Monday, February 23, 2015

Human Evolution- Multiple Opportunities For Migrating "Out Of Africa"

There really seems to be a connection between orbital mechanics and human behavior, though not in a way astrologers think it to be!

From the early online February issue of Geology-

The dispersal of human populations out of Africa into Arabia was most likely linked to episodes of climatic amelioration, when increased monsoon rainfall led to the activation of drainage systems, improved freshwater availability, and the development of regional vegetation. Here we present the first dated terrestrial record from southeast Arabia that provides evidence for increased rainfall and the expansion of vegetation during both glacial and interglacial periods. Findings from extensive alluvial fan deposits indicate that drainage system activation occurred during Marine Isotope Stage (MIS) 6 (ca. 160–150 ka), MIS 5 (ca. 130–75 ka), and during early MIS 3 (ca. 55 ka). The development of active freshwater systems during these periods corresponds with monsoon intensity increases during insolation maxima, suggesting that humid periods in Arabia were not confined to eccentricity-paced deglaciations, and providing paleoenvironmental support for multiple windows of opportunity for dispersal out of Africa during the late Pleistocene.

One popular compact version of the dispersal of Homo sapiens from Africa was that ecological opportunities and evolution of behavioral sophistication likely coincided beginning around 60,000 years ago or so. Human dispersal from that point on colonized the continents, replacing older human populations (descendants of Homo erectus).

This work provides evidence that it would have been possible during earlier times for modern humans to exploit opportunities in Arabia. Indeed there are tool assemblages in Arabia which are more than 100,000 years old and are very similar to the Nubian Middle Stone Age style found in Sudan suggesting a link between those peoples.

Take a look at the opportune time periods. They occur at around 20,000 year intervals. That aligns with climatic changes associated with the precessional  orbital cycles of 23,000 years frequency. And what are Marine Isotope Stages? The oxygen isotope ratio (O18/O16)  in sea water fluctuates in response to glacial buildup at the poles and to glacial melting and influx of more fresh water into the oceans. During glacial buildup more of the lighter isotope gets trapped in ice (since the lighter isotopes preferentially evaporates and falls as snow in the polar regions) and sea water becomes correspondingly "heavier". During glacial melting, influx of fresh water contributes O16 and sea water becomes "lighter". When  marine organisms like foraminifera  build their calcium carbonate (CaCO3) skeletons, they incorporate differing amounts of the oxygen isotopes in different climatic regimes (glacial versus non glacial). So, their skeletal isotope composition, which geologists measure, becomes a proxy for sea water composition.

If Arabia was colonized much earlier then did Homo sapiens enter India earlier (with a coastal route from Arabia onwards to India) than the unequivocal evidence around 45,000 years or so? Or China and other parts of Asia?  There is quite a controversy about this question. A team led by Michael Petraglia at the University of Oxford, UK think they have evidence of modern human presence in India much before 75,000 years ago based on tool assemblages found below a datable volcanic ash layer attributable to the Toba eruption. One problem has been that the skeletal record of hominins in India is very poor. Scientists have had to rely almost exclusively on the tool record and comparing particular tool assemblages across continents and assigning them as a handiwork of specific human species can be problematic.

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