Many early human fossils come from one area, distorting the view of evolution.

Research in human evolution is also subject to sampling bias—bias that arises because research material is not collected in the same way or in the same quantities everywhere. This is what two American paleontologists wrote. in Nature and Evolution Environment.

Almost all early human fossils come from the East African Rift Valley, often called the cradle of modern humans. But that’s not entirely certain, the researchers conclude by comparing it with the distribution of contemporary species.

About 6 to 8 million years ago, the human lineage—the human lineage—split from chimpanzees and bonobos. Since these African ape species are closely related to humans genetically and morphologically, the assumption that humans originated in Africa is clear.

fracture in the earth’s plate

Even Charles Darwin predicted this origin, based on body structure. Since the 19th century, fossil evidence has confirmed these suspicions. Remains of human ancestors have been found in limestone caves in South Africa and in the eastern branch of the Rift Valley (a geological depression created by a fault in the Earth’s plate, also known as the East African Rift Valley).

The latter site in particular has generated many new ideas about human evolution and the interaction of early humans with their living environment. Finding one elsewhere, such as an ancient one rational humanA skull in Morocco has not yet significantly shaken those assumptions about an East African origin.

But the authors stress that the Rift Valley—despite the fact that it runs through Ethiopia, Kenya, Tanzania and elsewhere—only covers about 1 percent of the entire African continent. The region is also not representative in terms of climate: it is much drier than neighboring regions. It is not surprising that an unusual number of human fossils have been found, since geological depressions are known to accumulate large quantities of fossils.

However, this introduces a sampling bias that could lead to incorrect conclusions about the origin (East Africa) and environment (dry) of humans. Or as the researchers write: “Although the exact geographic distribution of each group of early humans cannot be determined, it is unlikely to be limited to fossil excavation sites.”

To bolster their warning, they used a computer model that used the distribution of modern mammal species as a comparison. This model showed that of the 106 African species examined, almost none are found exclusively in the Rift Valley. Only the wild ass is a true “Rift specialist.” And of the three specific species of gazelle, the gynt and the baboon, at least a third of the population lives within the Rift Valley. All other larger mammal species have a much larger distribution area.

Baboons and meerkats

In addition, the scientists investigated whether it was possible to map changes in body structure by focusing only on the eastern branch of the Rift Valley, where the human fossils have been found. They did this by looking at differences in the skulls of vervet monkeys and baboons. This also shows that focusing on a very specific location leads to misconceptions: in fact, the morphology of baboons and vervet monkeys is much more diverse than can be inferred from populations in the eastern branch. It is therefore likely that the skull shape of early humans was much more diverse than current fossil collections suggest.

Evolutionary biologist Laura Van Holsteinof Clare College, Cambridge, describes the paper as interesting and innovative. “In my view, there is very little explicit thinking in my field about the limits of our knowledge and what we can do about it. This is a much-needed response to this paucity. Living primates are often used rather cowardly to make vague comments about what humans might do in terms of foraging, social behaviour or tool use. It is a very uncomfortable consequence that our samples are so biased. Hats off to this paper, I wish I had written it myself!”

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