The current (February 2006) issue of AJPA carries an article by Craig Stanford describing the context of bipedal posture for chimpanzees in the Bwindi Impenetrable National Park. When considering how bipedal locomotion evolved in early hominids, it is an essential comparison how chimpanzees (or other hominoids) use bipedal postures. Stanford writes:
As Hunt (1994, 1996) pointed out, hypotheses for the advent of bipedalism that involve behaviors in which prehominids may have frequently engaged offer the most plausible explanations for the adaptive shift from quadrupedal to bipedal posture (Stanford 2006:225).
Stanford was able to observe a large number of episodes of bipedal posture in the study group -- 179 cases in 247 observation hours. I find the context to be the most interesting result:
All 179 instances of bipedalism were recorded while chimpanzees were foraging in large trees. All but one instance occurred as postural rather than locomotor bipedalism, and 96% of all instances occurred in a feeding context....Chimpanzees appeared to forage bipedally most often when feeding in the upper portion of the crown, reaching up to branches emergent in the sunlight, and perhaps containing harder-to-reach ripe fruit. (Stanford 2006:227).
Studies of bipedal posture in wild chimpanzees have been rare, as Stanford reviews, but have typically found fewer instances of bipedality and have included some terrestrial cases. The key finding of all studies appears to be that foraging for fruit is the main reason why chimpanzees occasionally stand.
What do the chimpanzees tell us about early hominids? Here is the suggestion:
The behavior of wild chimpanzees suggests that several aspects of the positional behavior of earliest hominids may have been given less attention that they merit. First, arboreal bipedal posture is not dichotomous with arboreal quadrupedal posture. Bwindi chimpanzees moved fluidly between four-legged, three-legged, and two-legged postures while feeding in tree crowns. Their use of three-dimensional space in tree tops incorporated elements of positional behavior most often seen as binary states. This fluid quadrupedal-bipedal shifting may have occurred in the earliest hominids as well. Arguments about whether early hominids were fully adapted to bipedal walking, or facultatively arboreal, have been carried on for at least three decades (Susman et al., 1984; Lovejoy, 1988). Recent evidence suggests that knuckle-walking may have been employed by the immediate ancestors of the australopithecines (Richmond and Strait, 2000). Chimpanzee bipedal behavior suggests that early hominids likely engaged in a fluid variety of positional behaviors and postures, but provides little evidence for the adaptive advantage of terrestrial knuckle-walking in the last common ancestor of apes and humans (Stanford 2006:230).
Now, humans are fully adapted to bipedal walking, and we are facultatively arboreal (that is, we can climb trees), so there is no reason to think that early hominids were less facultatively arboreal than we are, and I would venture that they were probably a good deal more so.
The fundamental question about early hominids is why they abandoned the ability to be facultative quadrupeds. That is something that chimpanzee positional behavior isn't going to tell us -- after all, chimpanzees take on bipedal posture in ways that don't compromise their quadrupedal abilities.
The chief importance of the chimpanzee comparison is to illustrate the kinds of ways that locomotor diversity occur in hominoids. After a brief discussion of locomotor flexibility in gorillas, Stanford concludes with this:
Rose (1984) argued that there is no reason to view the origin of bipedalism as a progression from "poor biped" to "good biped." Instead, there was likely a diversity of forms of bipedalism in the earliest hominids. One such hominoid example may be Oreopithecus bambolii, a sup-
posed bipedal ape (Kohler and Moya-Sola, 1997). The bipedal evidence from Bwindi, Mahale, and Gombe supports this view of early hominid evolution. Instead of viewing the earliest bipedal adaptation as the lowest
rung on a posture/locomotion evolutionary ladder, it may be that early hominid species evolved a variety of forms of bipedalism in particular ecological contexts (Stanford 2006:230).
I guess that is one possibility to explain evidence of vertical posture in early hominids in the absence of good evidence of bipedality (from postcranial evidence).
The "diversity of forms" argument really suggests a stage during early hominid evolution when the ability to be effective quadrupeds had not yet been lost. Perhaps we will find these quadrupedal hominids. Perhaps we already have. On the other hand, this idea opposes the hypothesis that locomotor evolution may have either caused the origin of the hominid lineage or very closely followed it.
It seems to me that the level of species diversity of early hominids and this locomotor problem may be strongly linked. But I think they might be linked in the opposite direction than one might assume.
Suppose, for instance, that the hominid lineage arose as an adaptive radiation resulting from a significant new adaptation for bipedality. The "adaptive radiation" would be the origin of many new bipedal species spreading and adapting to different ecologies. Early hominid species diversity would be a consequence of their novel locomotor adaptation.
In contrast, if hominids originated as one among many quadrupedal apes in the Late Miocene, they might well have adapted over a long time as quadrupeds within a single ecology to which they remained limited. Perhaps the attainment effective bipedality would have spurred an adaptive radiation, but this event would have followed long after the origin of the hominids. Hominid species diversity might have always been low, or might have remained low until the Late Pliocene.
Now I don't think any of these arguments can be taken very far. It is always possible that bipedality arose early without any consequent adaptive radiation, or that there were multiple bipedal ape lineages other than hominids, or almost any other combination of events. There just isn't fossil evidence that could delimit hypotheses about hominid origins.
But I can't think about diversity without considering the mechanisms for it to have arisen. And while it is possible that many hominoid lineages were experimenting with bipedal posture and locomotion in diverse ways, I can't think of what would have caused the diversification of a large array of hominid species in the absence of bipedality.
Stanford CB. 2006. Arboreal bipedalism in wild chimpanzees: Implications for the evolution of hominid posture and locomotion. Am J Phys Anthropol 129:225-231. Abstract