It didn’t happen all at once.
That’s the main message of a new paper published in Science by the Director of the Smithsonian’s Human Origins Program, Rick Potts, and his colleagues Susan Antón, Professor of Anthropology at New York University, and Leslie Aiello, President of the Wenner-Gren Foundation for Anthropological Research.
The traditional “savanna hypothesis” postulates that many of the adaptations that distinguish our genus, Homo – such as walking upright, making tools, a large brain and body size, and a long period of maturation – were associated with cooler, drier habitats and the spread of grasslands. But this new study fine-tunes the timing of these traits to reveal a much more piecemeal process rather than a package deal. The climate framework they present, based on Earth’s astronomical cycles, demonstrates that these characteristic emerged against a backdrop of fluctuating climate that included both wetter and drier times of varying intensity.
It’s also clear that human evolution did not proceed in a linear fashion, with one species evolving after another. Soon after our genus (Homo) first evolved, by about 2.4-2.3 million years ago, there were at least three different species of Homo walking the earth: Homo habilis, Homo rudolfensis, and slightly later Homo erectus. In fact, these species overlapped in time with each other and even with Australopithecus and Paranthropus, two other genera of early humans. Some of their fossils have even been found at the same prehistoric sites. Anatomical traits and behaviors were mixed and matched, with some more human-like features found in some species, and others in different species, which according to the authors indicates that each species utilized a different strategy to survive. This diversity has since dwindled due to the extinction of all other human species aside from our own. We are the last biped standing, but currently stand at over 7 billion strong. Potts attributes our great evolutionary success to our adaptive versatility: the ability of the earliest members of our genus to innovate – to find new foods, make new tools, exploit new habitats, and eventually migrate out of Africa – in the face of ever-changing conditions.
By Dr. Briana Pobiner, Human Origins Program, Smithsonian's National Museum of Natural History