The Human Fossil Record, Part 8: Evolution in Early Homo
In the previous post, I detailed the arrival of early Homo on the landscape and the differences of these forms from contemporary australopithecine species. The australopithecines, while possessing bipedal locomotion and, perhaps, rudimentary tool use, were characterized by having small brains, largely ape-like faces, reduced stature and primitive characteristics reminiscent of their ape ancestry. The advancements of their successors, manifested in slightly increased cranial size, more flattened faces and demonstrated stone tool use were sufficient to cause most anthropologists to differentiate these hominins from the australopithecines, assigning them to the genus Homo.
There was, however, some controversy surrounding this decision and not all agreed (Strait, Grine, & Moniz, 1997; Walker, 1976). The general consensus, however, was that there were two species, Homo habilis and Homo rudolfensis present on the landscape between 2.2 and 1.8 million years ago (the above graph lists a third Homo species, ergaster, which we will discuss in greater detail later in this post).
So far, we have focused on early Homo remains in eastern Africa. Now we can turn our attention to the rest of the continent. In 1953, J.T. Robinson discovered skull fragments in an overhang at Swartkrans cave in South Africa, (the site of many australopithecine discoveries) that, when assembled (SK 847, Figure 2) some years later, demonstrated early Homo affinities. This find established the range of early Homo to at least 2500 miles, from East Africa down to the southern end of the continent. (Curnoe, 2010). It is thought that this cranium dates to between 1.8 and 1.5 million years ago but the fossil was not found on a habitation floor, the age remains uncertain.
In the 1970s, more work at the nearby cave site of Sterkfontein yielded a partial cranium that was also included within the genus Homo, although a firm designation was not given. It is now thought to be Homo habilis. This skull was found above a layer with Au. africanus remains, although it is not clear what relationship the two hominins may have had. Associated with the find were numerous Oldowan tools. This fossil was found on the surface of the cave and, based on associated fauna, it has been given a date range of between 2.0 and 1.5 million years ago. Sadly, at this time, these are the only specimens that can be attributed to early Homo in South Africa. This may reflect a limited migration of these forms to this area.
At least one investigator, Darren Curnoe, has examined the South African material and has argued that morphological differences exist between these finds and those from east Africa. In light of these differences, he has proposed a separate species, Homo gautengensis. This is based on the appearance of characteristics such as a mid-line keel (a ridge on the top of the head), a generally wider jaw, larger molars than those found in other early Homo remains, and a more rounded cranium. At present, however, this argument has received little support. Most researchers are content to label these forms Homo habilis.
If there was reason to doubt the Homo affiliation of the earliest finds, the same cannot be said of Homo ergaster, the third species of early Homo listed on Figure 1, which makes its appearance on the landscape at 1.8 million years ago in east Africa. This hominin species is quite distinct from other early members of Homo, with considerably expanded crania that had straighter walls, greater vertical height and the appearance of very large eyebrow ridges.
The first cache of these specimens were discovered at Koobi Fora near the shores of Kenya’s Lake Turkana (on the northern border of Kenya, see the map in this post) in the 1970s, the most notable of which were KNM-ER 3733 and KNM-ER 3883 (Figure 3).
The greatest change that Homo ergaster had attained over their precursors, however, was not known until the 1985 discovery, at Nariokotome, near Koobi Fora, of a twelve-year-old adolescent, who had the misfortune of falling into a swamp, only to be swallowed up and buried for over 1.5 million years. This find (Figure 4) is one of the most intact hominin fossils in existence, consisting of an individual that is 70% complete.
When we are born, our arm and leg bones are a mix of bone and cartilage, similar to what we have in our nose. As we grow, the cartilage is replaced by bone, which spreads from the middle to the ends of each bone, with the ends “fusing” just before adulthood. This is why it is very easy for us to break bones in childhood. Using the timing of these changes, we can determine how old an individual was at the time of death. In the Nariokotome skeleton, the ends of the bones of the arms and legs (long bones) were not yet fused, indicating that this individual had not reached adult status. The discoverers (Brown, Harris, Leakey, & Walker, 1985) estimated that his adult height would have been nearly six feet. This is significantly greater than the height of any known australopithecine or Homo habilis individual.
It also presents us with the first concrete example of a fossil hominin that has fully modern stature and proportions (Tattersall, 2007). While one can see traces of an arboreal past in australopithecines and even in the earliest Homo in the form of curved hand and toe digits and slight elongation of the arms, relative to the legs, (Rightmire, 1993) by the time of H. ergaster, such traces are gone.
At present, there are eight specimens of Homo ergaster as for which skull and jaw remains are present (Strait, et al., 1997). Many other specimens are represented by scattered teeth.
