Q. But what about the evidence of Homology?
Isn't this pretty much conclusive evidence for common ancestry and common
processes of adaptation over the generations?
A. By far the strongest primary evidence for evolution,
for common descent and for Darwinian processes of mutation and natural
selection, is that of homology. Homology is the name given to the anatomical
correspondences between different species that biologists and paleontologists
have noted and studied for centuries.
Darwin himself explained the significance of homology with eloquent simplicity
in The Origin of Species when he said; 'We have seen that the members of the
same class, independently of their habits of life,
resemble each other in the general plan of their
organisation. This resemblance is often expressed by the term "unity of
type"; or by saying that the several parts and organs in the
different species of the class are homologous.'
'What can be more curious than that the hand of a man,
formed for grasping, that of a mole for digging, the leg of
the horse, the paddle of the porpoise, and the wing of the
bat should all be constructed on the same pattern and should
include similar bones in the same relative position?'
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On the face of it, there can be only one rational
explanation for such similarities and that is descent from a common ancestor
from whom the similar features are a genetic inheritance. Some homologies are so
striking that it appears impossible to deny this interpretation. Every
four-footed vertebrate animal has the same five-fingered design with the same
set of bones in modified form. The bones of the arm, wrist and hand that are
found in humans can also be found in modified form in all other four-limbed
animals with backbones.
It is homology that leads Darwinists to put together isolated
fossil remains in ancestor-descendant relationships - often very convincing
ones. It is homology that Darwinists rely on to bridge the gaps in the fossil
record, as in the case of horses. It is homology that underlies the diagrams
drawn up by Darwinists showing how every living thing is related.
But as with all the other evidence for Darwinism, there are major problems when
you look closer. Consider this. The human hand is not only homologous with the
porpoise's paddle and the bat's wing, it is homologous with a structure much
closer to home -- the human foot.
But the human hand cannot possibly have evolved from the
human foot, or vice versa, as they are both part of the same organism. They must
have both evolved independently to become adapted to their present uses --
grasping and walking -- from different ancestral structures.
But if there is no scientific justification for claiming
that the human hand and human foot are homologous structures by reason of
descent from a common precursor organ, then what justification is there for
claiming such a relationship with the bat or the porpoise or the horse? The only
honest answer is 'none'.
There are even worse problems. Australian molecular
biologist Michael Denton has pointed out that if the doctrine of homologous
structures were valid, then it would apply not merely to developed organs like
the hand, but would also apply to embryology and genes. Homologous structures
should be specified by homologous genes and should follow homologous patterns of
embryological development.
However, this is often not the case. In embryological
development, for example, organs that appear identical in different animals do
not arise from the same site or group of cells of the embryo. Even a fundamental
structure such as the alimentary canal, found in all vertebrates, is formed
differently in different animals. In sharks it is formed from the roof of the
embryonic gut cavity, whereas in the lamprey it is formed from the floor of the
gut; from the roof and floor in frogs, and from the lower layer of the embryonic
disc, or blastoderm, in birds and reptiles.
The classic case of homology referred to by Darwin - that
of the forelimbs in vertebrates - turns out in fact to be flawed, since
forelimbs develop from different body segments in different species. In the
newt, the forelimbs develop from trunk segments 2,3,4 and 5; in the lizard from
segments 6,7,8 and 9; and in humans from segments 13,14,15,16,17 and 18. As
Michael Denton points out, from this evidence it could be argued that the
forelimbs are not strictly homologous at all.
Many other comparable examples can be given from embryology: in almost every
case they have been put into a file drawer labeled 'unresolved problems of
homology' and largely forgotten about.
It isn't only embryology that experienced such
disappointments. In the 1950s, when molecular biologists began to decipher the
genetic code, there was a single glittering prize enticing them. When they found
the codes for making proteins out of amino acids, they naturally assumed that
they were on the brink of discovering at the molecular level the same homologies
that had been observed at the macroscopic level in comparative anatomy.
If the bones of the human arm could be traced to the wing
of the bat and hoof of the horse, then the miraculous new science of molecular
biology would trace the homologies in DNA codes that expressed these physical
characteristics. At long last, biologists were on the brink ofng Pandora's
box and finding inside the final key to life: the chemical formula for an arm or
a leg or an eye.
Yet when biologists did begin to acquire an understanding
of the molecular mechanism of genetics, they found that apparently homologous
structures in different species are specified by quite different genes.
Pandora's box turned out to be empty.
The main problem with understanding the genetic code contained in the DNA molecule is that individual genes do not
appear to correspond to individual characteristics. The gene that controls the
colour of a mouse's coat also controls the mouse's size. The gene that controls
the color of the eye of the fruit fly Drosophila also controls the shape of the
female sex organs. Almost all genes in higher organisms have multiple effects of
this sort and Ernst Mayr has suggested that genes which control only a single
characteristic must be rare or nonexistent.
