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paleoanthropology, genetics and evolution

Photo Credit: From the catwalk at Sterkfontein. John Hawks CC-BY-NC-ND

Is Little Foot really 3.67 million years old?

Darryl Granger and colleagues report in Nature this week on the date of the StW 573 specimen, commonly known as “Little Foot”, from Sterkfontein, South Africa. They estimate the skeleton’s age at 3.67 million years old, an age slightly older than the footprints at Laetoli, Tanzania, which would make the South African skeleton roughly contemporary with the East African species Australopithecus afarensis, and earlier than Kenyanthropus platyops. The skeleton might thus be the oldest known hominin specimen in South Africa, although the fossils from the Jacovec cavern of Sterkfontein may be as old or older, as argued first by Partridge and colleagues (2003).

Does it matter? The age of the skeleton is irrelevant to its relationships, but may give some insight into the ecology of early hominins.

Stw 573 is an impressively complete skeleton of an early hominin, from the Silberberg Grotto deep within the Sterkfontein cave system. The skeleton was completely enclosed in a hard concrete-like breccia when it was first discovered, and the excavation has now taken more than 20 years. Some foot bones of the specimen were the first part to be uncovered, and were described in 1995 by Ron Clarke and Phillip Tobias. Later, other parts of the skeleton were discovered and a paper by Clarke (1998) presented basic details about the discovery. The excavators approached it very cautiously, taking years to extract blocks of breccia from the cave for preparation in the laboratory. As yet, only a handful of anatomical observations have been reported.

Without seeing the specimen, I cannot really say anything about its morphology or relationships. There are photos of the specimen but these all show the remains to be substantially distorted, apparently with some crushing and with plastic deformation of the breccia deposit. The fossil is beautiful, but studying the anatomy of this skeleton is going to be a tough job.

What about the name, Australopithecus prometheus? Raymond Dart gave this name to the first hominin specimen from Makapansgat, MLD 1, a piece of the posterior part of a cranial vault. As you can see from the bone itself, it gives very little information about the anatomy of the skeleton as a whole:

MLD 1, posterior view
Yeah, other people give you bootleg photos of Little Foot. I give you the real Australopithecus prometheus. MLD 1, posterior view. John Hawks CC-BY-NC-ND

Clarke (2008) has argued that the Sterkfontein and Makapansgat samples include two distinct fossil species. In Clarke’s view, Australopithecus africanus, which was identified by Dart at Taung, is a smaller-toothed species with a more elongated cranium, narrow nasal bridge, and posteriorly-placed cheeks. In contrast, Australopithecus prometheus includes those specimens that have anteriorly-placed cheeks, larger, bulbous molars and premolars, large canines and incisors, widely-spaced orbits and a sagittal crest. Clarke uses Au. prometheus for this set because the temporal lines of MLD 1 appear to have converged at the superiormost point on this cranial fragment. They may have formed a slight sagittal crest, although the crest itself is not preserved here. Clarke has argued (2013) that this feature and other aspects of the occipital morphology connect MLD 1 with some Sterkfontein specimens, including the StW 573 skeleton, and that in the StW 573 skull, those features of the posterior vault occur together with the bulbous teeth found in specimens like StW 252. In other words, for Clarke, Little Foot is the keystone that holds Australopithecus prometheus together.

I cannot evaluate that hypothesis without the evidence.

Other paleoanthropologists treat nearly all the South African material from Sterkfontein and Makapansgat as representatives of a single, highly variable species, Au. africanus. We know that these samples extend over a very long period of time, perhaps a million years. The largest number of specimens come from Member 4 of Sterkfontein, which includes an immense deposit of breccia in which it has been hard to establish a clear chronology. Given the extent of time involved, we might guess that a single species assemblage would sample populations that varied substantially from time to time. Clarke claims that the differences between his two proposed species Au. africanus and Au. prometheus are consistent, and that each includes clear male and female examples. I wait to see the evidence.