The Acheulean Tool Tradition
Just slightly after H. ergaster appeared on the landscape, a different style of stone tool technology, the Acheulean (Figure 5) also appears. This is based on the hand axe, which would serve as the technological staple for the next million years. These tools were more advanced than the Oldowan scrapers and are recognizable from site to site. Foley and Lahr (2003) suggest that these tools were developed during the tenure of H. ergaster and, at the beginning of this hominin’s appearance, are not very different from the late developed Oldowan tools. Several hundred thousand years following the appearance of H. ergaster, however, the Acheulean tool industry had become standardized. Rather than simple choppers, these tools demonstrate complex bifacial working and the process of “knapping” or using other rocks to create a finished tool.
The Rise of Humanity?
The presence of the Acheulean tools, the morphological changes and the increase in stature of Homo ergaster suggest that significant changes were occurring in the ways that hominins were utilizing the landscape around them and evolving in response to it. Based on analysis of the Developed Oldowan culture, Osvath and Gärdenfors (Osvath & Gärdenfors, 2005) argue that in H. ergaster, one can see evidence of anticipatory cognition, a trait only present in humans:
…we identify the niche as consisting of stone tool manufacture, of transports over long ranges of tools as well as food and of the use of accumulation spots. Our main argument is that this niche promoted the selection for anticipatory cognition, in particular planning for future goals. Once established, anticipatory cognition opened up for further cultural developments, such as long ranging migration, division of labour, and advanced co-operation and communication, all of which one finds evidence for in Homo ergaster/erectus.
Similarly (Isaac & Isaac, 1977), in a study of the African site of Olorgesaille, discovered hundreds of Acheulean tools associated with large animals, many of which had cut marks on them. Such evidence has also been found at Acheulean sites in the Middle Awash River Valley. Further, there is evidence that there was transport of raw materials from site to site, sometimes in excess of 100 kilometers. Wang and colleagues suggest that there was increased selection for longer legged individuals partly as a result of increased need for this longer transport (Wang et al., 2004). Given that there is a clear increase in brain size for these specimens over their Homo habilis predecessors, it is tempting to suggest that there was positive selection for both an increase in brain size and body size for Homo ergaster. (see side bar for more detail)(Aiello & Wells, 2002) and, as a result, hunting may have begun to play a role in subsistence. An example of a study which explores this is the paper by (Bramble & Lieberman, 2004) which examines the success of humans at endurance running. Since that success can be correlated with certain identifiable skeletal features, it appears that hominins became capable of endurance running about 2 million years ago. (see side bar for abstract from their paper ) This kind of subsistence pattern is not very different from modern-day African Bushman populations and suggests strongly that these hominins were acting in ways that were noticeably human.
Next: knocking on the gates of Europe.
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Bramble, D. M., & Lieberman, D. E. (2004). Endurance running and the evolution of Homo. [10.1038/nature03052]. Nature, 432(7015), 345-352.
Brown, F., Harris, J., Leakey, R., & Walker, A. (1985). Early Homo erectus skeleton from west Lake Turkana, Kenya. Nature, 316(6031), 788-792.
Curnoe, D. (2010). A review of early Homo in southern Africa focusing on cranial, mandibular and dental remains, with the description of a new species (Homo gautengensis sp. nov.). HOMO - Journal of Comparative Human Biology, 61(3), 151-177.
Foley, R., & Lahr, M. M. (2003). On stony ground: Lithic technology, human evolution, and the emergence of culture. Evolutionary Anthropology: Issues, News, and Reviews, 12(3), 109-122.
Isaac, G. L., & Isaac, B. (1977). Olorgesailie: archeological studies of a middle Pleistocene lake basin in Kenya: University of Chicago Press.
Osvath, M., & Gärdenfors, P. (2005). Oldowan culture and the evolution of anticipatory cognition. Lund University Cognitive Studies, 122, 1-16.
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Tattersall, I. (2007). Homo ergaster and Its Contemporaries. In W. Henke & I. Tattersall (Eds.), Handbook of Paleoanthropology (pp. 1633-1653): Springer Berlin Heidelberg.
Walker, A. (1976). Remains attributable to Australopithecus in the East Rudolf succession. In F. C. Howell, Y. Coppens, G. L. Isaac & R. E. F. Leakey (Eds.), Earliest Man and Environments in the Lake Rudolf Basin (pp. 484–489). Chicago: University of Chicago Press.
Wang, W., Crompton, R. H., Carey, T. S., Günther, M. M., Li, Y., Savage, R., et al. (2004). Comparison of inverse-dynamics musculo-skeletal models of AL 288-1 Australopithecus afarensis and KNM-WT 15000 Homo ergaster to modern humans, with implications for the evolution of bipedalism. Journal of human evolution, 47(6), 453-478.