Overreliance on Fossil Hotspots Creates a False Picture of Human Evolution

New research has identified a persistent problem that has hindered studies of the origins and patterns of human evolution. Specifically, there seems to be a mismatch between the spots where most ancient human fossils have been collected and the locations where most human ancestors lived in the greatest numbers.

In a new paper just published in the journal Nature Ecology & Evolution, researchers W. Andrew Barr and Bernard Wood from the Center for the Advanced Study of Human Paleobiology at George Washington University lay out the case that our understanding of human evolution is more limited than many scientists imagine. A problem they identify as ‘spacial sampling bias’ in the collection of fossils from eastern Africa is the source of the distortion, and they argue it has real consequences for those seeking a detailed and accurate picture of humanity’s true origins.

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“The eastern branch of the Eastern African Rift System (EARS) is the source of a large proportion of the early hominin fossil record, but it covers a tiny fraction (ca. 1%) of the continent,” they explained in their article. “Here we investigate how this mismatch between where fossils are preserved and where hominins probably lived may influence our ability to understand early hominin evolution, using extant mammals as analogues.”

The idea behind this new research project is that the living patterns of ancient humans likely would have matched those of currently living mammals, at least to some extent.

Since all species rely on a natural resources like food, water, and safe places to sleep to survive, they can be expected to gravitate toward ecosystems that can best supply those needs. While they might have a limited presence in less hospitable locations, the majority of their populations would never live there, or stay there for long if they were passing through.

In a summary of their work, Professors Barr and Woods claim to demonstrate that “the eastern branch of the EARS [where most ancient human fossils have been found] is not an environmentally representative sample of the full species range for nearly all extant rift-dwelling mammals.”

In other words, even though modern rift-dwelling mammals have a presence in this area, the range of territory they occupy is much more extensive. Presumably, the same would have been true for archaic humans living there two or three million years ago.

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Eastern Africa fossils: Left; the most complete skull of Paranthropus robustus, a female, ever discovered from Drimolen Main Quarry, South Africa. (DrHerries/CC BY-SA 4.0) Right; A 2.1-million-year-old Australopithecus africanus specimen from Sterkfontein cave, S. Africa. (José Braga; Didier Descouens/CC BY-SA 4.0)

Africa’s Eastern Rift Valley is Not the Only Place Where Ancient Humans Lived

It is an undisputed fact that much of the present understanding of human evolution comes from the study of fossilized skeletal remains recovered from a handful of fertile fossil beds. These are generally concentrated in the eastern section of the Eastern African Rift System, where geological conditions are quite favorable for the preservation of ancient bones.

But concentrating on the EARS leaves nearly 99% of the African landscape mostly unexamined.

Professor Barr states in a George Washington University press release:

“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… 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."

Seeking to comprehend just how significant this systematic bias was, Barr and Wood looked closely at the distribution of mammal populations across the rift valley today. They discovered that only a small number of medium or large-sized mammals spent the majority of their time in the rift environment. This section of land accounted for 1.6% of the total geographic range of these mammals on average, highlighting how difficult it would be to survive living exclusively in this region.

To complement these findings, the researchers examined the skulls of modern primates that had been collected in the rift valley, to compare them with primate skulls found elsewhere in Africa. As they suspected, there was quite a bit of diversity in the skulls, with what had been recovered in the EARS representing less than 50% of the continent’s primate skull variety.

Finding Fossils in the Places Where Our Ancestors Actually Lived

This is the first study to use modern mammal distribution in Africa as a model for evaluating geographical bias in ancient human evolutionary research, the study authors say. They aren’t claiming that this is a perfect method for comprehending archaic human living patterns, but rather a useful tool that can help illuminate the issues involved in studying human fossils recovered only from one unique landscape.

“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" Wood cautioned.

"Imagine trying to capture the social and economic complexity of Washington D.C. if you only had access to information from one neighborhood. It helps if you can get a sense of how much information is missing.”

Barr and Wood acknowledge the challenges involved in trying to find extremely old human fossils in areas outside the Eastern African Rift System, where such fossils are less likely to have been preserved. Nevertheless, this is an effort that must be made, if the study of human evolution is to make substantial progress in the future.

“There's a smaller number of people who work outside these traditional hotspots and do the thankless labor of trying to find fossils in these contexts that are really hard to work in, where the geology isn't favorable for finding fossils," said Barr, who is active in this type of research himself. "It's worth doing that sort of work to make our picture of mammal and human evolution from this time period more complete.”

Top image: Olduvai Gorge or Oldupai Gorge in Tanzania is a fossil hotspot. Source: Noel Feans/CC BY 2.0

By Nathan Falde