The robotic Lucy model

The BBC is running this article about a new study that evaluates the bipedality of A. afarensis using robotic design software:

Now, a team of scientists from around the UK have used computer robotic techniques to work out the most energy efficient gait for afarensis based on Lucy's skeleton and the Laetoli footprint trails.
They claim to have cleared up the debate by finding that, based on their model, Lucy almost certainly did walk tall.
There has been a long-standing debate about how human Lucy was
"Assuming that the early human relative Australopithecus afarensis was the maker of the Laetoli footprint trails, our study suggests that by 3.5 million years ago at least some of our early relatives - despite their small stature - could sustain efficient bipedal walking at absolute speeds within the range shown by modern humans," co-author Weijie Wang, from Dundee University, told the Scotsman newspaper.

So what we seem to have here is a computer software equivalent of early 1980's science. Perhaps they programmed Owen Lovejoy?

The paper (abstract) is a little more interesting than the BBC description. It does a kind of optimization modeling to find the speed and style of locomotion with the lowest energy cost. That lowest-cost speed was associated with a human-like gait at around 1 meter per second (m/s). Here is the logic:

Rather than trying to interpret the behaviour of such species by a combination of analogies to humans in certain anatomical regions with analogies to other apes elsewhere, it seems sensible to adopt a reverse-engineering approach and determine what kind of locomotion a particular set of body proportions were best 'designed' to perform. Since the locomotor system is concerned primarily with the application of external force by the body, simulation techniques drawn from mechanical engineering are a potential means of predicting the significance of differences in proportions for the motion and force characteristics of bipedal locomotion (Sellers et al. 2005: 2).

The authors relate their gait findings to the preserved Laetoli footprint trails, and find something very interesting. The trails have footprints that are very close together -- especially for the larger, possibly dual G2 trail. This has previously been interpreted as meaning that the walking speed of the larger individual who made this trail was very slow (Alexander 1984; Charteris et al. 1984) -- only around 0.7 - 0.8 m/s. The model in this study predicts a faster speed of around 1 m/s, which would be close to the optimal walking speed estimated for Lucy's proportions.

They do not address a relevant question, which is whether the larger trail was made by an individual larger than Lucy, who might have had a different optimal walking speed. Indeed, there is a basic assumption of monomorphism. It is partly covered by the fact that living people don't exceed 1.0 m/s for their average walking speed by very much, so the variation between large and small A. afarensis individuals may have been slight. The basic finding seems to be that shorter legs have an optimum gait that involves more, slightly shorter, steps, while longer legs take fewer steps more slowly. This isn't surprising based on a pendular model: shorter pendula have shorter periods, and longer steps with a shorter leg must require more energy-wasting up-and-down motion.

An important weakness of the model is that it considers costs only due to motion in two dimensions: forward and up-and-down. The wide pelvis of A. afarensis might be expected to exert greater costs in a side-to-side dimension compared to recent humans, and that energy effect is not considered.

But the bottom line:

Thus, within the limits of our model, and assuming that Taylor and Rowntree's (1973) data are reliable, the bipedal performance of Australopithecus afarensis, as predicted by our model is not only much closer to that of modern humans than to that of bipedally walking great apes, but at normal walking speeds, shows a clear speed/cost advantage over chimpanzee quadrupedalism. Climbing remains significantly more energetically expensive than terrestrial quadrupedalism for chimpanzees, despite their musculoskeletal adaptations (Pontzer and Wrangham 2004). Pontzer and Wrangham (2004) have shown that the costs of locomotion in chimpanzees are nevertheless dominated by terrestrial walking, because of very high daily travel distances, versus only limited use of climbing. If we make the major (and quite likely incorrect) assumption that the African apes existing at the time of A. afarensis were ecologically, morphologically and physiologically similar to modern common chimpanzees, then our data would tend to support Rodman and McHenry's (1980) argument that the adoption of bipedalism offered energetic advantages to early human ancestors (ibid., 9).

In any event, it won't quiet doubters:

However, Professor [Christopher] Stringer believes the controversy will not vanish overnight.
"There are still some people who argue that, looking at the anatomy of the foot bones of afarensis, that they were unlikely to have made the Laetoli footprints," he told the BBC News website.
"So it doesn't end the argument because there is still the possibility that there were different creatures around at the time."

No doubt soon, Kent State will have a droid afarensis army to finally crush this dissent and bring order to the galaxy.

References:

Sellers WI, Cain GM, Wang W, and Crompton RH. 2005. Stride lengths, speed and energy costs in walking of Australopithecus afarensis: using evolutionary robotics to predict locomotion of early human ancestors. Roy Soc Interface Online