We walk, therefore we are
Humans differ from all other primates, and indeed from virtually all other animals, in possessing a very weird adaptation. We don’t think of it as weird because we do it every day of our lives. It is so obvious that when I ask my students what makes humans unique, many of them, after citing large brains and culture, miss the obvious: We walk on our hind legs.
Charles Darwin thought bipedalism, along with big brains and tools, was part of a package that defined us as human and emerged when our ancestors descended from their ancient arboreal habitat. Darwin suggested that uprightness liberated our hands from locomotor needs, thereby enabling our ancestors to make the tools that their big brains had visualized. This trinity of attributes began to unravel, however, when discoveries of the most ancient bipedal ancestors were found to possess small brains and apparently did not manufacture stone tools.
Anthropologists have grappled with this quandary for nearly a century, for there is scant doubt that our ancestors were walking on their hind legs at least 1.5 million years before they began crafting stone tools. This understanding prompted anthropologists to refocus their attention and consider why our ancestors exchanged a fast, stable, evolutionarily enduring four-legged mode of locomotion (quadrupedalism) for a slow, unsteady, disease-ridden (fallen arches, bad backs, hernias etc.) two-legged strategy. It certainly wasn’t predestined that when our ancestors stood up, they would evolve into modern humans, but standing undoubtedly laid the groundwork. If reproductive success is a measure of the effect of a behavioral adaptation, then with more than 6 billion of us bipeds on the Earth today, this uncanny locomotor mode has had a big payoff.
(During my days as an undergraduate, I learned that our ancestors stood up to look over the tall savanna grass. And yet I had to wonder: What was the survival rate for a short, indefensible, small-brained ancestor, like Lucy and her ilk, after standing up to look over tall grass, thus announcing their presence to every hungry carnivore in the neighborhood, all of which were quadrupedal and very fast? Evolutionary success is about leaving genes, but the Savanna Hypothesis, as it came to be known, is really nothing more than a quick route to extinction.)
Jonathan Kingdon is uniquely skilled to embrace the daunting task of learning how we became human because of his intimate lifelong affair with Africa. Brought up in Tanganyika (now Tanzania) and sensing the pulse of Africa as a barefoot youth, he nourished his curiosity on a never-ending diet of close encounters in the crucible of human origins. The real message from Kingdon’s book, “Lowly Origin,” is that hominid fossils are only a part of the story. Thus, Kingdon ponders the origins of our singular adaptation within a richly textured framework of African animal behavior, geography and ecology.
Kingdon rejects single-cause explanations for upright walking, such as looking over tall grass, reaching up into trees for fruit, carrying food etc. Instead he proffers a scenario that paints a more nuanced picture of early hominid ancestors moving to the ground to exploit rich food sources. Couching his argument in a classic Darwinian framework of gradual evolutionary change, he outlines a more credible series of sequential postural and feeding adaptations that ultimately led to bipedal posture and locomotion.
Kingdon’s scenario will elicit widespread discussion. He contends that it was in eastern Africa and especially in the forests adjacent to the Indian Ocean that our ancestors experienced the primal glimmerings of uprightness. He envisions this process not as a rapid happening but as a gradual postural adaptation from having lived as ground apes and having incorporated a squat feeding phase, which is characterized by increasing verticality of the body as a precursor to bipedalism. It was the acquisition of uprightness, freeing the forelimbs and hands from locomotor tasks that set the stage for human modernity.
Kingdon’s model is decisively rooted in the ecology and biogeography of Africa’s past. He cleverly interweaves his firsthand insights into the behavior and adaptations not only of modern African primates like the great apes and monkeys but also of the other African mammals in the larger arena of evolution. With his unsurpassed knowledge of Africa, Kingdon views us not as having evolved from ape to angel but as having lived as “just another African mammal” in a rich ecological setting through a series of astonishingly successful behavioral adaptations.
Pinpointing precise geographical regions and ecological settings for hominid origins, Kingdon delivers a provocative hypothesis of when, where and how our distant ancestors became bipedal. In the initial phase, he sees the immigration into Africa of a group of generalized, woodland-adapted, non-equatorial Eurasian apes that crossed the land bridge from Arabia about 10.5 million years ago. This is a departure from the long-established view that the rootstock of humans and modern African apes was a yet-to-be discovered creature that existed in the dense rainforests of Africa some 10 million to 15 million years ago.
The next phase in Kingdon’s reasoning is that some of the descendants of the Eurasian apes evolved into modern chimps in the moister, more tropical forests of central Africa, while another group, which he calls the ground apes, made their home in the Indian Ocean littoral forests. The two groups maintained their isolation because of an arid zone that lay between them. (The more conventional argument postulates a gradual diminution of the tropical forests of central Africa, forcing our ape ancestors to retreat westward with the forests, thereby isolating our hominid ancestors in the open environments to the east.)
Because the eastern forests did not provide consistent arboreal food sources, the protohumans moved frequently into terrestrial habitats to augment their diets. This ground ape developed increased flexibility in the trunk and waist and started to use both upper limbs during squat feeding and foraging. These changes further modified the orientation and balance of the skull and gradually shifted the bulk of body weight to the legs, thus moving the center of gravity to the pelvic region rather than the chest (as it is in apes). The conditions in these forests and the behavioral changes to upright posture set the stage for the “unfolding of the legs” and becoming an upright walker.
Kingdon postulates that the ground apes, as they adopted this new stance, would have encountered more predator pressure, leading perhaps to richer and more diverse social interaction in an effort to ward off danger or competition through coordinated displays. As a result, eastern “squat-foragers” began to select mates based on behavioral flexibility.
In a final step, Kingdon further speculates that the diversity of the human fossil record is the result of the movements of these ground apes as they began to expand their ranges and adapt to a diversity of ecological niches.
“Lowly Origin” -- a reference to the last words in Darwin’s “Descent of Man”: “Man still bears in his bodily frame the indelible stamp of his lowly origin” -- is brimming with information, insight, experience and speculation about how we became human. For the nonspecialist, this thoughtful treatise presents a considerable challenge, almost demanding that a dictionary of biology be kept nearby. Those who make the journey through “Lowly Origin” will be rewarded with a comprehensive and evocative rendition of who we are and how we fit into the natural world. Kingdon, in an ambitious and far-reaching exposition, brings a wealth of personal observation, experience and learning to a core issue in human evolutionary studies: the origins of human walking.
In 1871, Darwin wrote that the primary aim for humans is “to discover our biological history and get to know our evolutionary nature.” The quest, he argued, is obligatory if we are to attain “a still higher destiny in the distant future.” In answering Darwin’s call, Kingdon explains not only how our ancestors became erect but also how this bizarre behavior laid the foundation for the evolution of a species in control of all future life on this planet. Rather than answering to a higher destiny, we are, according to Kingdon, “niche-thieves,” ever increasing our exploitation, domination and destruction of wider and wider ecological niches in the natural universe, to the point that we are now threatening the very essence of our survival.
