Paleoanthropologists from the United States, the United Kingdom, Norway, and South Africa have examined the fossilized hand of Ardipithecus ramidus, a species of hominid that lived in the east of the African continent around 4.4 million years ago. Their results, published in the journal Science Advances, provide clues about how early humans began to walk upright and make similar movements that all humans perform today.
Ardipithecus ramidus, a hominid that lived in Africa more then 4 million years ago. Illustration by Arturo Asensio, via Quo.es.
“Bone shape reflects adaptation to particular habits or lifestyles — for example the movement of primates — and by drawing connections between bone shape and behavior among living forms, we can make inferences about the behavior of extinct species, such as Ardi, that we can’t directly observe,” said Dr. Thomas Cody Prang, a researcher in the Department of Anthropology at Texas A&M University.
To make comparisons about the kind of locomotor behavior used by the earliest hominins, the researchers compared the shape of the Ardipithecus ramidus hand to hand specimens from 53 anthropoid primate species, including chimpanzees, bonobos, and orangutans.
“We found evidence for a big evolutionary ‘jump’ between the kind of hand represented by Ardi and all later hominin hands, including that of Lucy’s species,” Dr. Prang said.
“This jump happens at a critical time when hominins are evolving adaptations to a more human-like form of upright walking, and the earliest evidence for hominin stone-tool manufacture and stone-tool use, such as cut-marks on animal fossils, are discovered.”
“The fact that Ardi represents an earlier phase of human evolutionary history is important because it potentially shines light on the kind of ancestor from which humans and chimpanzees evolved.”
“Our study supports a classic idea first proposed by Charles Darwin in 1871, when he had no fossils or understanding of genetics, that the use of the hands and upper limbs for manipulation appeared in early human relatives in connection with upright walking.”
“The evolution of human hands and feet probably happened in a correlated fashion.”
Since Ardipithecus ramidus is such an ancient species, it might retain skeletal features that were present in the last common ancestor of humans and chimpanzees.
If this is true, it could help the scientists place the origin of the human lineage — in addition to upright walking — into a clearer light.
“It potentially brings us one step closer to an explanation for how and why humans evolved our form of upright walking,” Dr. Prang said.
“The big change in hand anatomy between Ardi and all later hominins occurs at a time, roughly between 4.4 and 3.3 million years ago, coinciding with the earliest evidence of the loss of a grasping big toe in human evolution.”
“This also coincides with the earliest known stone tools and stone cut-marked animal fossils.”
“It appears to mark a major change in the lifestyle and behavior of human relatives within this timeframe,” he said.
“We propose that it involves the evolution of more advanced upright walking, which enabled human hands to be modified by the evolutionary process for enhanced manual manipulation, possibly involving stone tools.”
Thomas C. Prang et al. 2021. Ardipithecus hand provides evidence that humans and chimpanzees evolved from an ancestor with suspensory adaptations. Science Advances 7 (9): eabf2474; doi: 10.1126/sciadv.abf2474