What the fossilized hand says about the last human and chimpanzee ancestor

Over a million years before the first hominids known as Lucy crossed Ethiopia’s lesser-known Afar region Ardipithecus ramidus roamed roughly the same area. Now a team of anthropologists have examined the 4.4 million year old fossilized hand a specimen (affectionately known as “Ardi”), and argue that the roaming of human ancestors may have involved more rocking through trees than previously thought.

Posted In the journal Science Advances on Wednesday, the team’s comparative analysis focuses on the morphology of Ardi’s hand in the context of missing and existing relatives. In a conversation with bones from elsewhere in our family tree, Ardi’s hand traits indicate when specific adaptations currently seen in different primates have occurred. Specifically, knowing how Ardi moved brings us closer to knowing how we (homo sapiens) became a group of terrestrial and bipedal primates.

The team of Thomas Cody Prang, a biological anthropologist at Texas A&M University and lead author of the recent article, noted some key features of Ardi’s hand that indicated how she may have moved. The specimen’s phalanges were long compared to its estimated body size. Tpipe bone too curved inward, suggesting that the hand was predisposed to grasping. “The fact that Ardipithecus overlaps the length and curvature of the fingers with the most suspensory primates strongly implies that Ardipithecus has been adapted for suspension,” said Prang.

“Now that doesn’t mean that, you know, humans evolved from an ancestor that looked exactly like a chimpanzee,” he added on a phone call. “This doesn’t mean that chimpanzees are living fossils, or that the chimpanzees themselves haven’t evolved. Instead, our study shows that Ardipithecus, and probably, the earliest fossil humans retain characteristics of an ancestor that most closely resembles chimpanzees and bonobos than any other living primate.

Initially excavated and described in the early 1990s by a team led by paleoanthropologist Tim White, Ardipithecus ramidus received more attention after the 2009 the description of a partial skeleton (nicknamed Ardi) in the journal Science. White does not agree with the findings of the recent article, based on the specific bones and specimens selected for analysis, and the exclusion of others.

“We do not dispute the well established fact that the human hand has evolved over time. However, there is no new data or interpretation here, ”White wrote in an emailed statement. “We cannot take these authors seriously until they become familiar with the unique anatomy of Ardi’s hand, rather than using limited and selected measurements in a post hoc argument supporting the discredited notion that our ancestors specifically looked like chimpanzees.

If you’re relatively new to the debate about the nature of human origins and how our ancestors might have walked on the ground or swinging in trees, get ready to step outside. In 2009, White’s team argued that Ardi lacked characteristics that would indicate he was suited to a monkey-like life; these missing traits included a morphological configuration for swinging and climbing trees, and walking on their knuckles. White’s team postulated that the last common ancestor between humans and chimps was probably quite different from any existing monkey.

Prang’s team argues the opposite in this article, claiming that the ancestor (who preceded Ardi) was closer to chimpanzees than anything else. They go on to report that more human hands appear first with more familiar ones. Australopithecus afarensis, Lucy’s species.

The scarcity of the fossil record doesn’t help much, although Ardi’s hand is more complete than that of young Lucy. Prang added that Ardi’s hand interpretations are useful in narrowing down the goals of what may have been the circumstances of his evolution (and by proxy, what happened before and after Ardi).

“This illustrates the fact that in science we don’t prove something to be true. Instead, we reject assumptions and alternatives that are probably wrong, ”Prang said. “In this case, the hypothesis that humans evolved from an ancestor devoid of suspensive characteristics and an ancestor who looked more like an ape, I think can be ruled out on the basis of Ardipithecus. “