The Human Fossil Record: The Rise of Early Homo
Today's entry was written by James Kidder. Please note the views expressed here are those of the author, not necessarily of The BioLogos Foundation. You can read more about what BioLogos believes here.
Our Journey Thus Far
Thus far, we have journeyed from the forests of the late Miocene/Early Pliocene at 4 and half million years ago to the open savannah at a little over one million years ago. We have seen perhaps our first forebears, Ardipithecus ramidus in Northeast Africa, walk upright, albeit awkwardly at first—the first primate to do so. Although not human as we would know it, this step was the first of many and, coupled with a smile that contained human-sized teeth, it signaled the arrival of a different kind of primate. The form that followed, Australopithecus, showed even more changes. With the use of true bipedalism and the first evidence of tool use, this form underwent rapid adaptive radiation across the Northeast, central east and South African landscape, eventually reaching an estimated nine different species, some small and slight, others larger and more robust. From approximately 4 million years down to 2 million years ago, the australopithecines were unchallenged on the savannah. It was at this time that another form appeared on the landscape—early Homo.
With the transition from Australopithecus to early Homo came changes as great as those that had come before. In early Homo came a hominin with a larger, rounder cranium and evidence of clear stone tool production.
From Whence Came These Beings?
Unfortunately, the path from Australopithecus to early Homo is shrouded in mystery, with no clear hominin form considered decisively to be the progenitor. One reason for this mystery is that the first specimens of our line are extremely varied in morphology—so much so that there is considerable disagreement about how many species are present (Kramer, Donnelly, Kidder, Ousley, & Olah, 1995). Although there is much material comprising early Homo, many of the finds are incomplete, consisting of partial crania, jaws, sets of teeth or long bone fragments. In the early stages, there was little emphasis on constructing species names for the early Homo fossils that were coming out of the ground from the 1960s on (Richard E. Leakey, 2009). It was easier to just call them Homo. As I will show in detail below, the differences in size and shape of these early Homo forms has led researchers to divide them into three separate species, Homo habilis, the smallest and most primitive form, Homo rudolfensis, the primitive but larger form, and Homo ergaster, the largest of the three. While both H. habilis and H. rudolfensis had faces that tended to be similar to late australopithecines, these traits were gone by H. ergaster, which exhibited a much flatter, more modern face and much larger cranium.
All three species, Homo habilis, Homo rudolfensis and Homo ergaster, (Figure 1) appear in the fossil record between 2.3 to 1.8 million years ago (Prat et al., 2005). They, therefore, overlap with the late, robust australopithecines, which were very specialized towards diets of grass, nuts and berries.
One possible path is from Au. afarensis to Homo. Au. afarensis is very generalized for an australopith and does not show the specializations of the later robust forms in the large rear teeth, massive chewing complex or wide, flaring faces. There is, however, considerable time in between this form and the appearance of early Homo—on the order of one and a half million years. It has been suggested that Au. afarensis gave rise to both Au. aethiopicus which shows some of the same characteristics: the crest on the top of the head and very scooped face, and Au. africanus, which is also a small form. Supporters of this argument then posit that Au. africanus gave rise to early Homo.
Others argue that Au. africanus is too specialized to have given rise to any of the early Homo forms. Au. africanus possesses very strong facial features such as the large anterior pillars that show up in modified form in the south African Au. robustus but not in Homo. This is a vertical ridge of bone that extends up from the canine to the outside edge of the nasal cavity and is distinctive for these hominins. The other problem is that the earliest Homo shows up in East Africa and Au. africanus is only found in South Africa.
Berger (Berger et al., 2010) argues that Au. sediba possesses traits aligning it with Homo and that it may have given rise to one of the later forms. Its late date of 1.8-1.,9 million years excludes it from being the progenitor of either Homo habilis or Homo rudolfensis, however.
It has even been suggested that early Homo derives from Kenyanthropus platyops, which if you will remember from the last post, is a fossil find that dates to 3.5 million years ago and that appears to have a very flat face in comparison to the early australopithecines which is argued to be a more modern trait. (Cela-Conde & Ayala, 2003). This position is not well supported because the find was so badly crushed post-mortem and is so badly distorted that very little can be said about it taxonomically and it has been described by other researchers as an Australopithecus afarensis variant.
In 1961, Louis Leakey’s team discovered the parts of a skull, and some hand and foot bones at Olduvai Gorge. Two years earlier, workers had found the spectacular robust Australopithecus boisei skull, OH5, at this same location. These remains, he concluded were different from the known samples of Australopithecus. The brain size of this individual, OH 7, was 630 cubic centimeters (cc), fully a hundred ccs larger than the just discovered, Au. boisei specimen (L. Leakey, 1961). Not only that but in comparison to the australopithecines, the crania were more lightly built, rounder and slightly larger. The back teeth, which were very large in the robust australopithecines, were smaller and more closely approximated our own dentition. The face was, overall, slightly flatter although it still had the “scooped” appearance of the gracile australopithecines (see sidebar). One such skull was OH 24 (short for Olduvai Hominin 24) (Figure 2), which demonstrates the evolved morphology. This cranium was estimated to have a cranial capacity of slightly less than 600 cc, still far larger than any australopithecine species.
