7-Million-Year-Old Femur Reveals Prehistoric Human Walked Upright
A 7-million-year-old femur reveals a prehistoric human walked upright.
In 2001, archaeologists found two forearm bones and part of a fossilized leg bone in the central African country of Chad.
A recent study says that these fossils come from the oldest known hominid.
This person walked upright on the ground and in the trees and lived about 7 million years ago.
S. tchadensis, which is seven million years old, is a contender for the title of first member of the hominin lineage, the evolutionary branch that connects the ancestors of modern humans and chimpanzees.
S. tchadensis was found by a French and Chadian team in the Lake Chad basin in July 2001.
The most important find was a skull that was almost whole but had a lot of damage.
The next year, Nature wrote about it.
The name Touma, which in the Chadian Daza language means "hope of life," was given to the skull.
Even though Touma's brain was the size of that of a chimpanzee, researchers at the University of Poitiers in France led by Michel Brunet said that his face and teeth were like those of later hominins.
Touma may have stood upright on two feet based on the angle at which its head and spine would have met.
During the excursion, the researchers gathered hundreds of other bone fragments, but they first missed the upper leg bone's shaft.
Roberto Macchiarelli, a palaeoanthropologist, says that Aude Bergeret-Medina, a graduate student at the University of Poitiers, found the piece of the blackened femur in 2004.
In his capacity as Bergeret-supervisor, Medina concurred with her conclusion that the bone belonged to a primate, most likely S. tchadensis.
In 2004, Brunet's team began investigating the femur.
The researchers emphasized field research and work on other Sahelanthropus remains, including a set of arm bones discovered in 2001, but the project didn't pick up momentum until 2017.
At a news conference, Franck Guy, a palaeoanthropologist from Poitiers who oversaw the study, stated in a news conference that "it was not our priority at the time."
One of palaeoanthropology's worst kept secrets over the years was the undescribed fossil, frequently referred to as Touma's femur, even though it's unclear whether the skull and leg bone belonged to the same person.
In 2020, a team led by Macchiarelli and Bergeret-Medina gave a brief description of the femur based on their research from 2004. This research took several days.
They came to the first guess that the bones probably did not come from a species that usually walked upright.
Guy's team arrives at the opposite conclusion in the report, which now includes the femur along with the two arm bones.
The researchers say that more than a dozen features of Touma's femurs show that his ancestors walked on two feet, and the fact that his arm bones look like those of apes shows that he would have had no trouble climbing trees.
“It is great that these specimens are finally published officially, since their presence has been known to a lot of us,” says Yohannes Haile-Selassie, a palaeoanthropologist at Arizona State University in Tempe.
He says the description “clearly shows that the femur assigned to Sahelanthropus here had most of the morphology that one would expect to see in a habitual biped” and confirms that Sahelanthropus was a hominin.
In an essay accompanying the paper, Daniel Lieberman, a palaeoanthropologist at Harvard University in Cambridge, Massachusetts, writes that "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.”
The analysis has less of an impact on other scientists.
One of the things Guy's team used as proof of bipedalism was the fact that the femur is supported by a bony ridge when the animal walks upright.
However, a 2022 study discovered that this trait, known as a calcar femorale, is occasionally absent in humans but is present in some apes like orangutans and should not be seen as a sign of bipedality.
In addition, lead author Marine Cazenave, a palaeoanthropologist at the American Museum of Natural History in New York City, questions whether the Sahelanthropus femur ever had such a characteristic.
Macchiarelli says that other things about the femur, like the twist in its shaft, that have been used as proof of bipedalism may actually be the result of being crushed by sediment for millions of years.
“They cherry-pick what they think is information which is consistent with the femur shaft being a biped, and they studiously ignore information to the contrary,” adds Bernard Wood, a palaeoanthropologist at George Washington University in Washington DC who co-authored the 2020 analysis with Macchiarelli and Bergeret-Medina.
Guy and his co-authors say that Sahelanthropus is an upright walker not because of a single trait, but because of a group of traits that are more typical of bipedal hominins than of knuckle-walking apes.
“There is no such thing as a ‘magic trait’ that can be used to say if a fossil species was bipedal or not,” they added.
Even so, Sahelanthropus, according to Macchiarelli, is more likely to be an ape than a hominin, and it likely existed in the last ten million years, not long after the two lineages split.
Macchiarelli says that the remains of Sahelanthropus, especially its well-known femur, may hold clues about how the ancestors of humans and chimpanzees looked and moved.
“These are extraordinarily important specimens.”
According to Madelaine Böhme, a palaeontologist at the University of Tübingen in Germany, the bones could help researchers better understand how traits like hominin-like teeth originated in a species with ape-like arms that were well-suited for living in trees.
She is glad that the arm and leg bones have been formally described, but she wishes that similar talks had started when the bones were found 20 years ago.
“This delay has slowed this field of science considerably,” she says.