Understanding the origins of the human lineage (hominins) requires reconstructing the morphology, behavior, and environment of the chimpanzee-human last common ancestor. In new research, paleoanthropologists from the American Museum of Natural History and elsewhere looked at the major discoveries in this area since Charles Darwin’s works and concluded that the morphology of fossil apes was varied and that it is likely that the last shared ape ancestor had its own set of traits, different from those of modern humans and modern apes.

The last common ancestor of chimpanzees and humans represents the starting point of human and chimpanzee evolution. Image credit: Christopher M. Smith.

Humans diverged from apes — specifically, the chimpanzee lineage (genus Pan) — at some point between 9.3 and 6.5 million years ago, and habitual bipedalism evolved early in hominins.

To understand hominin origins, paleoanthropologists aim to reconstruct the physical characteristics, behavior, and environment of the last common ancestor of humans and chimps.

“In The Descent of Man in 1871, Charles Darwin speculated that humans originated in Africa from an ancestor different from any living species. However, he remained cautious given the scarcity of fossils at the time,” said Dr. Sergio Almécija, a researcher in the Division of Anthropology at the American Museum of Natural History.

“150 years later, possible hominins have been found in eastern and central Africa, and some claim even in Europe.”

“In addition, more than 50 fossil ape genera are now documented across Africa and Eurasia.”

“However, many of these fossils show mosaic combinations of features that do not match expectations for ancient representatives of the modern ape and human lineages.”

“As a consequence, there is no scientific consensus on the evolutionary role played by these fossil apes.”

The evolutionary history of apes and humans is largely incomplete: whereas the phylogenetic relationships among living species can be retrieved using genetic data, the position of most extinct species remains contentious; surprisingly, complete-enough fossils that can be attributed to the gorilla and chimpanzee lineages remain to be discovered; assuming different positions of available fossil apes — or ignoring them owing to uncertainty — markedly affects reconstructions of key ancestral nodes, such as that of the chimpanzee-human last common ancestor. Image credit: Almécija et al., doi: 10.1126/science.abb4363.

There are two major approaches to resolving the human origins problem:

(i) ‘top-down,’ which relies on analysis of living apes, especially chimpanzees;

(ii) and ‘bottom-up,’ which puts importance on the larger tree of mostly extinct apes.

For example, some scientists assume that hominins originated from a chimp-like knuckle-walking ancestor.

Others argue that the human lineage originated from an ancestor more closely resembling, in some features, some of the strange Miocene apes.

In reviewing the studies surrounding these diverging approaches, Dr. Almécija and his colleagues discuss the limitations of relying exclusively on one of these opposing approaches to the hominin origins problem.

‘Top-down’ studies sometimes ignore the reality that living apes are just the survivors of a much larger, and now mostly extinct, group.

On the other hand, studies based on the ‘bottom-up’ approach are prone to giving individual fossil apes an important evolutionary role that fits a preexisting narrative.

Overall, the researchers found that most stories of human origins are not compatible with the fossils that they have today.

“Living ape species are specialized species, relicts of a much larger group of now extinct apes,” said Dr. Ashley Hammond, an assistant curator in the Division of Anthropology at the American Museum of Natural History.

“When we consider all evidence, it is clear that a human evolutionary story based on the few ape species currently alive is missing much of the bigger picture.”

“The unique and sometimes unexpected features and combinations of features observed among fossil apes, which often differ from those of living apes, are necessary to untangle which features hominins inherited from our ape ancestors and which are unique to our lineage,” added Dr. Kelsey Pugh, a postdoctoral researcher in the Division of Anthropology and the New York Consortium in Evolutionary Primatology at the American Museum of Natural History.

“Living apes alone offer insufficient evidence. Current disparate theories regarding ape and human evolution would be much more informed if, together with early hominins and living apes, Miocene apes were also included in the equation,” Dr. Almécija said.

“In other words, fossil apes are essential to reconstruct the ‘starting point’ from which humans and chimpanzees evolved.”

A paper on the findings was published in the journal Science.

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Sergio Almécija et al. 2021. Fossil apes and human evolution. Science 372 (6542): eabb4363; doi: 10.1126/science.abb4363