Re: Question: Population Bottleneck and "out of Africa"

From: "Paul Crowley" <slkwuoiutiuytciu...@xxxxxxxxxxxxxxxxx>
Now, habilis and erectus are now thought to be sister species
that overlapped in time.

I agree that latter day habilis, shortly before they became
extinct, were very likely sister to middle day erectus that lived
at the same time. But unless we can find erectus fossils all the
way back to the earliest habilis fossils, there's a chance that the
earliest habilis are ancestral to both of those later sister
species. Perhaps a small group of habilis became isolated from the
main group, experienced rapid drift and/or adaption, which by
chance allowed them to expand in number to rival the remaining
descendents in the habilis main group. There may have been many
small isolated groups of habilis, all but one of which drifted to
extinction, but that one drifted to success. This is the usual
punctuated equilibrium hypothesis, that there were large numbers of
both the main habilis group all the way along, and of the later
clade of erectus after their numbers grew due to their successful
drift or adaption, leaving fossils we can see, but the h->e
founding population was too small to leave any fossils for us to
find, so we have no h->e transitionals. Maybe some day we'll find
one or two to clinch the case. Or maybe it'll remain unresolved. Or
maybe we'll find DNA from both major groups at various times, and a
cladogram will look like this (vertical postion represents time,
oldest at top, while horixontal position represents genetic
distance along shortest path from final of each major group):
H
H
E H
E H
E H
E H
E H
E H
E
E
E
Note the gross anatomical changes came first, to allow erectus to
employ a different survival strategy from habilis, to live in the
new habitat initially, and later to co-exist with habilis in
overlapping territory without competing too much for the very same
resources. Then later adjustments to other biological features such
as metabolism or mating behaviour etc. followed, driving the
cladogram further away from habilis without much affecting the
morphospecies we observe in the fossil record. Thus we see two
distinct narrowly-peaked morphospecies in the fossil record, but
will see (when we get the DNA) a spectrum with only a small gap in
the genome span.

"Their co-existence makes it unlikely that Homo erectus evolved
from Homo habilis," said co-author Professor Meave Leakey,
palaeontologist and co-director of the Koobi Fora Research Project.
. . . . . "

I disagree. My cross-plot between time and genetic distance above
is entirely consistent with both co-existance and ancestry. If we
believe Gould's theory of punctuated equilibrium, we have to treat
my kind of scenerio as acutally quite likely, almost exactly what
we would expect. Too bad we don't have the DNA to test our
competing hypotheses.

.

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