What Fossilized Hand Says About Last Human and Chimpanzee Ancestor

More than 1 million years before the early hominin known as Lucy was striding across the Afar region of Ethiopia, the lesser-known Ardipithecus ramidus roamed approximately the same area. Now, a team of anthropologists have looked at the 4.4 million-year-old fossilized hand of one specimen (affectionately dubbed “Ardi”), and argue that the human ancestors’ roaming may have involved more swinging through the trees than previously thought.

Published on Wednesday in the journal Science Advances, the team’s comparative analysis focuses on Ardi’s hand morphology in the context of extinct and extant relatives. In conversation with bones from elsewhere in our family tree, the traits of Ardi’s hand indicate when specific adaptations currently observed in different primates came to be. Specifically, knowing how Ardi moved gets us closer to knowing how we (Homo sapiens) came to be a terrestrial, bipedal bunch of primates.

The team of Thomas Cody Prang, a biological anthropologist at Texas A&M University and lead author of the recent paper, noted some key traits of Ardi’s hand that indicated how it may have moved. The specimen’s phalanges were long relative to its estimated body size. Those bones also curved inwards, suggesting the hand was predisposed for grabbing. “The fact that Ardipithecus overlaps in finger length and curvature with the most suspensory primates strongly implies that Ardipithecus was adapted to suspension,” Prang said.

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

Originally excavated and described in the early 1990s by a team led by paleoanthropologist Tim White, Ardipithecus ramidus received more attention following the 2009 description of a partial skeleton (nicknamed Ardi) in the journal Science. White doesn’t agree with the findings of the recent paper, on the basis of 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 evolved through time. However, there are no novel data or interpretations here,” White wrote in an emailed statement. “We cannot take these authors seriously until they come to grips with the unique anatomy of Ardi’s hand, rather than using limited, selected measurements in a post hoc argument supporting the discredited notion that our ancestors were specifically chimpanzee-like.”

If you’re relatively new to the debate on the nature of human origins and how our ancestors may have strolled on the ground or swung through the trees, be prepared to hang out. Back in 2009, White’s team argued that Ardi lacked characteristics that would indicate it was suited for an ape-like life; such absent traits included a morphological set-up for swinging through trees and climbing them, and walking on their knuckles. White’s team posited that the last common ancestor between humans and chimpanzees was probably quite different from any extant ape.

Prang’s team argues the opposite in this paper, stating that the ancestor (which preceded Ardi) was closer to chimpanzees than anything else. They go on to report that more human-like hands first crop up with the more familiar Australopithecus afarensis, Lucy’s species.

The sparseness of the fossil record doesn’t help matters much, though Ardi’s hand is a more complete than the younger Lucy’s. Prang added that interpretations of Ardi’s hand are helpful in narrowing the goalposts of what may have been the circumstances of its evolution (and by proxy, what arose before Ardi and afterwards).

“It illustrates the point that in science, we’re not proving something to be true. Instead, we’re discarding hypotheses and alternatives that are likely to be false,” Prang said. “In this case, the hypothesis that humans evolved from an ancestor that lacked suspensory characteristics, and an ancestor that was more monkey-like, I think can be discarded on the basis of Ardipithecus.”