Three new studies highlight major environmental, ecological and technological changes that occurred in East Africa preceding the Middle Stone Age roughly 300,000 years ago, around the time that anatomically modern humans were evolving. The results hint that environmental factors may have spurred a change in human behavior, encouraging more widespread dispersal, trade and novel tool-making. First, a study by Rick Potts et al. analyzes well-preserved sediments in the Olorgesailie Basin, in Kenya, finding that beginning around 800,000 years ago the region underwent a transformation. The sediments suggest that the Olorgesailie Basin was mostly floodplains until roughly 800,000 years ago, when it increasingly exhibited fluctuations between moist and arid states. Furthermore, carbon isotopes of soil samples suggest that the region developed into a vast grassland. Around this time, the mammalian fauna experienced dramatic turnover, with many large-bodied, specialized grazing lineages, including some elephant and horse species, going extinct; in their stead, related taxa with smaller body sizes emerged - which the authors say is another sign of climate variability. They note that such climate variability makes food availability unpredictable for human hunter-gatherers, in turn driving greater mobility, information gathering, and perhaps trade. Such changes are evident in archeological evidence, Potts et al. note; whereas previously 98% of rock used to manufacture tools was from a tiny localized area of the Olorgesailie Basin (spanning just 5 kilometers), by about 320,000 years ago, tools were replaced with obsidian from regions farther away, an indication of travel and potentially trade. This represents a significant revision in African hominin behavior at or near the time of origin of Homo sapiens, the authors say.
A separate study by Alison Brooks et al. provides more detail on human-made artifacts excavated from the Olorgesailie Basin, including weapons (and also pigments) that shed life on early technology and trade. Notably, the authors base their work on evaluations at five sites spanning the period between approximately 500,000 and 298,000 years ago, finding distinct differences in the types of tools at older and younger sites. Whereas the older sites yielded larger, bulkier weapons such as hand axes, which were made from localized volcanic rock, one of the more recent sites contained much smaller and more refined weapons of a different style. About 42% of the latter tools were crafted from obsidian, of which there is no local source, the authors note. Furthermore, about 46,000 obsidian flakes were recovered from the site, indicating that obsidian was brought in as raw material and manipulated onsite rather than imported as finished artifacts. As well, the researchers found that the second-most common exotic raw material was green, brown, or white chert (colored rock). Of particular interest is a lump of ochre pigment with two perforated holes, which makes it among the oldest-known clearly worked pigments, the authors say, noting that exotic bright red and black rocks may have been valued, and worth transporting, for their intense color - used as symbolic communicators of identity or status. Lastly, Brooks et al. also discuss animal remains found within the vicinity of the sites, which suggest that early modern humans may have in part subsisted on small animals. A third study, by Alan Deino et al., provides detailed dating of sites within the Olorgesailie Basin, work that helps elucidate the critical transition between the Acheulean period and the Middle Stone Age. The researchers used argon and uranium dating techniques to determine the timeline of sites within the basin, confirming that older Acheulean sites contained larger tools; beginning around 320,000 years ago, sites lacked Acheulean-like tools, the authors report, noting that these results establish the oldest repository of Middle Stone Age artifacts identified in eastern Africa to date.