In looking for human origins, one starting point is to look at human anatomy, more specifically, how humans are anatomically different from our closest relatives, the chimpanzees (Pan troglodytes). It should be stressed in looking at these anatomical differences, that modern humans are not descendants of chimpanzees, but the relationship is more like distant cousins as we share a common ancestor in the distant past (perhaps 7 million years ago). Genetically chimpanzees and humans share about 99% of their genetic inheritance yet they are visually different species. Humans and chimpanzees are more closely related genetically than are dogs and wolves.
Chimpanzees and gorillas are quadrupeds which have a form of walking on the ground generally called knuckle-walking. When they walk, they are on all fours, supporting their weight on the knuckles of their hands. When they do walk on two legs, chimpanzees waddle: they walk with their legs far apart and their bodies sway from side to side. Their swaying walk reminds some observers of the gait of an inebriated human. Sober humans, on the other hand, have a striding gait in which there is little sway. Instead of having to expend energy on trying to stabilize the upper body, humans use their energy for forward motion.
Unlike other primates, humans are bipedal meaning that we walk on two legs. Bipedalism is generally considered to be the central feature of being human and has consequences for much of our anatomy. In his entry on human nature in The Oxford Handbook of Archaeology, Jonathan Marks writes:
“The adoption of bipedalism is uniquely human only insofar as it occurred on a primate frame, with considerable independence of movements of the pelvic and shoulder girdles.”
In looking at bipedalism from an evolutionary viewpoint, Daniel Lieberman, in his book The Story of the Human Body: Evolution, Health, and Disease, writes:
“If there was any one key initial adaptation, a spark that set the human lineage off on a separate evolutionary path from apes, it was likely bipedalism, the ability to walk and stand on two feet.”
Chris Stringer and Peter Andrews, in their book The Complete World of Human Evolution, write:
“Bipedalism is unusual because it slows us down and at first sight makes us vulnerable to faster-moving predators.”
They go on to say:
“So in adopting such a restricted form of locomotion humans must have had other means of defence against attack, and yet it seems that bipedalism was one of the first, if not the first, distinctly human attribute to evolve.”
In their book From Lucy to Language, Donald Johanson and Blake Edgar put it this way:
“Any explanation for why we became bipedal must also take into consideration that, compared with quadrapedalism, bipedal locomotion is slow, clumsy, and fraught with opportunities for injury. And contrary to much that has been written, walking on two legs is not energetically more advantageous than getting around on four legs.”
Speculation on how and why bipedalism first evolved is often driven by some inherent purpose—a discredited view of evolution proposed by Jean-Baptiste Lamarck in 1800. In his book The Accidental Species: Misunderstandings of Human Evolution, Henry Gee writes:
“For example, humans got up so that they could free their hands in order to make tools or grasp low-hanging fruit; or in order to cuddle babies close to their chest; or in order to see longer distances; or in order to live better in open country rather than in forests, as our ape cousins still do.”
While bipedalism certainly enhanced all these things, these things did not lead to bipedalism.
With regard to the evolutionary factors leading to bipedalism, Daniel Lieberman writes:
“It is impossible to know for sure why natural selection favored adaptations for bipedalism, but I think the evidence most strongly supports the idea that regularly standing and walking upright was initially selected to help the first hominins forage and obtain food more effectively in the face of major climate change that was occurring when the human and chimpanzee lineages diverged.”
In looking at the natural selective force that would have conferred advantages on bipedalism, Richard Klein with Blake Edgar, in their book The Dawn of Human Culture, write:
“With regard to what stimulated the shift to bipedalism, the most likely cause is environmental change. Between 10 million and five million years ago, global climate became cooler and drier, and grasslands expanded while forests shrank or thinned out.”
Richard Klein with Blake Edgar also write:
“Life on the ground presented new challenges and opportunities that favored those individuals whose anatomy and behavior gave them a reproductive advantage, however slight, over their peers. In retrospect, it appears that the most important anatomical advantage was an enhanced ability to walk and run bipedally.”
