The Human Fossil Record, Part 4: Australopithecus Conquers the Landscape
In my previous post, I described the discovery of the first Australopithecus in South Africa by Raymond Dart. Beginning with the work of Dart and venerable palaeontologist, Robert Broom, an extensive range of discoveries has been made that continues to the present day.
The earliest known species from the genus that Dart discovered is known as Australopithecus anamensis. Its remains are, sadly, still quite fragmentary. Working at the sites of Kanapoi and Allia Bay in Lake Turkana, in Kenya, Richard Leakey and others unearthed the partial remains of a number of individuals, which are securely dated to between 4.0 and 4.2 million years ago. By comparison, Ardipithecus ramidus (see Figure 1) is dated to around 4.4 million years. More recently 30 specimens of Australopithicus anamensis from at least 8 individuals have been found in Ethiopia and they have been dated at a similar age (see here for an analysis of the rock strata and age of the formations in which the specimens were found.)
Figure 1 Australopithecus in Perspective (from Science Magazine)
Editor's Note: The above diagram is intended as a means of comparison between Australopithecus and its close genetic relatives. It is NOT intended to present a strictly linear progression or imply that gorillas and chimpanzees evolved into present homo sapiens.
Currently, the remains attributed to Australopithicus anamensis consist of several jawbones, some lower faces, more than 50 isolated teeth, skull fragments, several sections of lower arm and a section of a tibia, just below the knee (See Figures 2 and 3). Nonetheless, it is possible, for reasons discussed below, to determine a great deal about how this species moved around, the kinds of things that it ate and, critically, how it differed from its predecessor, Ardipithecus, and the forms that would come after it.
It has been determined that this species inhabited sparse woodland, river bank environments and open grassland, a greater range than that in which Ardipithecus lived. This is based on the similarities between the deposits from Kanapoi, Allia Bay and those of Australopithecus afarensis, the hominin that follows Australopithecus anamensis, temporally. In addition there has been an extensive analysis of accompanying fossil species found at the Ethopian site where specimens from eight A. anamensis individuals have been found.
In addition to the increase in range, certain elements of the skeleton are present that show a clear trend toward the forms that would come later. These include significant changes in how the teeth and palate are arranged, the shape of the teeth (Figure 2), and the morphology of the tibia (Figure 3).
Figure 2 (above, left). Lower Jaw bone of two specimens of Australopithecus anamensis. Figure 3 (below, right). Different views of the same tibia bone from Australopithecus anamensis.
In Ardipithecus, the dentition was very similar to that of modern chimpanzees, with the exception of the size of the canine, which was shortened. The teeth were geared toward a fruit and leaf diet, with occasional meat thrown in, such as you would find in a forest environment.
In Australopithecus anamensis, however, the teeth had much thicker enamel, they were larger and had flatter grinding surfaces that would have been more suited to nuts and other hard foods. This suggests that this hominin would have been well-adapted for life in open grassland and savannah.
The orientation of the tibial shaft indicates that it was positioned directly up and down in relation to the foot and the femur, suggesting that the individual walked completely upright. The length of the radius fragment and its comparison to the tibial fragment further attests that this individual had arms that were elongated, like Ardipithecus. It seems likely, therefore, that like Ardipithecus, A.anamensis spent quite a bit of time off the ground.
It is tempting to speculate about the cognitive abilities of Australopithecus anamensis relative to Ardipithecus ramidus. We have no evidence that Ardipithecus existed outside the forest environment. Based on the taphonomic evidence, we strongly suspect that A. anamensis existed in the fringe and savannah. We do know that the vast majority of modern primates have home ranges that are restricted to one kind of biome. For example, Orang-utans only live in the forests of Borneo and Sumatra while Spider and Howler monkeys only exploit the forest canopies of Central America. While these examples certainly reflect a stable evolutionary response to particular environments, their inability to move beyond these environments and the need for conservation efforts to preserve them reflect a level of cognitive ability that appears to be restricted. If A. anamensis could survive in both the forest/fringe and savanna environments, it suggests an increase in cognitive abilities for this species. More evidence will be necessary to lend credence to this hypothesis.
In 1973, working with a local team of fossil hunters, Maurice Taieb went to an arid stretch of land in the Afar Triangle of Ethiopia to an area called Hadar. A year later, Don Johanson, a member of his team discovered one of the most famous of all fossil hominin discoveries ever made. Exploring 3.4 and 3.6 million years old deposits, he discovered the fragmentary remains that constituted 40% of the skeleton of a small adult female (Figure 4) . This individual was nicknamed “Lucy” after the Beatles' song "Lucy in the Sky with Diamonds" which played in the camp during the analysis of the remains. The team named the species she represented Australopithecus afarensis, because she had been found in the Afar triangle. Johanson, together with M. Edey, went on to pen the New York Times Bestseller Lucy: The Beginnings of Humankind. This book catapulted Lucy onto the national stage and fueled research into the biological origins of humankind (Johnanson and Edey 1980).
When Lucy was examined, it was found that the shape and position of the the teeth and jaw as well as the hip and long bone fragments put her almost perfectly intermediate be tween the ape position and the human position. Although she had the overall size and rib cage structure of a chimpanzee, her pelvis and leg bones were perfectly adapted for bipedalism (for a discussion on bipedalism, see this post and this video.) It was in the teeth and palate that the clearest transitional characteristics existed (Figure 5, right). In modern humans (5c), the dental arcade (tooth row) is in the shape of a parabola, like the Gateway Arch in Saint Louis. In apes (5a), it is a sharp “U” shape. In A. afarensis (5b), it is intermediate, tending toward the ape condition.
