Humans are thought to have diverged from primates 7 million years ago. Let’s look at the best candidates for our earliest ancestors. Questions remain as to whether these were early humans or just extinct apes.
In the famous 1968 film, Planet of the Apes, we were presented with an alternate vision of the planet Earth, one where the other primates have risen to dominance after humanity destroyed itself in a nuclear cataclysm. The new rulers of the planet are not quite human and yet not quite apes, they resemble something in-between. The world which existed when the first members of our genus emerged was perhaps something like that cinematic vision, a world in which various types of beings, resembling ape-men, represented the apex of evolution.
Prior to the emergence of the first examples of Homo there existed a long succession of more primitive beings, going all the way back to a common ancestor shared with the other living primates. Most of these predecessors are very hard to physically distinguish from chimpanzees and gorillas. Some of those identified may yet transpire to be in fact simply apes rather than true ape-men. Anthropomorphic (human-like) traits are looked for in the available fossil evidence.
The earliest identified potential human ancestor is Sahelanthropus tchadensis, known from a single skull (nicknamed Toumai) discovered in the southern Sahara Desert back in 2001. Dating methods used on the skull place it between 6 and 7 million years old, though most likely at the earliest part of that range.
Sahelanthropus tchadensis is believed to have been one of the first forms to emerge after the divergence from our primate ancestors. The small number of fossilized skull fragments, discovered at the archaeological site in Chad, exhibit morphological evidence (anatomical features) suggestive of bipedal movement, convincing most academics that it walked on two legs at least some of the time.
The main evidence for this claimed bipedalism is the placement of the foramen magnum (the hole from which the spine exits the skull), which is centrally placed in the Toumai skull. Most physical anthropologists adhere to the popular understanding that central placement of this hole is a feature typical of creatures that walk upright. Typically, animals that are quadrupedal (four legged) have this hole positioned further towards the back of the skull.
Fortunately, a few teeth were also found alongside the skull fragments, some of which were small, human-like, canines. These teeth help to add some merit to the claimed position for these being on the human family tree. However, unlike modern humans, Sahelanthropus tchadensis had a small brain, with a cranial capacity of around 350cc, which is about the same size as found in a modern chimpanzee.
A second being is currently recognised as a possible human ancestor from soon after the initial divergence event, Orrorin tugenensis. This species, known from just a handful of bones, was first discovered in the hills of northwest Kenya in 2000, the fossils are dated to around 6 million years before present. This discovery has remained controversial as several relevant scientists feel it is simply an unusual ape.
The fossil finds included several pieces of leg bones, these are of course preferable more useful than skull fragments when it comes to assessing the evidence of bipedalism. Once all the potential indicators of bipedalism had been evaluated, including joint size and thighbone shaft strength, they were then compared with like measurements from other early hominin fossils, also with those of modern humans and apes. The findings displayed a strong similarity to the legs bones of early humans living around 4 million years ago, human forms that are well accepted to have walked on two legs. The brain size was however once again a mere 350cc, or Chimp sized.
The acceptance of Sahelanthropus tchadensis as a human ancestor has been called into question by Aidan A. Ruth, a biological anthropologist at Kent State University in Ohio. Ruth has recently completed a study of the way in which the foramen magnum position and angle relates to animal types, whether bipedal or quadrupedal. This study found that there was no evidence the angle of this hole could reveal how any animal moved around. This calls into question the reliance on this feature for identification of hominin ancestors.
“The foramen magnum might tell you about how the neck is held, which might tell you about how the trunk is held — but there’s a chain of inferences from the bottom of the skull to locomotion, and each time you have to infer, your argument gets a little weaker.” – David Strait, physical anthropologist at Washington University in St. Louis
For the moment, it seems that Orrorin tugenensis most likely did stand on two stable legs, meanwhile Sahelanthropus tchadensis seems to be standing on some very shaky ground.
6-Million-Year-Old Human Ancestor 1st to Walk Upright? National Geographic News
Can The Skull Tell Us If An Ancient Human Walked Upright? Inside Science
Bruce Fenton is an ancient mysteries and human origins researcher based in Australia. He is author of the book The Forgotten Exodus – The Into Africa Theory of Human Evolution.