside of lobe-fins which suggest it could drag itself over land. It also had lungs and
it is thought that amphibians developed from fish along this line of descent. The
extinct Archaeopteryx is most likely a link between the reptiles and birds. An example
of a living transitional form would be the Peripatus which is a link between the
annelids and the arthropods.
(3)
Evidence from taxonomy
The characteristics of organisms differ in such an orderly pattern that they can be fitted
into a hierarchical scheme of categories. Our present, well-established classification
scheme, first developed by Carolus Linnaeus, shows relationships between groups of
organisms that follows evolutionary development. If plants and animals were not related
in evolution but had been independently created, their characteristics would most likely
be distributed in a confused, random fashion and a well-organised classification scheme
would be impossible.
(4) Morphology
There is much evidence for evolution from comparative anatomy. The existence of
homologous structures is particularly striking in vertebrates. All vertebrates show some
form of a basic five-digit plan in the bones of their limbs (the pentadactyl limb). It would
be difficult to explain why the limbs of such diverse creatures as a human, a whale, a bat
and a frog all shared a common basis for the construction of their limbs if there were no
evolutionary relationship between them.
As well, the presence of vestigial organs which are useless or degenerate structures
found in the modern organism points to the existence of some ancestral forms in which
these organs were once functional. The human tail bone at the base of the spine is such a
vestigial structure.
(5) Comparative embryology: “Ontogeny
recapitulates phylogeny”
First described by Ernst Haeckel in 1866, this principle suggests that embryos, in the
course of their development, recapitulate or repeat the evolutionary history of their
ancestors. The early human embryo resembles a fish embryo in that it has gill slits, pairs
of aortic arches, a fish-like heart with a single atrium and ventricle, a primitive fish-like
kidney and a tail that can wag. Later in its development, the human embryo resembles a
reptile with closed gill slits, fused vertebra, a new kidney and an atrium partitioned into
left and right chambers. Still later the human embryo develops the four-chambered heart
of birds and resembles a chick embryo more than an adult human. Even after seven
months of development the human embryo (if covered with hair) looks more like a baby
ape than a human being.
(6) Biochemical evidence
Chemical similarities at the cellular level, such as amino acid sequences in common
plasma proteins and tissue enzymes, can be used to confirm the evolutionary
relationships within groups of organisms. Cytochrome c, a respiratory chain enzyme, has
been studied extensively in this regard and many species have been compared. The
further apart any two species are evolutionarily, the greater the number of differences in
their amino acid sequences.
The universality of genetic information (i.e. human DNA can function perfectly in
bacterial cells) is strong evidence for a common ancestor for all living things.
(7) Geographic distribution
Particular types of plants and animals are found in certain continents and not others. For
example, the animals and plants of Asia and Australia are very different. This
phenomenon was first described by Alfred Russel Wallace who suggested Wallace’s Line
to demarcate the distribution of these organisms. It is widely accepted that Australia’s
unique mammals and flowering plants resulted from periods of evolution in isolation.