In apes, there is a space (diastema) between the canine and the second incisor (bicuspid, if you prefer) to allow room for the long lower canine when the ape closes its mouth. In A. afarensis, the canine is human-sized and the diastema, while still present, is smaller. In apes, the first premolar is rotated relative to the tooth row and has a very high cusp so that it creates a sharpening surface for the opposite canine when the two teeth come together. In Lucy, the cusp is somewhat lower and the premolar is only slightly rotated. In humans, the cusp does not extend above the tooth row and there is no rotation at all.
The case for habitual bipedalism received added support from a site much further south than where Lucy had been found. In 1976, Andrew Hill, a digger in Mary Leakey’s team, working at the site of Laetoli, in Tanzania, unearthed a set of hominin tracks that had been covered by a now extinct volcano. These footprints, which extend approximately 80 feet across the plain, have been securely dated at 3.6 million years and show where two individuals walked side-by-side (Figure 6). The tracks are significant in that they demonstrate that the individuals who made them had arches and practiced the characteristic “toe-off” pattern of gait practiced only by hominins. The presence of A. afarensis skeletal remains nearby at the same level provided the link to the footprints.
By the time A. afarensis appears in the lineage all clear evidence of spending time in the trees was gone. On the foot, the arch had become prominent and the big toe, which had been slightly off-set in Ardipithecus and possibly Australopithecus anamensis, was firmly in-line with the other toes. The presence of the arch allowed for easy toe-off locomotion and would have been disadvantageous in climbing trees because it contributes to the rigidity of the foot. This was a species that was not just optimized for bipedality, it had become its only form of locomotion Furthermore, as noted, taphonomic evidence indicates that this species could exist in the forest and fringe environments. Its presence in both the Afar triangle and near Lake Turkana (nee Lake Rudolf) further suggests that it had a range of several thousand miles (Figure 7, left).
Kimbel and others have suggested that the similarities in traits between A. anamensis and A. afarensis represent an ancestor-descendant relationship, with A. afarensis representing the more advanced stage in hominin evolution(Kimbel et al. 2006).
Stone Tools: A Cognitive Shift
With Australopithecus afarensis came another striking evolutionary development. While A. anamensis and A. afarensis have been shown to have adapted to the forest/fringe and savannah environments, it is with A. afarensis that we have the first evidence of behavior that is directly cognitive in nature: the use of stone tools. At the site of Dikika, in Ethiopia, very near the site of recovery of an almost complete A. afarensis skeleton, animal bones were discovered dated to at least 3.39 million years ago that demonstrate distinctive signs of human action: cut marks. As McPherron et al. write:
The cut marks demonstrate hominin use of sharp-edged stone to remove flesh from the femur and rib. The location and density of the marks on the femur indicate that flesh was rather widely spread on the surface, although it is possible that there could have been isolated patches of flesh. The percussion marks on the femur demonstrate hominin use of a blunt stone to strike the bone, probably to gain access to the marrow. (McPherron et al. 2010) (For video summary of discovery, see here)
These authors are quick to point out that there is no way to determine whether or not these marks were made by tools that were modified for this purpose or made with the first available sharp rock, although the authors note that rocks found in association with the bones had been transported over six kilometers from their original location. This reflects advanced behavior by these hominins, though, including the first consumption of meat known in the fossil record.
Figure 8. Cut Marks on 3.35 Million year old Bovid Femur (from Nature)
With Australopithecus afarensis, however, a new hominin was on the landscape—a hominin that could adapt to new environments and, to a limited degree, adjust the environment around them to meet their needs. Without the adaptations necessary to remain in the trees for any length of time, and with the ability to balance and walk in a truly human fashion, the fact that they no longer needed to use the hands for support or grasping, freed them up for other uses. As A. afarensis moved about the landscape, these uses became evident: they modified what they had at their disposal.
It is unfortunate that we do not know whether these hominins created the tools to make these marks or used what was available to them. If they brought stones with them to butcher animals, though, it means that they were the first hominins that did more than react to their environment; they modified it to their uses.
Next, the diversification and extinction of the australopithecines and the rise of the genus Homo.
1. Johnanson, D. & M. Edey. 1980. Lucy: The beginning of humankind. New York: Simon & Schuster.
2. Kimbel, W., C. Lockwood, C. Ward, M. Leakey, Y. Rak & D. Johanson (2006) Was Australopithecus anamensis ancestral to A. afarensis? A case of anagenesis in the hominin fossil record. Journal of human evolution, 51, 134-152.
3. McPherron, S. P., Z. Alemseged, C. W. Marean, J. G. Wynn, D. Reed, D. Geraads, R. Bobe & H. A. Bearat (2010) Evidence for stone-tool-assisted consumption of animal tissues before 3.39 million years ago at Dikika, Ethiopia. Nature, 466, 857-860.
4. Ward, C., M. Leakey & A. Walker (1999) The new hominid species Australopithecus anamensis. Evolutionary Anthropology: Issues, News, and Reviews, 7, 197-205.