Some 7 million years ago, the edges of the Sahara desert may have been one of the first places our distant relatives stopped scampering around on all-fours and started strutting around on two feet.
Ancient arm and leg bones found in present-day Chad suggest that one of the oldest known species in the human family tree – Sahelanthropus tchadensis– was walking on two feet at least 7 million years ago, according to a new study published in the journal Nature today.
The discovery comes from the remains unearthed at Toros-Menalla in the Djurab Desert, northern Chad. Among the finds was a skull of an individual nicknamed Toumaï, which means "hope of life" in the local Daza language. It was identified as a new species that evolved very shortly after humans and chimpanzees diverged, approximately 7 million years ago.
When these fossilized remains were first discovered in 2001, a study of Toumaï’s skull indicated that this extinct hominin had an upright spinal column and an upright posture, suggesting they were perhaps bipedal. However, a huge amount of debate surrounded this question. In 2020, another study looked at a femur bone and concluded that this individual was “not habitually bipedal.”
In this latest study, researchers have taken a deep look at two ulnae (forearm bone) and a femur (thigh bone) also discovered at the site, which may or may not have belonged to Toumaï.
The team used a bunch of advanced measuring methods to examine the shape of the bones, then compared them to bones of chimpanzees, gorillas, orangutans, Miocene apes, and members of the human group, including Orrorin, Ardipithecus, australopithecines, ancient Homo, and Homo sapiens.
The anatomy of the femur suggested that S. tchadensis was bipedal on land. Simultaneously, the forearm suggests that this early ancestor was also equipped to clamber up trees. As you might expect from a relative of both humans and chimps, it appears this species could walk on two legs, but could also be found climbing around trees.
Compared to modern Homo sapiens, we would consider S. tchadensis to be fairly ape-like, bearing physical similarities to a chimp with a relatively puny brain to match – which is partially why the question of bipedalism in the species is so controversial.
While this latest research doesn’t mean the debate is done and dusted, it’s been said to provide some of the strongest evidence yet that this very important character in the human story walked on two legs.
“The Sahelanthropus femur doesn’t have ‘smoking-gun’ traces of bipedalism, but it looks more like that of a bipedal hominin than that of a quadrupedal ape. When considered in conjunction with the orientation of the foramen magnum, which is compatible only with bipedalism, it seems reasonable to infer that Sahelanthropus was some type of biped,” Daniel E Lieberman from the Department of Human Evolutionary Biology at Harvard University wrote in an accompanying News & Views article.