Evolution Basics: From Primate to Human, Part 2
Note: This series of posts is intended as a basic introduction to the science of evolution for non-specialists. You can see the introduction to this series here. In this post we discuss early discoveries in hominin paleontology and their reception in the scientific community.
In the last post in this series, we examined species known from the fossil record that are near the last common ancestral population we share with our closest living relatives (chimpanzees). As we have discussed, species more closely related to us than to chimpanzees are known as hominins. As we will see, the lineage from which humans emerged is the sole remaining branch of what was once a diverse group of related species.
The paleontology of this group is interesting not only for its scientific aspects, but also for the history surrounding it. The intense interest surrounding our evolutionary relatives, preconceived ideas about how and where the transition from ape to human came about, and human pride and deception would all play a significant role in shaping the discovery and interpretation of the first hominin fossils.
Seeking the “missing link”
When Darwin published On the Origin of Species in 1859, and later The Descent of Man in 1871, scientific knowledge of the hominin fossil record was nearly non-existent. While the first Neanderthal bones had been discovered by the 1850s, an understanding of what these finds (and subsequent Neanderthal discoveries in the 1880s) represented would be decades in coming. Following on from Darwin’s work there was great interest in identifying transitional forms between humans and living apes. Neanderthals, however, were close enough in form to modern humans for scientists to doubt that they were a distinct species. What was expected, and sought, were hypothesized forms commonly viewed as “missing links” in a ladder-like progression from ape to human. The appreciation of the hominin group as a bushy tree rather than a ladder would elude scientists for some time, to say nothing of the general public.
It was in this context that Eugene Dubois would set out for Indonesia in the late 1880s to search for fossils linking humans and apes. (Despite Darwin concluding from the available evidence that human origins would be found in Africa, the early, largely preconceived consensus was that humans had originated in Asia.) Dubois was the first scientist to deliberately look for such fossils, and through great effort and personal hardship he was successful. Dubois discovered the first fossils of what would later be known as Homo erectus in Indonesia: a fossil he provocatively named Pithecanthropus erectus (“the ape-man that stands upright”). Unfortunately for Dubois, his finds were fragmentary – a few teeth, the cap of the skull, and a femur – and generally not well received by the scientific community of his day. Many suspected – given the small brain size of his specimen and the human-like leg bones – that Dubois’ “ape man” was nothing more than the coincidental discovery of human remains mixed with that of an extinct ape. This skepticism was fueled in part by the assumption at the time that it was brain evolution that had set humans apart from apes. The expected “missing link”, therefore, was expected to have an ape-like body and a more human-sized skull. It was not until decades later that Dubois’ interpretation of Pithecanthropus – that it indeed was a species with a human-like body and small ape-like brain – would be vindicated.
Dubois’ views would face a further setback in 1912, when a fossil discovery conveniently matching the expected set of intermediate features would be discovered: the infamous Piltdown Man. This “find” would later be shown to be a forgery constructed from a crushed modern human skull with the jaw of an orangutan, complete with teeth filed down to match expectations of the time for an intermediate between humans and living apes. As before, critics did raise doubts that perhaps the fossil was an amalgamation of an extinct ape jaw and a human skull, but these criticisms were largely silenced a few years later when a second skull matching the first was also “found” at a nearby site. Whereas chance might place the remains of two distinct creatures together once, it was highly unlikely it would do so twice – and so Piltdown gained acceptance for a time.
It would take considerable work to dethrone Piltdown as the “missing link” between humans and apes, but ultimately suspicion would grow to the point where it was exposed as a fraud. That work was the ongoing discovery of legitimate hominin remains – discoveries that increasingly singled out Piltdown Man as an oddity that did not fit within the growing picture of hominin evolution.
Enter the Australopithecines
One such discovery that would eventually contribute to exposing Piltdown was a second hominin species that combined human-like traits with a sub-human braincase volume. In 1924, anatomist Raymond Dart [http://en.wikipedia.org/wiki/Raymond_Dart] would examine fossils from Tuang, South Africa and discover a juvenile hominin with a braincase volume too large for an ape, but below that even of Homo erectus, precluding it from being human. As Dart would later recount in his memoirs,
I knew at a glance that what lay in my hands was no ordinary anthropoidal brain. Here in lime-consolidated sand was the replica of a brain three times as large as that of a baboon and considerably bigger than that of an adult chimpanzee. The startling image of the convolutions and furrows of the brain and the blood vessels of the skull were plainly visible.
It was not big enough for primitive man, but even for an ape it was a big bulging brain and, most important, the forebrain was so big and had grown so far backward that it completely covered the hindbrain.
Dart published his work in 1925 in the influential journal Nature under with the title “Australopithecus africanus: The Man-Ape of South Africa”. Like Dubois before him, however, Dart’s seminal discovery would meet a cold reception – including the opposition of prominent anthropologists convinced not only that human evolution was a brain-first affair, but also that human origins were in Asia rather than Africa. These critics noted that unless an adult member of the species could be found, that the juvenile Dart had described was likely to be a mere ape – and not a species intermediate between apes and humans. Once again, the shadow of Piltdown Man – with its “correct” ape-like teeth and human-like braincase – hung over this new discovery with its incongruous mix of human-like teeth and a sub-human braincase volume. Like Dubois before him, Dart would have to await further discoveries for the tide of scientific opinion to turn.
In the next post in this series, we’ll explore how continued work in hominin paleontology provided a clearer picture of hominin evolution and eventually exposed Piltdown Man as a fraud.
For further reading
- Brain, C.K. (2003). Raymond Dart and our African origins. In A Century of Nature: Twenty-One Discoveries that Changed Science and the World. (Garwin, L. and Tim Lincoln, T., eds). University of Chicago Press.
- Fossil Hominids, Human Evolution: Thomas Huxley & Eugene Dubois.
- James Kidder: The Human Fossil Record.