Nevertheless, we find the argument for a substantial component of arboreality in the locomotor behavior of A. afarensis more credible than that for exclusive terrestrial bipedalism. There also appears to be good reason to suppose the Hadar hominid fossils represent more than one species. Furthermore, we favor the view, espoused by Louis and Richard Leakey, that no australopithecine, including A. afarensis, warrants being labeled a human ancestor.
Just as today we find true humans coexisting with various categories of apes, some more humanlike than others, the same was true in the past, as far back as our research can carry us. In fact, an objective review of the evidence yields signs of anatomically modern human beings tens of millions of years ago, a fact distinctly incompatible with any current evolutionary model.
11.10 The Laetoli Footprints
The Laetoli site is located in northern Tanzania, about 30 miles south of Olduvai Gorge. Laetoli is the Masai word for red lily. The area was first explored by the Leakeys in 1935. Later, Mary Leakey returned to Laetoli and discovered some hominid jaws, which she regarded as early Homo.
One day in 1979, Dr. Andrew Hill of the Kenya National Museum and several other members of Mary Leakey’s expedition were playing around, throwing pieces of elephant dung at each other. In the course of this sport, Hill noticed some marks on the ground. They proved to be fossil footprints of animals. Subsequently, Peter Jones and Philip Leakey, the youngest son of Louis and Mary Leakey, discovered among the footprints some that appeared to have been made by hominids. The prints had been impressed in layers of volcanic ash, dated by Garniss Curtis, using the potassium-argon method, at from 3.6 to 3.8 million years old.
National Geographic magazine featured an article by Mary Leakey titled “Footprints in the Ashes of Time.” A caption to a photo of some hominid prints read: “The best-preserved print shows the raised arch, rounded heel, pronounced ball, and forward-pointing big toe necessary for walking erect. Pressures exerted along the foot attest to a striding gait” (M. Leakey 1979, p. 452). Dr. Louise Robbins, a footprint expert from the University of North Carolina, observed: “They looked so human, so modern, to be found in tuffs so old” (M. Leakey 1979, p. 452).
Readers who have accompanied us this far in our intellectual journey will have little difficulty in recognizing the Laetoli footprints as potential evidence for the presence of anatomically modern human beings over 3.6 million years ago in Africa. We were, however, somewhat astonished to encounter such a striking anomaly in the unexpected setting of the more recent annals of standard paleoanthropological research. What amazed us most was that scientists of worldwide reputation, the best in their profession, could look at these footprints, describe their humanlike features, and remain completely oblivious to the possibility that the creatures that made them might have been as humanlike as ourselves.
Their mental currents were running in the usual fixed channels. Mary Leakey (1979, p. 453) wrote: “at least 3,600,000 years ago, in Pliocene times, what I believe to be man’s direct ancestor walked fully upright with a bipedal, free-striding gait. . . . the form of his foot was exactly the same as ours.”
Who was the ancestor? Here we once more confront the debate, between the Leakeys on one hand and Johanson and White on the other, about the number and type of species represented by the fossil materials from Hadar and Laetoli.
Taking the Leakeys’ point of view, the Laetoli footprints would have been made by a nonaustralopithecine ancestor of Homo habilis. Taking the JohansonWhite point of view, the Laetoli footprints would have been made by Australopithecus afarensis. In either case, the creature who made the prints would have had an apelike head and other primitive features.
But why not a creature with fully modern feet and fully modern body? There is nothing in the footprints that rules this out. Furthermore, we have compiled in this book quite a bit of fossil evidence, some of it from Africa, that is consistent with the presence of anatomically modern human beings in the Early Pleistocene and the Late Pliocene.
The most prominent set of tracks at Laetoli represented the footprints of three hominids, one larger than the others. Applying an anthropological rule of thumb that a hominid’s foot length represents 15 percent of the creature’s height, Mary Leakey (1979, p. 453) calculated that the largest hominid stood 4 feet, 8 inches tall, whereas the next largest stood 4 feet tall. The smallest would have been still shorter. Leakey hypothesized that the largest individual was an adult male, the next largest an adult female, and the smallest a child. Admitting this was only a guess, she suggested the alternative possibility that the second largest set of prints might represent a juvenile male (M. Leakey 1979, p. 453). One cannot, however, be certain that the largest tracks represent a fully adult form either. Even so, the heights of the creatures that made the two larger sets of tracks, as estimated by Mary Leakey, fall within the modern human adult range.
Are we perhaps exaggerating the humanlike features of the Laetoli footprints? Let us see what various researchers have said. Louise M. Robbins, who provided an initial evaluation of the Laetoli prints to Mary Leakey in 1979, later published a more detailed report. Several sets of tracks, identified by letters, were found at Laetoli. In examining the “G” trails, representing the three individuals described by Mary Leakey as a possible family group, Robbins (1987, p. 501) found that the prints “share many features that are characteristic of the human foot structure.”
Robbins (1987, p. 501) noted: “Each hominid has a non-divergent great toe, or toe 1, and that toe is about twice as large as toe 2 beside it.” She found the spacing between toes 1 and 2 “no greater than one finds in many people today, including individuals who habitually wear shoes” (1987, p. 501). Robbins also found “the ball region of the hominids’ feet is of human form” and added that the feet displayed “a functionally stable longitudinal arch structure” (1987, p. 501). Finally, she observed that “the heel impressions in the hominids’ footprints appear human in their form and in their locomotory performance” (Robbins 1987, p. 501).
Robbins (1987, p. 501) therefore concluded that “the four functional regions—heel, arch, ball, and toes—of the hominids’ feet imprinted the ash in a typically human manner” and that “the hominids walked across the ash surface in characteristic human bipedal fashion.”
Concerning the size of the prints, Robbins (1987, p. 502) stated: “The assumed dimensions of the G-2 footprints do indeed fall well within the adult male range of a sample of American subjects, and the measurements of G-3’s footprints fall in the lower portion of the range for adult females in the American sample. The dimensions of the G-1 footprints, however, are well below dimensional ranges for American adults but within foot length and width ranges for a small sample of immature individuals. . . . Nonetheless, it is mere conjecture at this stage of hominid footprint investigation to suggest that the Site G hominids may have been a male, a female, and an offspring who were walking from an area of falling volcanic ash.”
M. H. Day studied the prints using photogrammetric methods. Photogrammetry is the science of obtaining exact measurements through the use of photography. Photogrammetric methods are extensively used by cartographers in making accurate contour maps from aerial photographs. Day (1985, p. 121), having found the same techniques useful on the miniature geography of footprints, stated: “What these footprints, and their photogrammetric analysis, show is that bipedalism of an apparently human kind was established 3.6 million years ago. The mechanism of weight and force transmission through the foot is extraordinarily close to that of modern man.” His study showed the prints had “close similarities with the anatomy of the feet of the modern human habitually unshod; arguably the normal human condition” (Day 1985, p. 121).