The Mystery of the Extinct Apes That Preceded Humans

Fossils of our African ape ancestors are virtually nonexistent for a period of about 8 to 9 million years. This long period lasted from about 16.5 million to 7 to 9 million years ago, during the Miocene geological epoch.

However, fossil remains in Europe and Asia show an abundance of ape species thriving and developing new traits during this vacuum period for African apes.

The unanswered question is: How were today's monkey species discovered? The ape species currently living in Africa are the closest evolutionary relatives of humans. That's why it's important to know where they came from. Are they descendants of the apes that migrated to Eurasia during the Miocene and then returned to Africa? Or is the lack of African fossil remains from that period simply the result of local conditions, such as wet, acidic soils, that may have destroyed them?

Paleoanthropologists have approached the problem from different angles. One hypothesis for the disappearance of great ape fossils in Africa is that great apes originated in Africa and then migrated to Eurasia in the mid-Miocene, where they developed the preconditions for their evolution into humans. In this scenario, the best-adapted great ape species that withstood the climate change of the late Miocene would then have returned to Africa, where the human lineage would have evolved.

Reconstitution of monkey lineages

Another approach is to infer the missing great ape evolution that might have occurred in Africa, but left no fossil remains, by using genomic evidence from living great ape species to reconstruct the missing lineages. University of Cambridge paleontologists Robert A. Foley and Marta Mirazón Lahr proposed a model of what this possible great ape evolution might look like in a January 2024 paper in Trends in Ecology & Evolution.

Foley and Mirazón Lahr call the inferred evolutionary tree of extinct great apes “ghost lineages.” “Ghost lineages are species or groups of species that have not been observed directly, either in fossils or living species, but have been inferred from genetic sequences,” the authors said in an interview.

The search for these ghosts is made possible by modern genomics, particularly DNA analysis.

“We learn about these ghosts by building phylogenetic or evolutionary trees of known species—like apes, humans, Neanderthals, etc.—and looking for points in those trees that can only be explained if other, unknown species were involved,” they said. “These are the hypothetical ghost lineages. Research has shown ghost lineages in the hominid, chimpanzee, and gorilla lineages, as well as in many other taxa.” (Taxa is the technical term for species divisions.)

“In hominid evolution, they are 'ghosts' in the sense that we think they must have existed and been part of our history, leaving only traces in our DNA.” (Hominid is the term used to refer to humans and our fossil relatives.)

The Ghosts of African Monkeys

Foley and Mirazón Lahr discuss two plausible ghost models for extinct African great apes based on existing genomic evidence: a low divergence model and a high extinction model.

In the low divergence model, the Gorilla lineage descended from the last common ancestor of Stove(ancestor of the monkey) and Homo (human ancestor) about 7.2 to 11 million years ago. Then the two Stove And Gorilla The ancestors of today's ape species split from their ghost lineages. According to this theory, the ancestors of today's ape species experienced only two periods of growth, about 3 to 3.5 million years ago, and later, less than 2 million years ago.

As Foley and Mirazón Lahr point out, this rate of divergence “contrasts sharply with that of hominins,” but it remains possible. “It seems unlikely, given the geographic distribution, environmental diversity, and time period involved, that such a small divergence occurred.”

The other hypothesis is the ghost model of high extinction, which suggests that the evolution of African great apes followed a trajectory similar to that of hominids. The great apes diversified and spread to new regions, but earlier lineages died out. This is a scenario that Foley and Mirazón Lahr consider “more likely.” This high extinction model fits with what is known from hominid fossils discovered in the last 50 years.

Foley and Mirazón Lahr stressed the importance of learning more about the missing evidence for great ape evolution. “Chimpanzees and bonobos are the closest living relatives of humans, with a last common ancestor about 7 million years ago. Humans and their ancestors must have evolved alongside African great apes. However, in the absence of fossils, we know virtually nothing about the evolution of African great apes.”

“There is a tendency to think that, unlike us, they have changed little since their last common ancestor,” they said. “However, this is almost certainly not true, and we need ape fossils to tell us when, where and how modern apes evolved. Similarities and differences with the evolutionary history of humans will be groundbreaking discoveries.”

Pieces of the puzzle

Paleoanthropologists have proposed several possible explanations for the puzzle of the extinct late Miocene ape fossils. For one, climate and geographic changes in Africa may have made it difficult to preserve ape remains at that time. It is also possible that the forest soil in the ape habitats was too acidic for the fossils to survive. Another factor could be that paleontologists did not look in the right places for ape fossils.

But late Miocene ape fossils are not entirely absent from Africa. A partial lower jaw and 11 teeth from a now-extinct ape species that lived about 10 million years ago were discovered in Nakali, Kenya, in 2005 by an international team of paleontologists, who named it Nakalipithecus of Nakayama.

The authors of a paper describing the discovery write that it “suggests that it is highly likely that large-bodied hominoids survived from the Middle to Late Miocene in Africa, giving rise to the last common ancestor of African great apes and humans.” (Hominoids include humans, their fossil ancestors, and the anthropoid apes, which include chimpanzees, bonobos, gorillas, and orangutans.)

Another monkey fossil discovered by another international team comes from the Chorora Formation, near the southern part of the Afar Fault, named Chororapithecus abyssinicusThe fossil is dated to about 8 million years old and, as the newspaper reports, Nature According to the reports, “the attribution to the gorilla lineage seems all the more important because it helps to constrain the separation between gorillas and the lineage leading to hominids and chimpanzees, and suggests that this separation occurred in Africa.”

A heated debate

The paleoanthropological community is not unanimous in its solutions to the problem of great ape evolution, and some believe that it is likely that the great apes made a round trip, triggered by climate change in the Miocene. According to this theory, the great apes originated in Africa and some dispersed to Eurasia for a few million years, where they developed some proto-human characteristics, before returning to Africa about 7 to 9 million years ago.

Asked about the Eurasian scenario, Foley and Mirazón Lahr said: “That’s a tricky question! The idea that the last common ancestor of African apes and hominids was recent (in the geological sense, about 9 million years ago) is hotly debated. On the one hand, it’s tempting to see a long line of ape evolution in Africa—there’s a very rich fossil record of apes. But on the other hand, the fossils most similar to African apes are found in western Asia and the eastern Mediterranean.”

And here the collaborators' views diverged: Robert A. Foley said that “this makes the 'African entry' hypothesis more likely,” but co-author Marta Mirazón Lahr said that “the absence of late Miocene fossils in Africa—a big blank—leaves the question open.”

Future fossil discoveries are likely to shed more light on some of the ghost monkeys and help solve the puzzle.

This article was produced by Human bridges.