Was Bipedalism in Hominids Originally Postural?
This hypothesis states that bipedalism originally evolved as a postural feeding adaptation rather than a locomotor adaptation. I’m not sure who originated it, but the first article I read about this during grad school was this one by KD Hunt:
“…Chimpanzees fed bipedally from such trees either by reaching up to pick fruit while standing on the ground, or from within the tree, in which case bipedalism was frequently stabilized by grasping an overhead branch. The food-gathering function of chimpanzee bipedalism suggests that hominid bipedalism may have evolved in conjunction with arm-hanging as a specialized feeding adaptation that allowed for efficient harvesting of fruits among open-forest or woodland trees. Such evidence is particularly valuable when it is in accord with fossil anatomy. Australopithecus afarensis has features of the hand, shoulder and torso that have been related to arm-hanging in chimpanzees. The australopithecine hip and hind limb clearly indicate bipedalism, but also indicate a less than optimal adaptation to bipedal locomotion compared to modern humans. Locomotor inefficiency supports the hypothesis that bipedalism evolved more as a terrestrial feeding posture than as a walking adaptation. A bipedal postural feeding adaptation may have been a preadaptation for the fully realized locomotor bipedalism apparent in Homo erectus.”
Hunt gives anecdotal evidence in the above paper, which is later substantiated by Craig Stanford’s 2005 paper detailing frequency and conditions under which this posture occurs. Namely, that “[b]ipedalism was observed only on arboreal substrates, and was almost all postural, and not locomotor. Bipedalism was part of a complex series of positional behaviors related to feeding, which included two-legged standing, one-legged standing with arm support, and other intermediate postures.” Further information as to how this postural behavior may have become habitual, and eventually anatomical comes from a paper by Videan and McGrew in which they tested multiple situations in which a bipedal posture might be expected among captive chimps. Visual obstacles (such as tall savannah grass) did not encourage chimps or bonobos to adopt a bipedal posture; however, both foods that could be carried, and foods elevated relative to the chimps resulted in adoption of a bipedal posture and/or locomotion. Dr. De Waal mentions the proclivity of Pan paniscus for bipedal walking while carrying food in his book Bonobo: The Forgotten Ape* as he entreats us to look to them, not just chimps, to clues about the life and times of our common ancestor.
On a strictly logical basis, I found Hunt’s hypothesis to be extremely plausible in light of the differences in limb proportion between early hominids and H. ergaster/erectus onward:
Figure 1: Roughly to scale, but not quite. left to right: KNM-WT-15000 or Nariokotome Boy, a 12-13 year old specimen of H. ergaster. ‘Lucy’, Australopithecus afarensis. A female Homo sapiens skeleton.
Chimpanzees when knuckle-walking (jogging?) have a similar stride length and speed–though not efficiency–as a modern human. The same can’t quite be said of Australopiths (and putatively H. habilis). Thus, it simply doesn’t seem plausible that early hominids would’ve adopted an upright posture for reasons relating to locomotor patterns. In light of the fact that they would necessarily have traded off both speed and efficiency of movement, it is simply more likely that bipedalism did not evolve for reasons relating to speed and efficiency of travel.
However, if we make the mistake of teleological reasoning, it can be all too easy to fall into the trap that since modern humans move more efficently than modern chimpanzees, the early hominids adopted a bipedal gait as a locomotor adaptation.
The lesson today, kiddies, is that what structures do today may not be what they did in our evolutionary past. The mammalian middle ear bones formed the jaws of our ancestors. Terrestrial vertebrate lungs are actually evolved from Sarcopterygians’ swim bladders; lungfish lungs and our own lungs have two different embryological origins, something not expected given the fact that they’re the closest living relatives to terrestrial vertebrates. Heck, the hallmark of the tetrapods, our digited limbs, did not even evolve as adaptations for life on land. As Jennifer Clack and her colleagues showed, Acanthostega, one of the earliest tetrapods, had ‘arms’ that could not possibly have been brought underneath the body to support its weight, instead sticking straight out like the hydroplane-ish pectoral fins of a Shark (details of both the evolution of the middle ear and of limbs can be found in books by Jennifer Clack* and Henry Gee*).
The bipedal posture of Australopithecus probably conferred different fitness benefits than it does for us. It’s what’s known as a pre-adaptation. If Lucy hadn’t stood up to eat, or learn to walk on her hind legs while carrying things, H ergaster would never have stretched his legs. As to whether, as Dan Lieberman argues, later hominids evolved their distinctive body proportions as an adaptation running remains to be seen.
I’ve had the good fortunte to talk with Dr. Lieberman personally about this hypothesis after a seminar he gave at grad school, so expect a long post on it sometime in the future after I unpack my notes (blech).
*Books mentioned in this post (I don’t link books I haven’t read, and don’t link books I didn’t think were both with scientific merit and entertaining, unless I specifically say so):
Jennifer Clack: Gaining Ground. I highly recommend this one. I’m a huge fan of books about ‘beginnings’, and this one chronicling the rise of the tetrapods is among the best.
Franz de Waal: Bonobo. It’s de Waal, so not as ‘hard’ as most pop sci writing. But he’s one of the few who talks about proximate mechanisms of behavior in nonhuman primates. So he’s typically a must read for me. Also, a lot of people know a lot about chimps, and thus think they know all about our ‘closest living relatives’. Which is problematic, since Bonobos are a sister taxa. It’s an important gap to be filled for many.
Henry Gee: In Search of Deep Time. Will get a full review later. This is an excellent book covering various aspects of vertebrate evolution from proposed echinoderm-chordate common ancestors, to early tetrapods, dinosaurs, birds, and hominids. It gives a good view of the difficulty of pursuing a historical science when much of the record is obliterated. He waxes eloquent about the influence of cladistics, but that’s excusable given its influence and importance in recent years.