Bipedalism was the first evolutionary step which would eventually lead to the aspects of our anatomy which enable speech. In his book Thumbs, Toes, and Tears and Other Traits that Make Us Human, Chip Walter writes:
“Standing up straightened and elongated our throats so that our voice box dropped. In time that made speech possible, but we also needed a brain that could generate the complex mental constructions that language and speech demand.”
There are some researchers who feel that the origins of language lie in gestures made by the hands. In his book Nature via Nurture: Genes, Experience, and What Makes Us Human, Matt Ridley reports:
“Bipedalism freed the hands not just to carry things, but to talk. The front limbs of most primates are too busy propping up the body to get into conversation.”
There are some people, such as Owen Lovejoy, who have suggested that bipedalism was driven in part by sexual selection. Bipedalism shows off the human genitalia effectively, including the large male penis and the large female mammary glands. Lee Berger, writing with Brett Hilton-Barber in their book In the Footsteps of Eve: The Mysteries of Human Origins, report:
“Herein lies the evolution of sexual attraction, and therefore reproductive behavior.”
With regard to the consequences of bipedalism, Chris Stringer and Peter Andrews write:
“The head has to be balanced on top of the backbone instead of being slung in front; the backbone has developed curves to withstand stresses and function as a spring; the hip was broadened and wrapped around the sides of the body to give better leverage to the muscles that maintain us in an upright position; the legs become longer and angled inwards to keep the centre of gravity along the midline of the body; and the feet have developed arches and the big toe has rotated in line with the rest of the foot to provide additional leverage.”
In his book Humans: From the Beginning, Christopher Seddon summarizes the anatomical adaptations to bipedalism:
- “A curved lower spine;
- A shorter broader pelvis and thigh bone angled inwards, allowing it to swing under the body (the chimpanzee thigh bone is not angled);
- Longer lower limbs (chimpanzees have short lower limbs);
- The ability to fully straighten the leg (chimpanzees cannot do this);
- An enlarged, in-line big toe (chimpanzees have opposable big toes);
- A repositioning of the foramen magnum from the rear to the centre of the skull; with present day apes, it is positioned at the rear, but with humans and other hominins, it is centrally-placed.”
With regard to the position of the foramen magnum, Chip Walter reports:
“If you sat a gorilla and a human side-by-side and looked straight down through the tops of their heads, you would see that while the spine of a gorilla enters at the back of the cranium, ours enters at the middle, so that our heads tilt forward and sit squarely on top of our bodies, making it the final piece of skeletal statuary to be stacked in one straight line from the tips of our toes to the tops of our necks.”
Feet
Evolution from a quadruped—that is, walking on all fours—to a biped also means that the foot changes. When we look at the foot of a chimpanzee, we see that it resembles the chimpanzee hand in that the big toe splays out from the other digits. In addition, the chimpanzee foot does not have an arch: when apes stand, the whole length of the foot is on the ground. The foot of the chimpanzee is designed for climbing, not walking. Jonathan Marks writes:
“We see many similarities between the chimpanzee’s foot and the human foot, for they are built of roughly the same parts, in roughly the same relations. Yet the chimp foot is a graphing structure and the human foot is a weight-bearing structure.”
In comparing the feet of humans with those of chimpanzees and other apes, Daniel Lieberman writes:
“Walking humans usually land first on the heel and then, as the rest of the foot makes contact with the ground, we stiffen the foot’s arch, enabling us to push the body upward and forward at the end of the stance, mostly with the big toe.”
Lieberman also reports:
“The feet of chimps and other apes lack an arch, preventing them from pushing off against a stiffened foot, and their toes are unable to extend as much as humans.”
More about Human Origins
Human Origins: Sexual Selection
Ancient Africa: Australopithecus
Ancient Africa: Homo Naledi
Ancient Humans: Fire
Early Homo Sapiens: A Brief Overview of DNA
Ancient Europe: Climate Change and Neandertal Extinction
Human Antiquity
Ancient Humans: A Short Overview of Homo Ergaster