Leakey was so convinced that these represented a new, advanced species that he called it Homo (For an excellent review of the earliest finds, see (Schrenk, Kullmer, & Bromage, 2007). As more remains were discovered at Olduvai Gorge, in 1964, Homo habilis (handy man) was formally named. (L. S. B. Leakey, Tobias, & Napier, 1964). This was the first time that such a designation had been given to remains that were so manifestly primitive compared to modern humans and it was very controversial. For example, the cranial capacity was fully 900 cc less than the 1500 cc capacity of modern humans. Even now, fifty years later, controversy still swirls around this species, with some authors arguing that it belongs in the genus Australopithecus. Its primitive face and other skeletal features (for example the finger bones are curved as in Au. Aafricanus.) suggest that it is more like members of the australopithecines than it is like Homo.
In the late 1960s, Richard Leakey joined his father in the hominin adventure. Richard had come into palaeoanthropology somewhat reluctantly having rejected his father’s encouragement to follow in his footsteps. Instead, he had made a name for himself as a big game safari leader. As Don Johanson writes:
He proved to be extremely good at this. As a result, by the time he was in his early twenties he had learned a great deal about how to get about in rough country. It was that experience, and obvious talent for organization, which persuaded Louis that Richard should handle the Kenya end of the expedition, even though he had no training in anthropology or geology whatsoever. He knew quite a lot—how could he not, growing up with Louis and Mary Leakey as parents?—but his formal education had stopped in high school (Johanson & Edey, 1981)
After a few years working at Omo, in the early 1970s he took an expedition team to the Koobi Fora region of Lake Rudolf (now Lake Turkana, See Figure 3, a map of site locations in East Africa) and, in 1972, unearthed one of the most famous of all hominin fossils, the KNM-ER 1470 skull (Figure 4) (Day, Leakey, Walker, & Wood, 1975; R.E. Leakey, 1973). Although he initially dated the find to around 2.9 million years old by palaeomagnetism, discrepancies arose in the form of faunal correlations. The pigs that were found in the same level as ER 1470 were securely dated to around 1.8-1.9 million years at other sites. The level was subsequently re-dated by Potassium-Argon and found to be 1.9 million years old, making it coeval with the robust australopithecines of the region (Johanson & Edey, 1981). This has become the settled date of this fossil, relying not just on biostratigraphy (see sidebar) but radiometric dating.
The initial problem this skull created was that it presented very different morphology from any of the Olduvai specimens, having a much larger supraorbital region and larger, inflated cheeks. Originally subsumed within Homo habilis, the discoverers felt that the difference in size between the Olduvai hominins and the larger fossils from Koobi Fora could not be accommodated within one species. The new specimen had a rounder cranium, flatter face and cranial capacity between 700 and 750 cc., a full half again as much as the largest australopithecines and between 50 and 100 cc greater than the Olduvai Homo habilis sample.
It has been suggested by Walker and others (Strait, Grine, & Moniz, 1997; Walker, 1976) that the rush to call these remains Homo belies their generally australopithecine-looking faces, introducing an unnecessarily large amount of variation into the genus. They suggest that these forms should be called large-brained australopiths. However, cladistic analyses carried out by several workers involving both quantitative (skull measurements such as skull length, width and facial height) and qualitative traits (whether or not a particular trait was present) demonstrate that early Homo forms represent a group that is highly distinctive compared to all australopithecine species (Strait & Grine, 2004; Bernard A. Wood, 2009).
In addition to the large Homo crania found at East Rudolf, a smaller form was found that was similar in size to the Olduvai Homo sample with the same characteristics as the larger East Rudolf sample. One, in particular, KNM-ER 1813 (Figure 5), had a cranial capacity similar to that of the largest australopithecines, around 510 cc. It has been suggested alternately that the large size differences between the larger and smaller forms at this site represent sexual dimorphism (the difference in size between males and females of the same species) (Rightmire, 1993) or that they represent the presence of H. rudolfensis and H. habilis at the same location (Schrenk, et al., 2007).
The Oldowan Tool Tradition
As controversial as the taxonomy (see sidebar for definition) of early Homo is, there remains another vexing problem: who was the creator of the Oldowan tools that are found at many sites, dating to between 2.6 and 1.7 million years (Figure 6) As Roche et al. (Roche, Blumenschine, & Shea, 2009) note, based simply on chronostratigraphic context, these tools are found in indirect association with Australopithecus boisei, Australopithecus garhi, and all three species of early Homo. They consist of edge and end scrapers and are, by modern standards, crudely made. Arguments have been put forth by several different workers in support of both australopithecine and Homo manufacture. The argument for australopithecine manufacture is based on several assertions: The earliest evidence of stone tools is at the site of Gona, in East Africa at 2.3 million years ago (Prat et al., 2005) and the most prevalent hominin around at the time was Au. boisei. Additionally, stone tools are found in the same chronological level as Au. boisei (especially at Olduvai). Finally, Susman has argued that Au. boisei possessed a precision grip necessary for tool manufacture (Susman, 1991).
The argument that early Homo made the tools, on the other hand, rests not just on stratigraphic associations but also on the premise that increased brain size would confer greater cognitive thought, and, coupled with reduction in back teeth may reflect greater reliance on meat, the procurement of which would require tools like those found in the Oldowan assemblages.
All we know at the moment is that the stone tools were present and that they were made by a hominin species that had the cognitive thought level to construct them.
Next: The South African evidence and major changes for early Homo.
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James Kidder holds a Ph.D. in Biological Anthropology from the University of Tennessee (UT). He currently employed as an instructor at UT, and as a science research librarian at Oak Ridge National Laboratory. He has been involved in the Veritas Forum at UT and runs the blog "Science and Religion: A View from an Evolutionary Creationist/Theistic Evolutionist."