For those who follow the theory of evolution and accept the idea that humans and other primates had common ancestors, one of the turning point events that signified a clear branch in the evolutionary tree is when the first of these beings stopped moving around on all fours and assumed an upright and locked position for getting around – thus being able to see better to find food and plants to relieve the back pain from walking upright. The date of transition to bipedalism has been debated vociferously -- some scientists use the shape of footprints as a sign that an early human stopped using their hands to get around, while others studied the changes in the shape of spines and leg bones for a sign of upward mobility. A generally accepted timeframe for the transition has been 2 to six million years ago, and the species who finally put all of the pieces together and mastered walking upright was named Homo erectus in honor of his skill. Researchers think the H. erectus era began two million years ago and ended around 108,000 years ago in Java, putting “upright man” between Homo habilis and Homo bodoensis, Homo sapiens, Neanderthals, Denisovans and possibly Homo floresiensis and Homo luzonensis. Well, that’s what most scientists thought until a new discovery of leg bones in Africa which potentially came from a walking human ancestor who lived – get ready – seven million years ago. Does this mean Homo erectus has to give up his title?
“A battered fossil leg bone discovered more than 20 years ago in Chad is finally making its scientific debut. Researchers say that remains, described today in Nature, show that a species called Sahelanthropus tchadensis was an ancient human relative that walked on two feet.”
According to the new study published in the journal Nature, this potential major change to the evolutionary tree began in 2001 in the Lake Chad basin in Central Africa (Lake Chad borders Chad, Cameroon, Niger, and Nigeria) when a French and Chadian team led by palaeoanthropologist Michel Brunet of France’s University of Poitiers found a small, nearly complete but damaged skull that matched no known human or primate species. Along with the chimp-sized skull, a number of bone fragments were recovered and taken to labs for study. The skull got the most initial attention – it was nicknamed Toumaï, which means ‘hope of life’ in the Chadian Daza language, and the researchers debated whether it was an early human or an ape. Brunet pointed out that the teeth and face were human-like and the species was named Sahelanthropus tchadensis.
Meanwhile, the other bone fragments received less attention. Two forearms were found and a blackened femur was eventually identified in 2004, but the researchers were not sure if they belonged to the Sahelanthropus tchadensis. However, no other primate bones were found so process of elimination linked them at least to the same species. Unfortunately, the femur appeared to have been gnawed on by animals and was missing the joints at either end – a neck which connects to the hip socket would show if it could bear all of a body’s weight on one leg, while the end connected to the knee would show if the leg was aligned with the body’s center of gravity … both would be clear signs of a walker, not a crawler or tree swinger. Without the ends, it’s easy to see why the femur was ignored.
That was the extent of the research on them until 2020 when there was renewed interest in the fragments. Roberto Macchiarelli, a palaeoanthropologist at the University of Poitiers, amd Aude Bergeret-Medina, the researcher who first identified the femur, examined it again and decided Sahelanthropus tchadensis was not an upright species. Fortunately, palaeoanthropologist Franck Guy, a co-author of the study, took a more open look and identified more than a dozen features of the femur that point upward to uprightedness. Those include the external shapes and curves, internal structures and thicknesses which they compared to early hominin bipeds like Orrorin tugenensis and Ardipithecus Ramidus as well as early ape species. No apes had femurs like this one. While there was no ”A-ha!” feature, the sum of the parts added up to Sahelanthropus probably walking around Lake Chad 7 million years ago. For a piece of non-femur supporting evidence, the researchers noted that the hole for the spinal cord in the Sahelanthropus tchadensis skull shows that it was on the top of the spine – that same hole in non-bipedal apes is more forward in the skull.
One monkey wrench (no pun intended) in the argument for this seven-million-years-old dead man walking (or woman) is the arm bones. Their shapes were very ape-like and suggested to some of the researchers that the species still had the ability to move comfortably in trees as well as on the ground. The press release also notes that one of the features said to support the bipedal argument -- a bony ridge on the femur called the calcar femorale that supports it for walking – was recently found in orangutans and is sometimes NOT found in humans. Should such a questionable feature be included in the supporting evidence? That’s for other researchers to decide.
Finally, Smithsonian Magazine points out that some scientists question whether Sahelanthropus is indeed our oldest known human ancestor or even if it is a hominin at all. Could it be a missing link between hominins and apes? Could it be a separate species that is more like a cousin than an ancestor to either? There is debate whether a single species developed the ability to walk upright or whether it occurred among multiple species at various times in our evolution before H. erectus perfected it.
That brings us to the last feature in favor of a 7 million year old ancestor – its teeth. The skull (remember the skull?) contained reduced canine teeth in the jaw that restructured the mouth -- a trait found in Orrorin tugenensis, which lived around six million years ago, and Ardipithecus Ramidus, which lived about 4.4 million years ago. That’s not necessarily an “A-ha!” but definitely an “OK.”
The search for the oldest human ancestor and the last common primate ancestor goes on as the evolutionary debate rages on. Let’s hope all accept the final answer if and when it is found.