Two recent papers take our minds from lofty thoughts to looking down at the ground.
In the journal Science, Bennett et al. (cited below) report on an analysis of 1.5 million-year-old footprints found in Kenya in what amounts to petrified silt, while in the Journal of Experimental Biology, Rolian et al. (also cited below) discusses why modern humans have evolved short, as opposed to long, toes.
Both papers are parts of a long series of debates on the timing of the change to upright walking (the Nature article demonstrably proves it has been going on for at least a million and a half years) and on what combination of evolutionary pressures selected for our upright gait, and on the route of body adaptations that has taken us there.
Looking at Our Evolutionary Cousins for Guidance
A great many arguments begin by looking at the ways that monkeys and apes get around, and which group, monkeys or apes, best represents the best living example to study the evolution of walking upright.
While virtually all monkeys are overwhelming arboreal, getting around by climbing and swinging from the trees, many of them can be trained in the lab (or in circuses) to be able to walk reasonably upright.
Spider monkeys are regarded as arguably the best natural walkers among the clan. And when either climbing or walking, they use their hands more as balancers than as components in a propulsion system.
But apes might provide an even better comparison , because on the whole, they are distinguished by spending relatively more time on the ground and ambulating there more efficiently than even those monkeys that have been trained in the lab to walk.
Furthermore, much of the fossil record supports the idea that modern apes and humans share more of the same ancestors with each other than do with the monkeys, and that humans and apes together diverged from monkeys, long before we finally diverged from the apes.
Looking at different modern species of apes like the panins (bonobos and chimps), the gorillins (gorillas), and the pongids ( orangutans), gives us a variety of how-we- evolved-to-walk scenarios, and most discuss knuckle-walking.
Historically, it has been suggested that early humans were at least partly knuckle walkers, before they became upright walkers, in part because while virtually all the apes today with which we claim the closest common ancestry, can walk on two legs without the use of knuckles (to push themselves forward), they make much better speed using them, and this is their preferred means of terrestrial locomotion.
While there are a great many points in favor of this knuckle path to upright walking, it seems to be at odds with the ape’s own evolution of hips and shoulders, which have actually become more forward-pitching, and with the fact that most ape knees are too deeply flexed (bent forward), to support long range upright walking.
But orangutans, which are ironically the most arboreal of the great apes, and spend the least time on the ground, often show a behavior and means of locomotion that is not dependent on knuckle-walking.
They can actually walk on tree branches with a significantly more upright posture than do other apes, and when on the ground, actually use the palms of their hands as balancers, rather than as a means of propelling themselves forward. In a sense, they are the ape’s best match for the spider monkey.
What Other Anatomical Adaptations Have to Be in Place for Upright Walking to Succeed?
One of the most common of paleoanthropological finds are skulls and skull fragments. This is because the skull, jaws, and teeth, have some of the densest bone and are able to stand greater crushing or erosive pressure during the changes in the matrix during their millions of years of fossilization.
(In a sense, to paraphrase a commercial for Ford pickup trucks, “They are built tough, so they do last longer”).
Today, owing to tremendous advances in visualization software and model-building, a complete skull can be reliably reconstructed from fragments, and cranial capacity can be estimated very accurately. A certain high ratio of cranial capacity to the rest of body size, has long been thought to be necessary for successful bipedality.
Cranial reconstructions can also tell us a great deal about the relative position of the skull with relation to the vertebral column, and we can predict bipedality partly from this. Skulls that are mounted relatively straight on top, suggest a better ability to see ahead and at greater distances.
The study of crania can also let us infer some advances in vestibular balance mechanisms, much as we modern humans have. Walking upright requires a better ability to maintain oneself upright, and to be able quickly to adjust one’s gait to walking over uneven ground, or to self-correct oneself during a instance of tripping or falling through the walking surface owing to soft ground or washed away underlying soil.
Why Was Upright Bipedality Favored for Human Evolution Anyway?
There are two great argument for a slow transition form an arboreal and largely knuckle-walking interregnum before a completed evolution to bipedality. Both depend to a large extent having the “great toe” extends out at an angle from the remaining toes, so that the toes and the feet can serve as a better grabbing tool, in addition to their being able to be tucked in when knuckle-walking,
· Existing knuckle-walkers are far better tree-climbers, and tree-climbing would seem to have been an excellent way to evade being eaten by some predators (wolves, polar & grizzly bears, cheetahs and some African lions) but not others (black bears, tigers, leopards, etc.).
· Tree climbing also allows for better fruit gathering, and most apes prize ripe fruit above all other foods, and it would not be surprising if early man did so as well. (It should be noted that both knuckle-walking and tree-climbing benefit from.)
But there are also three counter arguments.
· The first is from biomechanical studies of caloric expenditure. Walking upright bipedally is far less costly in terms of calories than various forms of knuckle-walking, or climbing for humans, and by extension, the argument goes, likely for early hominids.
· The second point is an unusual, and yet well-argued one. Bipedality is a more secure way for adults to carry their babies over wide expanses on relatively flat or broken ground, particularly since the abundant hair or fur of most apes and monkeys is not available to modern humans (and by extension to early hominids) as an anchor for their infants. While it is not clear which came first, relative hairlessness on the body’s trunk, or bipedality, it is pretty clear that evolution has not favored the return of pelage or knuckle-walking for hominids.
· The third is based on yet another biomechanical study. Toes that are all parallel, facing forward, and short, relative to the length of the rest of the foot, and leg, allow for better running, particularly at distances. The immediate counterargument is that humans are routinely outrun by both their predators and their game, but this is based on short chases. The fossil record makes the point that early hominids were not only gatherers but hunters. Reasoning by analogy, endurance running, rather than bursts of speed, characterizes successful hunters among aboriginal peoples today. They outlast their prey, not outrun them. A wide range of arguments of modern biomechanical arguments also show that short, front-facing toes that are virtually useless in climbing, and as found in those 1.5 million-year-old-footprints, are significantly better at letting the foot push off during running and do so with less muscle effort and tendon strain.
The debates on upright posture and locomotion will likely go on for some time to come, but this blogger has gotten new respect for the constant admonition of the nuns in my school days, 50 years ago. The stern sisters were always telling me to sit, stand, and walk “up straight.”
Little did I suspect then that they were merely trying to prevent me from being characterized as an anatomically atavistic knuckle-walker!
Tony Stankus tstankus@uark.edu Life Sciences Librarian & Professor
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