Free Masters of the Planet by Ian Tattersall

we must always remember that this is not something we can observe directly in the bones. Rather, we have to infer it from Lucy’s anatomical structure. A footprint, however, is different, in that it is truly fossilized behavior. And the trackways at Laetoli are as eloquent of bipedality as it’s possible to get. At one site an arrow-straight double trail of prints some 80 feet long, more or less like those anyone might leave walking along a wet beach, attests clearly to a purposeful bipedal gait. What is unusual is that the Laetoli environment at the time these prints were made was quite open; the hominids were slogging across a flat plain largely devoid of trees, and they must have felt pretty vulnerable as they did so. But they were heading directly for the Olduvai Basin, only a few miles away, which at that time would have offered all the hospitable resources of a forest surrounding a shallow lake.
    The footprints themselves are clear evidence of bipedality: there is no indication that the hominids steadied themselves using their forelimbs, and the way in which weight was transmitted from one end of each print to the other seems to reflect the way we walk—which is to say, it went from the heel, along the side of the foot and across the ball, with a final thrust concentrated on the big toe. This was not the lurching gait of a bipedal bonobo. The feet that made the prints were structured essentially like ours, with longitudinal and transverse arches and a short big toe set in line with the others. The short distances between successive footfalls suggests that even the bigger individual was of fairly diminutive stature, although it seems the pair was not moving very fast—hardly surprising, given the slushy surface across which they were making their way.
    While there is nothing to cast doubt on the bipedality of these 3.6-million-year-old hominids, there has been some debate about the exact gait they employed. Did they, for example, fully extend the knee with each step? Or did they retain some vestige of the bent-kneed gait that today’s apes use when moving upright, and which, at some remove in time, the hominid ancestor must also have employed? A recent experimental study, using human subjects moving both straight- and bent-kneed, has confirmed that if you don’t fully extend your knee, the impressions your toes make in wet sand are deeper than those made by your heel. And the Laetoli prints clearly show heel and toe depressions that are about the same depth, arguing for a straightened knee. Clearly, in these footprints we have evidence of a serious biped.
    The scientists who carried out the experimental work suggest that adopting upright locomotion on the ground allowed the Laetoli hominids to increase their ranging distances without expending extra energy, during a period when the forest was diminishing. Indeed, it’s very unlikely that any hominid could have made a decent living in the rather barren ancient environment adjacent to the trackways, making it all the more plausible that the prints in the wet ash had caught them in the act of aiming straight for the forests that lined the nearby Olduvai Basin.
    Just who those bipeds were is another matter. Not far from the footprint tracks at Laetoli are rocks of about the same age that yielded the handful of hominid fossils already mentioned. In an unusual collaboration, the scientists who initially studied the Hadar and Laetoli specimens eventually decided that they were all from the same new sort of hominid. This new species,
Australopithecus afarensis,
was named for the Afar region of Ethiopia in which Hadar is situated, and from which most of the fossils in question came. But under standard zoological procedure, every new species has to be based on a “holotype,” a single specimen to which every other individual assigned to that species has to be compared. And to emphasize their conviction of unity, the scientists chose a lower jaw