Climate instability 2.5-1.5Ma (million years ago) drove the evolution of a number of traits we think of as distinctively human, according to a new paper in Science. The research challenges the notion that characteristics like long legs evolved later, along with large brains and extended childhoods. In the same paper the authors have also challenged the categorization into species of early human fossils.

Professor Susan Anton of New York University, the Smithsonian's Dr Richard Potts abd Professor Leslie Aiello of the CF tried to put early human fossils into a wider context. They used advances in reconstruction of paleoclimates and studies of other mammals to gain a better understanding of the forces operating on humans that led to the first expansion out of Africa 1.85Ma.

“Until recently, the evolution of the genus Homo has been interpreted in the context of the onset of African aridity and the expansion of open grasslands,” the paper notes. However, the authors argue, “New environmental data sets suggest that Homo evolved against a background of long periods of habitat unpredictability that were superimposed on the underlying aridity trend.”

Moreover, fossil evidence increasingly suggests that there human evolution looks more like a bush than a clean stemmed tree, with multiple early species of the Homo genus that, as the paper notes, “overlap in body, brain, and tooth size.”

These two observations are probably related. More diverse and changeable environments would suit the appearance of multiple related species, rather than a single one that would dominate the open grasslands previously thought to represent our genus' first home.

Potts says, "The narrative of human evolution that arises from our analyses stresses the importance of adaptability to changing environments, rather than adaptation to any one environment, in the early success of the genus Homo.” The same flexibility has turned out to be humanity's great advantage, allowing us to flourish in more diverse environments than any other animal.

 
^{Anton et al. The new proposed outline for human evolution shows Australopithecus in green, Paranthropus in yellow and Homo in green. The images in the behavioral column represent stone tools, dispersal to Eurasia and the  more advanced Acheulean technology. The authors note, “The cultural milestones do not correlate with the known first appearances of any of the currently recognized Homo taxa.”}

The authors also point out that many skulls from the era do not fit into the generally accepted categorizations. Five 1.8 Ma skulls from the Republic of Georgia represent some of the earliest humans outside Africa. However, they differ from traits seen as defining Homo erectus, while 1.98Ma Australopithecus sediba skeletons from South Africa “posses a bizarre combination of features”.

The authors argue that 2Ma two human “morphs” existed, which the paper terms the 1470 and 1813 group. The two are distinguished by their facial anatomy. The bones divided between these groups have traditionally been thought of as belonging to Homo habilis and Homo rudolfensis, defined by size. The decision to informally group by numbers, rather than propose new species, comes from a lack of certainty as to how many species make up each morph.

^{Rick Potts, Susan Antón and Leslie Aiello Large brains, tool use and long childhoods did not evolve all at once but in separate places and times.}