Many people often view evolution as a smooth and gradual change from one form into another. For humans, that would involve a reduction in facial projection (prognathism), decreasing size of the teeth and increasing brain size.
This week will hopefully demonstrate that these patterns can also be reversed – it’s time to meet the robust australopithecines. they evolved after dental reduction began so had to re-evolve massive teeth, jaws and facial skeleton in general, surpassing any hominin before or since:
Paranthropus aethiopicus is one of the earliest fossil species considered to be a robust australopith, known to be in existence between 2.3 and 2.5 million years ago (Mya). Its genus name Paranthropus means ‘alongside man’, hinting at the fact that it is not believed to be a direct ancestor of modern humans, while its species name aethiopicus or ‘Ethiopian ape’ indicates the place of discovery of the first fossil assigned to this species. Sadly there are very few fossils of anything other than skull or jaw fragments so not much can be said about its ability as a biped. It does however resemble the earlier Au. afarensis which means that it was probably capable of facultative bipedalism (Szpak, 2004).
The most interesting thing about the robust australopiths is the sheer size of their food processing parts (masticatory apparatus). They all possessed greatly expanded molar teeth and huge muscle attachments as shown by the sagittal crest running down the middle of the skull above, and the highly flared cheekbones.
Many people argue that Paranthropus doesn’t count as its own genus since not all of the species that are mentioned here are likely to share a close common ancestor with one another (they are a polyphyletic group – see David Attenborough Disappoints). That could mean that the similarities in terms of their masticatory apparatus were developed separately in each robust australopith as adaptations to similar diets – the wonder of convergent evolution.
The only other real option is to lump them all in last week’s genus of choice Australopithecus or ‘southern ape’. So. Australopithecus aethiopicus it is then.
Fig. 2 shows the cranial remains of the even-less-understood Australopithecus garhi which dates to around 2.5 Mya.
Its species name means ‘surprise’ in the local Afar language (Wikipedia) and is not quite a fully-fledged robust australopith, although the shape of the preserved upper jawbone (maxilla) suggests that the cheekbones would have been highly flared, and the teeth are fairly large. Some people like to think that Au. garhi was in fact ancestral to our own genus, Homo, although that would have involved some very rapid changes over the course of about 200 000 years to decrease tooth size and scale down the robustness of the facial skeleton. A single femur (see pt. ii) may or may not belong under the Au. garhi banner, but if it does it would seem that this species was an effective, if not habitual, biped (eFossils).
More fossil evidence exists for the two remaining species of robust asutralopith, the first of which we’ll meet is Au. boisei (fig. 3) which was around between 1.4 and 2.3 Mya in eastern Africa. The species name boisei is a mark of thanks to the man who funded the expedition which discovered the first fossil assigned to this species (Wikipedia).
In Au. boisei, the foramen magnum is in a forward position making it easier to balance the (particularly hefty) skull on top of a vertical spinal column. There is also a tibial fragment demonstrating expansion at the bottom (distal) end near the ankle to cope with higher and more frequent loads of weight-bearing in bipedal locomotion. A near-modern ratio of the length of the humerus compared to the femur suggests that the legs of Au. robustus were relatively elongated to help increase stride length for efficient walking.
Au. robustus (the name speaks for itself here, I think) is the name given to robust australopiths from various sites over South Africa dating from between about 1.5 and 2.0 Mya (fig. 4).
Some useful and not-so-useful skeletal fragments exist for working out how bipedal Au. robustus was. A very distorted pelvis (fig. 5) is used as evidence that they possessed flared ilia to increase the efficiency of the balancing muscles while walking (see pt. i).
More reliably, fossilised lumbar vertebrae are somewhat wedged, and also seem to be enlarged to cope with a greater weight-bearing role when standing and walking upright (see pt. iv).
Furthermore, an isolated non-specific first metatarsal has large joint surfaces also suggests that the big toe would have been heavily load bearing in robust australopiths (Zipfel & Kidd, 2006), although it is distinct from human first metatarsals in a more comprehensive shape analysis. This means that robust australopith bipedalism was not yet fully modern in terms of weight transfer or was possibly only facultative, with some time being spent in trees.
There’s loads more to say about the robust australopiths that has nothing to do with bipedalism so that can all wait for a future post.
Next week, we study the so-called ‘transitional’ hominins which take us right to the base of our own genus, Homo.
Brill, 1999. Australopithecu garhi. College of Visual Arts – Human origins. Available from: http://www.shortstreet.net/humorig.htm. Accessed: 14/05/14.
eFossils. Australopithecus garhi. Austin, Texas: Department of Anthropology, University of Texas. Available from: www.efossils.org. Accessed: 14/05/14.
Szpak, 2004. Tree of Life Web Project: Evolution of the Australopithecines. Available from: www.tolweb.org. Accessed: 14/05/14.
Columbia.edu. The Genus Paranthropus. New York, NY: Columbia University. Available from: www.columbia.edu. Accessed: 15/05/14.
Wikimedia Commons. Available from: http://upload.wikimedia.org/wikipedia/commons/4/43/Paranthropus_aethiopicus.JPG. Accessed: 14/05/14.
Zipfel, B., & Kidd, R. (2006). Hominin first metatarsals (SKX 5017 and SK 1813) from Swartkrans: a morphometric analysis. Homo: Journal of Comparative Anatomy, 57, 117–31. doi:10.1016/j.jchb.2006.01.001