Most theories of human origins are inconsistent with known fossils

In the 150 years since Charles Darwin suggested that humans came from Africa, the number of species in the human family tree has increased, but so has the level of debate about early human evolution.

Fossil apes often find themselves at the center of the debate, with some scientists rejecting their importance to the origin of the human lineage (“hominins”) and others assigning them a major evolutionary role.

A new review published May 7 in Science reviews major discoveries about the origins of hominins since Darwin’s work and argues that fossil apes can tell us about crucial aspects of ape and human evolution, including the nature of our last common ancestor.

Humans evolved from apes-particularly chimpanzees-at some point between 9.3 and 6.5 million years ago, at the end of the Miocene epoch. To understand the origins of hominins, paleoanthropologists seek to reconstruct the physical characteristics, behavior and environment of the last common ancestor of humans and chimpanzees.

“When you look at the origin story of hominins, it’s just a big mess — there’s no consensus,” said Sergio Almecia, a senior researcher in the anthropology department at the American Museum of Natural History and lead author of the review. “People are working within completely different paradigms, and that’s something I don’t see in other areas of science.”

Comment: Clearly, not everything in science has been solved, and the controversy is likely caused by some working from a position of ideology rather than objectivity.

There are two main approaches to solving the problem of human origins: “top-down,” which is based on analysis of living primates, especially chimpanzees, and “bottom-up,” which attaches importance to the larger tree of extinct primates. For example, some scientists suggest that hominins evolved from a chimpanzee that looks like a walking ancestor leaning on his toes. Others argue that the human lineage descended from an ancestor more similar in some respects to some strange Miocene monkeys.

In a review of research on these divergent approaches, Almezia and her colleagues with expertise in fields ranging from paleontology to functional morphology to phylogenetics discuss the limitations of relying exclusively on one of these opposing approaches to the hominin origin problem.

Top-down studies sometimes ignore the reality that living primates (humans, chimpanzees, gorillas, orangutans, and hylobatids) are only surviving members of a much larger and now largely extinct group. On the other hand, studies based on a bottom-up approach tend to give individual fossil apes an important evolutionary role that fits into an already existing narrative.

In The Origin of Man in 1871, Darwin suggested that humans originated in Africa from an ancestor distinct from all currently living species. But he remained cautious, given the paucity of fossils at the time,” says Almezia.

“One hundred and fifty years later, possible hominins–approaching the time of the divergence of humans and chimpanzees–have been found in eastern and central Africa, and some claim even in Europe. In addition, more than 50 genera of fossil apes have been found in Africa and Eurasia. However, many of these fossils exhibit mosaic combinations of features that do not match expectations regarding ancient representatives of modern ape and human genera. As a consequence, there is no scientific consensus on the evolutionary role played by these fossil apes.”

Comment: A previously unknown species of “protominins” suggests that the ancestor of humans evolved in Europe, not Africa

Overall, the researchers found that most stories about human origins are inconsistent with the fossil record we have today.

“Living ape species are specialized species, relics of a much larger group of now extinct apes. When we look at all the evidence – that is, both living and fossil apes and hominins – it becomes clear that a human evolutionary history based on a few living ape species misses much of the big picture,” said study co-author Ashley Hammond, assistant curator in the museum’s Anthropology Department.

Kelsey Pugh, a postdoctoral fellow at the museum and co-author of the study, adds: “The unique and sometimes unexpected features and combinations of features observed in fossil apes, which often differ from those of living apes, are necessary to find out which features hominins have inherited from our ape ancestors and which are unique to our lineage.”

Living apes alone, the authors conclude, are insufficient evidence.

“Current disparate theories of ape and human evolution would be much more valid if Miocene monkeys were included in the equation along with early hominins and living apes,” Almezia says.

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

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