Some theoretical speculations spurred by the recent discussion of
Out of Africa.

The oxygen content of the air reached a minimum over the past 250K
years between 145 and 118,000 years ago (see the jpg chart
attached).  This time just precedes or coincides with the earliest
findings of homo sapiens.  It was a very warm period as well,
representing a 30,000 year peak temperature that is much warmer
than the preceding 20,000 years and the ensuing 100,000 years.
Starting about 118,000 ybp (years before present), the climate
cooled dramatically.  And the oxygen content of the atmosphere
increased from about 32 per cent to closer to 40 per cent.  The
present warm, interglacial period shows few temperature and oxygen
deviations and the oxygen content is about 36 per cent.

The high oxygen content of the atmosphere during the period of the
emergence of homo sapiens may have been a factor contributing to
homo's adaptation.  An atmosphere with high oxygen content
produces a free radical load.  Free radicals are a well-known
source of mutations to the RNA and DNA and may have increased the
rate of mutation and hence the diversity of early homos and their
rate of adaptation to the changing climate and atmosphere.

The Eve bottleneck occurs near this period as does the appearance
of anatomically modern humans.  The ability to adapt rapidly to
the changing environment of falling, rising and then falling
temperatures and a highly variable oxygen level are likely to have
been the selective pressures that led to homo sapiens

If African Eve lived about 140ky ago it would coincide with the
abrupt change in temperature and oxygen density that took place
then. In just a few hundred years, the temperature jumped about 22
degrees and oxygen density plummeted from about 40 percent to 32
percent.  This could have produced the population bottleneck that
the Eve hypothesis proposes.

A notable change in the shape of the human rib cage occured during
this time which coincides with the appearance of anatomically
modern humans.  The upper thorasic area of the chest expanded
relative to homo sapiens' predecessors, probably in response to
the rapidly diminishing oxygen content from 150,000 to 130,000
years ago.  The ability to remain mobile and active in a world of
diminishing oxygen would have been an enormous selective
advantage.  Coupled with the evolution of sweat glands, the
widening thorasic crest and expanding lung capacity would have
given our first modern ancestors enormous competitive advantages
over their predecessors.  With the ability to utilize more oxygen
would come a concommitant need to shed heat.  A human adapted to a
lower oxygen density would have been able to move more rapidly and
for much longer periods than prey that had not so quickly adapted
to low oxygen levels.  Along with the increasing grassy areas and
large game, the mobility advantage is leveraged into hunting
prowess.

Art De Vany