The human family tree is a very bushy one, with several offshoots developing and dying out over the course of our evolutionary history. Most people are familiar with many of these offshoots, such as neanderthals, the hobbit and recently the new Chinese hominin. Less are familiar with one of the earlier branches: the Paranthropines.
The Paranthropines were to our Australopithecine ancestors (such as Lucy) what neanderthals are to you and me: a closely related sister lineage that ultimately died out. However, they’re interesting for the opposite reason neanderthals are. Our big-browed relatives are fascinating because they’re so close to being human, the Paranthropines are interesting because they’re so far from it. Despite being closely related to the lineage which eventually became humans they look incredibly alien, more like a Gorilla than any other primate. With massive cheek bones to accommodate strong jaw muscles, a crest on the top of their skull (for the jaw muscles to attach to) they look like no other member of the hominin family.
Of key interest are the evolutionary relationships between our ancestors and the Paranthropines. How did something en-route to becoming human become so distinctly non-human? Basically, the answer is convergent evolution. For one reason or another natural selection favoured similar traits in both Gorillas and Paranthropines, driving them to look more similar (and less human). In particular, dietary influences seem to have been key. Microwear evidence suggests that the Paranthropines ate very hard food, hence why many of Paranthropus‘s adaptations are to increase jaw strength.
However, if convergant evolution could transform an Australopithecine into a Gorilla once, could it have done so multiple times. There are 3 species of Paranthropus, might they be distinct lineages which independently developed from Australopithecine ancestors? Preliminary data does seem to lend credence to the existence of wo Paranthropine lineages. Paranthropus boisei and Paranthropus aethopicus both lived in East Africa and are both very robust whilst Paranthropus robustus lived in South Africa and is distinctly more gracile. Could P. robustus be another lineage evolving similar dietary traits to the other species?
Evolutionary development (“evo-devo”) may offer some clues here, explaining how either an Australopithecine or East African Paranthropine could become P. robustus. Evo-devo studies how small changes to the development of an organism can produce significant changes to the finished product. For example, increasing the rate at which a bone grows will change its thickness, size and shape. Since all Paranthropines and Australopithecines have the same bones, such developmental changes could explain how they came to look so different. So Philipp Gunz – an evo-devo reasearcher with the Max Planck Institute for Evolutionary Anthropology – generated two hypotheses:
- Paranthropus robustus falls on the allometric trajectory of the other Paranthropines (allometry being the relationship between scale, shape, thickness etc.). Specifically, it is a “scaled down” variant of the East African species
- Paranthropus robustus falls on the allometric trajectory of Australopithecus africanus (the South African Australopithecine from which P. robustus would’ve evolved had it developed separately)
He then tested these hypotheses, running a range of statistical and computational analyses on Paranthropine and Australopithecine skulls in an effort to work out their allometric trajectories and on which trajectory Paranthropus robustus fell. However, here he ran into his first major obstacle. As anyone with an understanding of statistics will tell you a large sample size is crucial to getting accurate results. Yet there are few complete skulls for the species in question, raising eyebrows as to whether his results can be trusted. To prove they could be, Gunz conducted similar analyses on small samples from modern species whose allometric trajectory is known. He found that, despite the sample size, the results were still accurate.
Confident that his results would be reliable, Gunz ran a developmental simulation of Australopithecus africanus to see what it would look like if the face “over-grew.” The results look surprisingly similar to a typical Paranthropus, with a “dish” shaped face and more pronounced eyebrow ridges. However a detailed statistical analysis of this simulation reveals that despite these superficial similarities, a warped Au. africanus is still not a good match for P. robustus. There are numerous other differences that allometry alone cannot explain. This is a very definite strike against hypothesis 2.
The second strike against hypothesis 2 is the vindication of the first hypothesis! The statistical analysis confirmed that the other Paranthropines were an almost perfect match for P. robustus, with its skull falling on the allometric trajectory of the other Paranthropines. To double check the results they ran five alternate analyses using different methods, including/excluding certain measurements and so forth. The results were all consistent, confirming that the measurement was not just an artefact of the analytical method used but a very real (and very reliable) result.
This is an excellent study, with a rigorous methodology and thus reliable conclusions. It unequivocally shows that P. robustus falls on the allomteric trajectory of the other Paranthropines and so is a member of their lineage. Despite superficial similarities to Australopithecus africanus (especially a warped one) P. robustus cannot be explained as an Australopithecine who underwent convergent evolution. Paranthropus robustus is a proper Paranthropine.
|Grine. 1986. Dental evidence for dietary differences in Australopithecus and Paranthropus: a quantitative analysis of permanent molar microwear. Journal of Human Evolution, 15(8):783–822|
|Philipp Gunz (2012). Evolutionary Relationships Among Robust and Gracile Australopiths: An ‘‘Evo-devo’’ Perspective Evolutionary Biology DOI: 10.1007/s11692-012-9185-4|