New research suggests that one of the first known hominids, a partial skeleton of a female named Ardi, 4.4 million years old, had the right hands to climb trees and swing from branches.
These results, based on statistical bone comparisons of the hands of fossil hominids and current primates, sparked an ongoing debate not only about how Ardi moved (SN: 22/02/19) but also what appeared to be the last common ancestor of humans and of chimpanzees. as (SN: 31/12/09).
“The last common ancestor of humans and chimpanzees was more like chimpanzees than any other living primate,” says paleoanthropologist Thomas Prang of Texas A&M University at College Station. That ancestor, who lived about 7 million years ago, had hands designed like those of chimpanzees and tree-loving bonobos, which walk on their knuckles, he and his colleagues say. The hand design was retained by early hominids such as the East African species of Ardi, Ardipithecus ramidus, the team reported on Feb. 24 in Science Advances.
Hand fossils showing a more human design and grip first appeared in a later hominid, Australopithecus afarensis, according to Prang's group. That fossil species, best known for Lucy's partial skeleton, has inhabited East Africa for about 3.9 million to 3 million years.
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It was not after Lucy's species had died that bonobos diverged into a species apart from chimpanzees, between 1.6 million and 2 million years ago (SN: 27/10/16). This makes the lineage of older chimpanzees a closer relative of early hominids. Still, Prang warns, chimpanzees have evolved over the last million years and do not represent “living fossils” that can be used as substitutes for the ancient ancestor of humans and chimpanzees.
To assess which species possessed especially similar hands, Prang’s team analyzed the sizes and dimensions of four fossils from Ardi’s hands. The researchers then compared those measurements with comparable ones from other fossil hominids and living primates.
Using the same statistical approach, Prang has already argued that Ar. ramidus had a foot that most closely resembled current chimpanzees and gorillas. If so, then Ardi and his compatriots, who were very close to the chimpanzees, most likely split their time between walking on all fours and moving through trees, he argued April 2019 on eLife.
In stark contrast to Prang's findings, paleoanthropologists who discovered and studied Ardi's remains argue that Ar. ramidus was constructed neither as chimpanzees nor humans (SN: 9/9/15).
Ardi’s finger bones look somewhat like those of chimpanzees, says Morgan Chaney of Kent State University in Ohio. Chaney works with Owen Lovejoy of Kent State, one of the scientists who originally studied Ardi's remains. But the fossil female's palm and forearm were much shorter than those of chimpanzees, Chaney says. Combined with their distinctive wrists, the arms would only allow them to grab branches as they moved slowly through the trees.
Chaney states that the structure of Ardi's forearm was not that of a gunner.
Prang’s previous analysis of Ardi’s feet also cannot demonstrate a chimney-like design, Chaney and colleagues argue on Jan. 10 in the Journal of Human Evolution. The scientists indicated that Ardi’s relatively long middle foot, which does not adapt to climbing, was considered in Prang’s statistical analysis. The similarities in body mass between Ardi and chimpanzees, rather than a close evolutionary relationship, at least partly explain the chimney-like foot measurements that Prang cites.
Based on his overall body design, Ardi walked upstairs, argue Chaney and his colleagues. She combined a long lower pelvis that stabilized a straight leg posture with a thick, opposing toe. Investigators claim Ardi climbed the trees cautiously and rarely hung or swayed from the branches.