Evrim Yazgin
Cosmos science journalist
The significance of ancient human fossils found in Africa is undeniable. But new research questions whether African fossil sites tell the whole story.
A brief history of uncovering human history
Scientists first found and identified ancient human fossils in the mid-19th century. Neanderthal and ancient Homo sapiens remains were found in Europe. Homo erectus was found in Indonesia in 1891. Homo heidelbergensis in Germany in 1908.
All of these finds pointed to the existence and evolution of humans over hundreds of thousands of years.
But in the 1920s and 1930s, with the development of archaeological and palaeontological work on the African continent, a new cradle of humanity began to emerge. Australopithecus africanus, first uncovered in 1925, and Paranthropus robustus remains, first dug up in 1938, were much, much older.
New ancient human ancestor species have been discovered in the decades since all around the world.
The fossil evidence shows that the birthplace of humankind, however, is in Africa. Among the oldest hominin (the group which includes modern humans, extinct human species and all our immediate ancestors) fossils have been found in Ethiopia, Tanzania, Chad and surrounding regions.
Among them is “Lucy” – Australopithecus afarensis female whose the skeleton found in Ethiopia in 1974 dates to about 3.2 million years ago.
Even older hominins have been found, dating to more than 4 million years ago.
But the new study raises questions about how much we can rely on the African fossil sites to build a picture of human evolution over millions of years.
Hotspots may be biasing evolutionary maps
The fossil record is far from complete.
Conditions which allow for bones to survive hundreds of thousands, let alone millions, of years are very rare. Often, this leads to “hotspots” where geological conditions are just right for bones to be fossilised.
The East African Rift System, stretching from Ethiopia to Mozambique, is one such hotspot. But it only covers only about 1% of the continent’s total area.
The new statistical study, published in the journal Nature Ecology and Evolution, argues that such bias should be accounted for when archaeologists and palaeontologists try to develop an evolutionary map of human development.
“Because the evidence of early human evolution comes from a small range of sites, it’s important to acknowledge that we don’t have a complete picture of what happened across the entire continent,” says lead author W. Andrew Barr, an assistant professor of anthropology at George Washington University in the US.
“If we can point to the ways in which the fossil record is systematically biased and not a perfect representation of everything, then we can adjust our interpretations by taking this into account.”
Barr, with colleague Bernard Wood, tried to determine the extent of the bias reflected in the fossil record.
Very few medium- and large-bodied mammals are “rift specialists”, they found. In fact, the region only represents about 1.6% of the total geographical range of modern mammal species.
They also looked at the variation of modern primate skulls in the area and found that the rift valley represents less than half of the variation of primate skulls in Africa.
Josephine Joordens from the Naturalis Biodiversity Center, Leiden, in the Netherlands, says the research is “timely and important”.
“The authors address an often overlooked but key issue in hominin evolution, spatial bias, in a clever and creative way using available primate databases and statistical analyses,” Joordens, who was not involved in the study, adds.
Beyond the rift
While scientists have long understood that the rift valley is a small sample, the authors of the new research suggest that using modern mammals reveals just how strongly the bias might be affecting current models of human evolution.
“We must avoid falling into the trap of coming up with what looks like a comprehensive reconstruction of the human story, when we know we don’t have all of the relevant evidence,” says Wood. “Imagine trying to capture the social and economic complexity of Washington, D.C., if you only had access to information from one neighbourhood. It helps if you can get a sense of how much information is missing.”
The researchers emphasise the importance of expanding archaeological digs in Africa beyond the rift valley to build a fuller picture of human evolution.
“There’s a smaller number of people who work outside these traditional hotspots and do the thankless labour of trying to find fossils in these contexts that are really hard to work in, where the geology isn’t favourable for finding fossils,” says Barr, whose own work involves looking for fossils beyond the hotspots.
“It’s worth doing that sort of work to make our picture of mammal and human evolution from this time period more complete.”
