THEORIES OF HUMAN EVOLUTION

Jonathan Paul Carson jpc4e at dayhoff.med.Virginia.EDU
Sat Sep 24 12:26:12 EST 1994


Darwinism says that mutations slowly arise over time within the
gene pool of a species.  A mutation(or polomorphism) is a change in
a gene.  A gene pool is the total number of genes shared by all
the organisms of a species.  A species is the sum of all its
organisms which can mate and produce viable offspring.

If a mutation or polmorphism is favorable to a population
*within* a species, over time, the mutation will be found in
greater frequency within the population.  That is, if a gene is
present that improves the reproductive fitness of one
organism--chances are--it will improve the reproductive fitness
of its offspring.  

An experimental approach has been set up in the last twenty
years that looks at the genetic basis for evolution.  As far as
this relates to humans, it has involved the comparison of
protein and DNA sequences between humans and other animals.  (It
has probably been more useful to compare amino acid sequences,
due to a lower signal to noise ratio.)  The analyses have shown
what we had expected: protein and DNA sequences between humans and
chimpanzees are more similar than between any other two
species.  Hence, there existed a species of ape which gave rise
to both chimpanzees and humans; judging by the estimated rates of 
change  found  among  protein  sequences  of  recognized
homology, this divergence occured roughly 4.5 to 7 million
years ago.

There is some criticism directed at the establishment of more complex
phylogenetic trees, because the criteria of determining which
species are "more primitive" are a bit arbitrary.  Also, it is not
easy to *definitively* draw a map of the branching of species,
because we do not have many (or any) protein or DNA sequences
from tens of millions of years ago!  However, on a small scale, 
this seems to shed light on things nicely.

Evolution, of course, is a dynamic.  At one time and place, a
genetic mutation may improve the reproductive fitness of its
population members.  In some cases, the effect of a mutation is
subtle and unrecognizable in the phenotype.  In others, the
mutations are lethal.

The jist of all this is that the ancestor population of chimps
and humans split off into two populations about 5 million years
ago.  Why?  Climate, availability of food, and geography all
played a role.  It seems that the ancestors of humans had to
become more adapted to a life on the plains and savannah, not
in the hills and forests of Africa.  Perhaps the grasslands
expanded into areas that were once forest, and human ancestors
"chose" not to follow the line of receding trees along with
their chimp cousins.  In any case, this new environment was one
that lended itself to tool use more than in the forest.  Also,
there might have been a pressure for a more closely-knit social
structure that could *communicate* and plan with one another.

In time, enough mutations accumulated such that the ancestors
of humans and chimps could no longer mate with one another and
produce viable offspring.  Presumably, by this time, they were
behaving very differently and adapted to very differnt
ecological niches.  Australopithicus was born.  

Well, i have to run, now.  I cannot write a part two.  So i'll
sum it up.  The current belief is that australopithicines
branched out, one of them into Homo--our genus.  Homo also
branched out into various forms, one of them sapiens--our
species.  There is dispute as to when and where Homo sapiens
emerged.  Probably it was around 100 to 200,000 years ago.
Initial mtDNA studies showed that it was in Africa, but they
set up their tree with some arbitrary criteria.  Subsequent
trees showed that the common ancestor of all living humans
might have come from Southeast Asia or Australia or Africa.

I cannot talk more.  There are interesting questions on the
physical nature of DNA which gives rise to evolution.  For
example, the presence of transposons, retroviruses, and exon
shufflings add new variables to questions in human evolution.
Transposons might be responsive to the external environment and
not just random chance.  Carnes talks about this, but with
bacteria.  Is anything Carnsian going on with mammals?

				bye

				Jon



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