The number of species in the human tree has exploded since Charles Darwin specified the origins of humanity in Africa 150 years ago; still, it has the level of disagreement regarding early human evolution. Fossil apes are frequently at the centre of controversy. Some scientists dismiss their significance in the origins of the human lineage (the “hominins”). Others were bestowing them evolutionary starring roles.

Dr. David Alba, Professor Salvador Moyà-Solà, Dr. Sergio Almécija from the Universitat Autònoma de Barcelona; Dr. Kelsey Pugh, Dr. Ashley Hammond from the American Museum of Natural History; and Dr. Nathan Thompson from the New York Institute of Technology published a new review in the journal Science. Researchers examined the significant advances in our understanding of hominin origins since Darwin’s time and argue that fossil apes can shed light on critical aspects of ape and human evolution, including the nature of our last common ancestor.

Humans split from apes — specifically, the chimp lineage — between approximately 9.3 million and 6.5 million years ago, near the end of the Miocene epoch. Paleoanthropologists strive to reconstruct the physical characteristics, behaviour, and environment of the last common ancestor of humans and chimps to gain a better understanding of hominin origins.

“When it comes to hominin origins, the narrative is a complete mess — there is no consensus,” said Sergio Almécija, a senior research scientist in the Division of Anthropology at the American Museum of Natural History and the review’s lead author. “People are operating under radically different paradigms, which I do not see in other branches of science.”

There are two primary approaches to resolving human origins: “top-down” analysis of living apes, particularly chimps, and “bottom-up” analysis of the enormous tree of mostly extinct apes. For instance, some scientists believe that hominins evolved from a knuckle-walking ancestor resembling a chimp. Others argue that the human ancestor resembled some of the strange Miocene apes in some ways.

Almécija and colleagues, whose expertise ranges from paleontology to functional morphology and phylogenetics, discuss the drawbacks of relying exclusively on one of these divergent approaches to the hominin origins problem studies involving these divergent approaches. Top-down studies frequently overlook that living apes (humans, chimps, gorillas, orangutans, and hylobatids) are descended from a much larger and now primarily extinct group. On the other hand, bottom-up approaches are prone to assign significant evolutionary roles to individual fossil apes that fit a preexisting narrative.

“In 1871, Darwin hypothesized in The Descent of Man that humans descended from an ancestor unlike any other living species in Africa. However, given the scarcity of fossils at the time, he remained circumspect,” Almécija explained. ” And after all years, possible hominins — ancestors of humans and chimps — have been discovered in Central and Eastern Africa, and some claim even in Europe. Furthermore, over 50 fossil ape genera have been identified in Africa and Eurasia. However, many of these fossils exhibit mosaic combinations of characteristics that do not correspond to human lineages and ancient representatives of the modern ape. As a result, scientific consensus regarding the evolutionary function of these fossil apes is lacking.”

In general, the researchers discovered that most stories about human origins do not fit the fossils we have today.

“Living ape species are specialized individuals that are relicts of a much larger group of now-extinct apes. When we consider all available evidence — that is, living and fossil apes and hominins — it is clear that a human evolutionary storey based on the few ape species that are still alive is missing a significant portion of the larger image,” said study co-author Ashley Hammond.

“Unique and often unexpected features and combinations of features discovered among fossil apes, which frequently differ from those reported among extant apes, are crucial for determining which characteristics hominins got from the ape ancestors and which are unique to homo sapiens lineage,” added Kelsey Pugh, a Museum postdoctoral fellow and co-author of the study.

The authors conclude that evidence provided by living apes alone is insufficient. “Contemporary disparate theories about ape and human evolution would be significantly more informed if Miocene apes were included in the equation alongside early hominins and living apes,” Almécija says. “In other words, fossil apes are critical for reconstructing the ‘evolutionary origins of humans and chimps.”

Journal reference:

Fossil apes and human evolution Sergio Almécija, Ashley S. Hammond, Nathan E. Thompson, Kelsey D. Pugh, Salvador Moyà-Solà and David M. Alba, 7 May 2021, Science. DOI: 10.1126/science.abb4363

Main image credit: James St. John, Flickr