Fossil deposits in Africa obscure a fuller picture of human evolution

In a new study published today in the journal Nature Ecology and EvolutionResearchers at George Washington University show how the concentration of sites in hotspots like the East African Rift System biases our understanding of human evolution and why scientists must take this bias into account when interpreting ancient human history.

“The evidence for early human evolution comes from a limited number of sites, so it’s important to recognize that we don’t have a complete picture of what happened across the continent,” said W. Andrew Barr, assistant professor of anthropology at GW and lead author of the study. “If we can highlight the systematic biases in the fossil record and how imperfect it is, then we can adjust our interpretations to take those things into account.”

To determine the extent of the bias in the fossil record, Barr and his co-author Bernard Wood, a professor of human origins at the University of Washington, studied the distribution of modern mammals that currently live in the Rift Valley. They found that very few medium- and large-sized mammals are “rift specialists,” and that the rift environment actually represents an average of 1.6 percent of the total geographic range of modern mammal species.

In a second analysis, Barr and Wood compared modern primate skulls collected in the Rift Valley with those of the same primates from other parts of the continent. They found that the Rift Valley skulls accounted for less than 50% of the total variation in primate skulls in Africa.

While the scientific community has long recognized that the rift represents only a small sample of where ancient humans likely lived, the researchers say previous studies have not used modern mammals as analogues for human fossils to try to quantify the magnitude of the bias. Information from modern mammals can’t tell us exactly where or in what kind of environments our human ancestors lived, but it can provide clues that help us better understand the environments and physical differences of ancient humans, the authors say.

““We have to avoid falling into the trap of proposing what seems like a complete reconstruction of human history when we know we don’t have all the relevant evidence,” Wood says. “Imagine trying to grasp the social and economic complexity of Washington, D.C., if you only had access to information from one neighborhood. It’s helpful to have a sense of how much information is missing.”

The researchers also stress the need for the scientific community to look beyond the rift to identify new fossil sites and extend the geographic reach of the fossil record.

“There are a smaller number of people who work outside of these traditional hotspots and do the thankless work of trying to find fossils in these really difficult settings to work in, where the geology is not conducive to finding fossils,” says Barr, whose job is to look for fossils beyond the hotspots. “It’s worth doing this kind of work to round out our view of mammalian and human evolution at this time.”