Why Fossil Apes are Vital to Understanding Human Evolution

by AMNH on

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Three skeletons on display at the entrance to the Spitzer Hall of Human Origins. D. Finnin/© AMNH

In 1871, Charles Darwin made a cautious speculation that humans originated in Africa. Since then, the number of species in the human family tree has ballooned, but so has the level of disagreement concerning early human evolution.

“When you look at the narrative for hominin origins, it’s just a big mess—there’s no consensus whatsoever,” said Sergio Almécija, a senior research scientist in the Museum’s Division of Anthropology.

Almécija is the lead author on a review out this week in the journal Science that examines the major discoveries in hominin origins in the 150 years since Darwin’s works and argues that fossil apes can inform us about essential aspects of ape and human evolution.

Humans diverged from apes—specifically, the chimpanzee lineage—at some point between about 9.3 million and 6.5 million years ago, towards the end of the Miocene epoch. To understand hominin origins, paleoanthropologists aim to reconstruct the physical characteristics, behavior, and environment of the last common ancestor of humans and chimps.

There are two major approaches to resolving the human origins problem: “Top-down,” which relies on analysis of living apes, especially chimpanzees; and “bottom-up,” which puts importance on the larger tree of mostly extinct apes.

In reviewing the studies surrounding these diverging approaches, the researchers 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—a group that includes humans, chimpanzees, gorillas, orangutans, and hylobatids—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 a starring evolutionary role that fits a preexisting narrative.

Illustration depicts the profile of an ape on the left transitioning to the profile of a human on the right.
The last common ancestor of chimpanzees and humans represents the starting point of human and chimpanzee evolution. Fossil apes play an essential role when it comes to reconstructing the nature of our ape ancestry.
Printed with permission from © Christopher M. Smith

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

“Living ape species are specialized species, relicts of a much larger group of now extinct apes. When we consider all evidence—that is, both living and fossil apes and hominins—it is clear that a human evolutionary story based on the few ape species currently alive is missing much of the bigger picture,” said study co-author Ashley Hammond, an assistant curator in the Museum’s Division of Anthropology.

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