Denton gives an example of the multiple effects of a
single gene in the case of the domestic chicken. There is a degenerative
mutation known for a single gene that causes a wide range of defects: no proper
development of the wings; no claws on the feet; underdeveloped covering of downy
feathers; lungs and air sac absent. The significance of this case is that some
features affected are unique to birds (wings, feathers) while others, such as
the lungs, occur in many other vertebrate species including humans.
Denton points out that; 'This can only mean that
non-homologous genes are involved to some extent in the specifications
of homologous structures'.
The remarkable discoveries of biochemistry and molecular
biology since the 1950s have provided much evidence that, on first reading,
appeared to support many of the premises of Darwinism. For example, there are
some proteins that are widely used in many organisms, such as the proteins
cytochrome C and haemoglobin. Research showed that the sequences of amino acids
comprising these proteins varied slightly from species to species. This seemed
enormously promising for it appeared to show a variation at the molecular level
between species that would mirror the morphological differences in the anatomy
of those species. Although fossils and comparative anatomy had failed,
biochemistry could perhaps provide the evidence Darwinists sought of patterns of
evolutionary inheritance.
It was discovered, for example, that the similarity
between the haemoglobin sequences of animals thought by Darwinists to be more
closely related was greater than that of creatures thought to be distantly
related. This confirmed the Darwinian view of genetic relationships. When the
hemoglobin sequence of two mammals such as a human and dog were examined, they
were found to have a divergence of only about 20 per cent, whereas when the
haemoglobin of human and a fish were examined, they were found to diverge by more
than 50 per cent.
Perhaps by compiling a table of sequences of all the
common proteins for all species we could get a quantified numerical picture of
how closely or distantly related each species is?
This hope, too, was dashed. According to Michael Denton; "However, as more protein sequences began to accumulate during the 1960s, it
became increasingly apparent that the molecules were not going to provide any
evidence of sequential arrangements in nature, but were rather going to reaffirm
the traditional view that the system of nature conforms fundamentally to a
highly ordered hierarchic scheme from which all direct evidence for evolution is
emphatically absent."
What biochemists found when they compiled their table of proteins (such as
cytochrome C) is that it is possible to classify species into groups and that
these groups do indeed correspond exactly to the groups that have been arrived
at by comparative anatomy.
However, what is most striking about such a protein
'atlas' is that each of these identifiable groups or subclasses is isolated and
distinct from the others. There are no transitional or intermediate classes,
just as there are no transitional species in the fossil record or in the living
world today.
Denton points out that published tables of divergence of
the cytochromes, such as the Dayhoff Atlas of Protein Structure and Function,
illustrate this dramatic absence of intermediates.
The most primitive organisms are bacteria whose cells do
not contain a nucleus. All higher organisms, from yeasts to humans, whose cells
do contain a nucleus, are called eukaryotes. If all eukaryotes have descended
from bacteria, then you would expect to find a graduated divergence in their
proteins like cytochrome C.
In fact what you find is that all the main classes, from
man to kangaroo, from fruit fly to chicken, from sunflower to rattlesnake and
from penguin to baker's yeast, are all equidistant
from bacteria with around 65 to 69 per cent divergence.
According to Denton; . . . eucaryotic cytochromes, from
organisms as diverse as man, lamprey, fruit fly, wheat and yeast, all exhibit a
sequence divergence of between sixty-four per cent and sixty-seven per cent from
this particular bacterial cytochrome. Considering the enormous variation of
eucaryotic species from unicellular organisms like yeasts to multicellular
organisms such as mammals, and considering that
eucaryotic cytochromes vary among themselves by up to about forty five per cent,
this must be considered one of the most astonishing findings of modern science.'
Even more extraordinary is the complete absence of evidence from biochemistry
for the most basic Darwinian evolutionary scheme of fish to amphibian to reptile
to mammal. When the protein divergence of land-dwelling vertebrates -
amphibians, reptiles, mammals - are compared with those of fish, they are all
again equally isolated. There is no graduation of divergence as one would expect
in an evolutionary sequence.
The horse, the rabbit, the frog, and the turtle are all 13
per cent divergent in their cytochrome C from the carp. 'At a molecular level',
says Denton, 'there is no trace of the evolutionary transition from fish to
amphibian to reptile to mammal. So amphibia, always traditionally considered
intermediate between fish and the other terrestrial vertebrates, are in
molecular terms as far from fish as any group of reptiles or mammals.'
Perhaps the most baffling finding of all is that radically
different genetic coding can give rise to animals that outwardly look very
similar and exhibit similar behavior, while creatures that look and behave
completely differently can have far less genetic divergence. There are, for
instance, more than 800 species of frogs, all of which look superficially the
same. But there is a greater variation of molecular structure between them than
there is between the bat and the blue whale.