Coming back to the issue of the date: Granger and colleagues used a method of dating based on the exposure of surface rocks to cosmic rays, which is known as cosmogenic nuclide dating. Oxygen-16 and silicon-28 are among the most common isotopes in the Earth’s crust, and both are components of the crystalline quartz found commonly in sand grains and chert. On the Earth’s surface, atmospheric particles and rocks are occasionally bombarded by high-energy cosmic rays, and these microscopic high-energy collisions create cascades of secondary particles. Some of these smash into quartz, converting oxygen-16 into beryllium-10, and silicon-28 into aluminum-26. Both these isotopes are radioactive and decay at different rates. If the quartz crystalline material is then buried deep underground in a cave breccia, the creation of new aluminum-26 and beryllium-10 stops, and the ratio between the two elements steadily changes as both isotopes decay. The ratio is a natural measure of the length of time since the material was introduced into an underground environment. Granger and colleagues examined this ratio in many rock samples from nearby the StW 573 skeleton, all of them together allowing a more precise estimate of the date that the rock samples were buried than any could individually.

Of course, to accept the date for the skeleton we must assume that it entered the deep chamber at the same time as the rock samples. If the rock samples had been reworked from earlier sediment deposits in the cave, then they might have much older ages than the fossils. In this paper, 9 quartz samples together are consistent with the same age (the 3.67 million year age is estimated as an isochron from this consistency across samples, not independently from the different samples). That makes it seem unlikely that the breccia composition has been systematically reworked from sediment that had lain underground for much longer than the fossils included in the breccia. Still, the geology is complex. Flowstone around the fossil yields a much lower age, around 2.2 million years, but Bruxelles and colleagues (2014) have argued that the flowstone formed much later, after the breccia bearing the skeleton had cracked and settled.

Accepting an early date for StW 573 does not weigh either toward or away from recognizing Au. prometheus as a valid species. The anatomy is the anatomy, whatever its age, and only the anatomy can determine whether StW 573 and possibly other specimens can be distinguished from the variability of Au. africanus. A phylogenetic analysis that includes that anatomy might be very interesting—I could even imagine that a reduced Au. africanus might look much more like Homo than the expanded sample, and StW 573 and a handful of other specimens might be more different from Homo than was Au. afarensis.

From the point of view of ecology, I cannot see any reason why early hominins did not enter southern Africa early in the Pliocene. They were established from Tanzania to Ethiopia by 3.6 million years ago, and the 3.4-million-year-old Bahr-el-Ghazal mandible shows that at least one australopith had spread across the Sahel region during the same time period. They should have reached southern Africa as well.

What happened to them there? An early date for Little Foot might help to establish whether southern hominins had already become different from their eastern counterparts. An early date would help us to evaluate whether climatic or other environmental factors had influenced the early habitation of southern Africa by hominins.

References

Granger DE, Gibbon RJ, Kuman K, Clarke RJ, Bruxelles L , Caffee MW. 2015. New cosmogenic burial ages for Sterkfontein Member 2 Australopithecus and Member 5 Oldowan. Nature doi:0.1038/nature14268

Clarke, R. J., & Tobias, P. V. (1995). Sterkfontein Member 2 foot bones of the oldest South African hominid. Science, 269(5223), 521-524.

Clarke, R. J. (1998). First ever discovery of a well-preserved skull and associated skeleton of Australopithecus. South African Journal of Science, 94, 460-463.

Clarke, R. J. (2008). Latest information on Sterkfontein's Australopithecus skeleton and a new look at Australopithecus. South African Journal of Science, 104(11-12), 443-449.

Clarke, R. (2013). Australopithecus from Sterkfontein Caves, South Africa. In The paleobiology of Australopithecus (pp. 105-123). Springer Netherlands.

Partridge, T. C., Granger, D. E., Caffee, M. W., & Clarke, R. J. (2003). Lower Pliocene hominid remains from Sterkfontein. science, 300(5619), 607-612.doi:10.1126/science.1081651

Bruxelles, L., Clarke, R. J., Maire, R., Ortega, R. & Stratford, D. Stratigraphic analysis of the Sterkfontein StW 573 Australopithecus skeleton and implications for its age. J. Hum. Evol. 70, 36–48 (2014)