According to long-established opinion among scientists, the human lineage emerged a little more than 2 million years ago, as the forests on the African continent retreated and the dry savanna expanded. In order to survive, our ape-like ancestors were forced to adapt key human traits such as longer legs, a bipedal gait, larger brains and the use of stone tools.
Now a trio of anthropologists are claiming that these evolutionary changes did not take place in one big leap, as had been previously believed, but came together in a piecemeal, or patchwork, fashion. As reported on NPR’s Morning Edition this weekend, the scientists–Rick Potts of the Smithsonian’s National Museum of Natural History, Susan Anton of New York University and Leslie Aiello of the Wenner-Gren Foundation–have analyzed fossils found over the past few decades. They found that at least three different types of early humans inhabited Africa around the period in question (around 2.5 million to 1.5 million years ago), all with different features that would end up in more modern humans.
According to Aiello, “…[T]he whole package that makes us human–long linear bodies, very large body size, delayed growth and development for the kids–didn’t evolve at the same time.” She and her colleagues, who lay out their findings in this week’s issue of the journal Science, point to two species, Homo rudolfensis and Homo habilis, both of which preceded the large-brained, more modern-proportioned Homo erectus but shared its uniquely “human” skull, teeth and jaw shapes. In addition, they call attention to Australopithecus sediba, a bipedal species discovered in 2008 at Malapa, South Africa that lived 1.98 million years ago. A relative of the famously well-preserved Australopithecus known as “Lucy,” it had apelike arms and a small brain, but also boasted such modern human traits as small teeth and longer legs.
The fact that such a mosaic of traits can be seen in different species living around the same time suggests that the evolutionary process progressed in patchwork style, not in one big leap as previously believed. Chris Stringer of London’s Museum of Natural History, who was not involved with the new research, tells National Geographic that What we consider as ‘modern’ traits seem to have been assembled piecemeal in Africa over a long period of time.”
Potts argues that what drove this type of evolution wasn’t a single event–such as the drying out of Africa’s climate–but a pattern of frequent climate change, or a “kind of unpredictability and uncertainty in the environment.” Thanks to rapid fluctuations between wet and arid conditions, he and his colleagues believe, early humans evolved the ability to adapt quickly to whatever new circumstances such changes might bring. For example, their growing brains and increasing use of tools allowed them to become generalists in their diet, consuming millet and grasses that would not have been on the menu for their predecessors.
Some fellow anthropologists urge caution, saying the new research is based on relatively little physical evidence. Stringer argues that climate change may not have been the driving factor in such evolutionary processes, while William Jungers of the State University of New York at Stony Brook questions why evidence of this “super-adaptability” hasn’t been found in other mammals subject to the same pattern of climate change. In the end, though the new report may have done little to resolve things, there’s no doubt its authors have added another piece to the complex puzzle of human evolution.