The Human Fossil Record, Part 5: The Dispersal of the Australopithecines
As we learned in the previous post, up until approximately three million years ago, australopithecines were restricted in variation to Australopithecus afarensis, the successor to Australopithecus anamensis. This hominin has been found in the north at Hadar, Ethiopia, and as far south as Tanzania. Subsequent to this time period, however, the australopithecines as a genus underwent a dramatic expansion and, eventually, would be found in all of eastern and possibly central Africa.
The Lack of Acceptance of Australopithecus and the Piltdown Forgery
Raymond Dart, discoverer of the first australopithecine, the Taung child skull, met with lukewarm to tepid response when he described his find in the journal Nature. One of the reasons for this is that, to the early 20th century eye, it looked very ape-like, and it was hard for many to grasp that there was any connection between it and the forms that followed. The other reason is that the path to humanity was thought to already exist elsewhere in the form of Eoanthropus dawsoni, the fossil remains from the Piltdown Commons, in England.
The Piltdown forgery ranks as one of the best scientific hoaxes of all time. Charles Dawson unearthed purported hominin fossil remains from a gravel pit at Piltdown Commons, East Sussex County, in 1912 that consisted of a mostly complete skull and partial jaw in association with extinct mastodon and hippopotamus fossils (See Figure 1). This was published in the Quarterly Journal of the Geological Society (Dawson and Woodward 1913) and became the crown of English anthropology, eliciting the support of most of the top anthropologists and anatomists of the day, including Sir Arthur Keith, Sir Arthur Smith-Woodward, Grafton Eliot Smith and William King Gregory.
The find, which was dated biostratigraphically to the Middle-Pleistocene, showed that evolution of the braincase preceded evolution of the rest of the head and jaw. Consequently, when Dart’s australopithecine find was described with its human-sized teeth and small braincase, it didn’t fit the pattern established by Piltdown and was, thus, denigrated by the researchers in the field. However, as more human remains were found in the 1930s and 1940s that resembled Dart’s find, Piltdown’s uniqueness became peculiar. Further, nothing else emerged from England itself that resembled Piltdown. As researchers around the globe began to assemble their human origins charts and timelines, Piltdown became increasingly hard to accommodate within any evolutionary framework.
In the early 1950s, Kenneth Oakley and Josef Weiner secured the rights to examine the remains using a new relative dating method that had been recently been calibrated, fluorine analysis. The basis behind this technique is simple: as organic material sits in the ground, it soaks up fluorine. The longer it is there, the more fluorine it soaks up. In this way, some fossils could be said to be older than others and rough comparisons could be made. Oakley’s analysis initially suggested that the find was much more recent than originally thought (Oakley and Hoskins 1950) This sowed the first seeds of doubt about the find. Eventually, a more detailed analysis was undertaken by Weiner, Oakley and Wilford Le Gros Clark, resulting in the publication “The Solution of the Piltdown Problem” (Weiner, Oakley and Le Gros Clark 1953). They discovered that the teeth had been filed down to make them look old, the jaw and isolated teeth were not the same age as the cranium but were, rather, modern in age, and that the remains had been stained to give them an old appearance. These findings led to a public outcry and the whole house of cards came down. Subsequent analyses revealed that the jaw was, in fact, that of an orangutan, the mastodon and hippo remains had metal knife marks, and the fossil mastodon remains were from Tunisia.
To this day, the identity of the Piltdown forger remains unknown. The weight of suspicion has fallen, in recent years, on the original discoverer, Charles Dawson, who likely had the means to carry out the prank. Sadly, Dawson contracted septicemia and passed away in 1916, possibly carrying this information with him to his grave. One of the best accounts of this forgery is by John Walsh, called Unraveling Piltdown (Walsh 1996). Perhaps one of the most important lessons from the Piltdown experience is that it demonstrates a classic example of the self-corrective capacity of science. Once the weight of Piltdown had been removed from the field, the australopithecine discoveries in Africa began to make sense in the context of the larger picture of human origins.
Australopithecine systematics is, like that of the earliest birds, a confused business, with many different competing hypotheses about where each species fits in the grand scheme of things (See Figure 2 below). To top this off, it is not clear from which of these lines Homo emerged.
Based on the best current weight of evidence, it is now commonly thought that between two and three lineages of australopithecines emerged from the line that came from A. afarensis. It is not possible to infer direct ancestor-descendent relationships between these forms. All that we can surmise at the moment is that at around 3.0 million years ago, A. afarensis was the only hominin on the landscape and at 2.5 million years ago, there were multiple forms, spanning the geographical distance between Ethiopia and South Africa with likely remains in Chad.
The East African Forms
Australopithecine species tend to be found in two general forms: a gracile, or smaller and lightly built, form and a robust, or larger and more heavily built, form. While some have argued that the robust australopithecines belong in a separate genus, Paranthropus (Grine 2007), this is a minority position within the field. For our purposes, they will be subsumed within the genus Australopithecus.
At around 2.5 million years ago, a new form was found in Ethiopia that appeared to be a scaled up version of A. afarensis. Called Australopithecus aethiopicus, this species had a long, low cranium with flared cheekbones and attachment areas for very powerful chewing muscles on its face (See Figure 3). To go with this, A. aethiopicus also had, on the top of its head, a large sagittal crest. Work with modern-day gorillas, which also have this feature, has shown that this is not a genetic trait but appears as a result of bone deposition on the top of the head through continuous grinding of nut and plant substances. Unlike the gorilla version, however, which is focused directly on the top of the cranium (vertex), the A. aethiopicus manifestation is toward the back of the cranium. This form had an average cranial capacity of around 410 cubic centimeters. For comparison purposes, the average modern human cranium is approximately 1450 cubic centimeters in capacity while chimpanzees average 375 cubic centimeters.
Also from this time period is a form similar to A. afarensis in brain size and gracility, Australopithecus garhi (“surprise” in the Afar language). The teeth are slightly larger than those of afarensis (See Figure 4). In overall body shape, however A. garhi was far more modern, with the ratio of upper arm length to upper leg length much closer to those of later hominins (Asfaw et al. 1999).
Slightly later in time, from around 2.2 million years ago, came Australopithecus boisei. This form was originally found in 1959 by Mary Leakey, in the Olduvai Gorge area near the Serengeti Plains, in Tanzania, where she and her husband, Louis, had been digging since 1951. This is known as a hyper-robust australopithecine. Comparisons with A. aethiopicus strongly suggest that the facial architecture of A. boisei is very similar, yet larger in size. This has led researchers to surmise that there is an ancestor/descendent link between the two species. A. boisei has a slightly larger cranial capacity of 510 cubic centimeters but otherwise retains all of the characteristics of A. aethiopicus, including the sagittal crest, the scooped-out facial appearance and wide, flaring cheek bones (see Figure 5). It has been hypothesized that these forms subsisted primarily on a vegetative diet consisting of hard nuts, roots and berries because the rear teeth were much larger than the front ones and considerably larger than those of A. afarensis. This eventually led to the nickname “nutcracker man.” This hypothesis has recently been challenged, however, by research that suggests that the primary diet of this hominin was grasses (Cerling et al. 2011). A. Boisei is found down to approximately 1.2 million years ago.
In our next post we will look at the South African forms, as well as the extinction and importance of Australopithecus.