Fossil deposits in Africa obscure fuller picture of human evolution, study finds

Many of the earliest human fossils come from a few locations in Africa, where favorable geological conditions have preserved a treasure trove of fossils that scientists use to piece together the story of human evolution.

One such hotspot is the eastern branch of the East African Rift System, home to important fossil sites such as Oldupai Gorge in Tanzania. Yet the eastern branch of the rift system only covers 1% of Africa’s land area, giving an idea of ​​how much information is missing for scientists relying on such small samples.

In a study published 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.

“Given that the evidence for early human evolution comes from a small number of sites, it’s important to recognize that we don’t have a complete picture of what happened across the entire continent,” says W. Andrew Barr, assistant professor of anthropology at GW and lead author of the study.

“If we can highlight the biases that are systematically present in the fossil record and why it doesn’t perfectly represent everything, then we can adjust our interpretations to take that into account.”

To determine the extent of the bias in the fossil record, Barr and co-author Bernard Wood, a professor of human origins at GW University, looked at 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 on average 1.6% 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.

Although the scientific community has long recognized that the rift represents only a small sample of where ancient humans likely lived, 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 cannot tell us exactly where and in what type of environment 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 must avoid falling into the trap of proposing what looks 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 have access to information from one neighborhood. It’s helpful to get 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 settings where it’s really difficult to work, where the geology is not conducive to finding fossils,” says Barr, whose own work involves searching for fossils beyond the hotspots.

“It’s worth doing this kind of work to make our picture of mammalian and human evolution at this time more complete.”