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