The Age of STW 573: The Debate Continues

An article appearing in Science (abstract here) continues the debate surrounding STW 573 (aka Little Foot). The STW 573 australopith (species hasn’t been decided yet) was found in the late 1990’s by R. J. Clarke at Sertkfontein.

For background on the find you can read First ever discovery of a well-preserved skull and associated skeleton of Australopithecus and Discovery of complete arm and hand of the 3.3 million-year-old Australopithecus skeleton from Sterkfontein. For background on the dating controversy you can read Hawks and the following two articles:
The new hominid skeleton from Sterkfontein, South Africa: age and preliminary assessment
Brief Communication: Revised Age Estimates of Australopithecus-Bearing Deposits at Sterkfontein, South Africa
STW 573 is a remarkably complete fossil, here is a picture of the skull:
and here is one of the lower limbs:
At issue is whether the fossil dates to 3-4 MYA or about 2MYA. The Berger paper – linked to above – argues for about 2 MYA and so does the new Science paper. The authors use Uranium-lead dating and come up with a date of 2.17-2.24 MY for the fossil. Both Science Daily and the press release for the paper focus on the fact that this new date (if it holds up) removes STW 573 from contention as a human ancestor:

Ancient remains, once thought to be a key link in the evolution of mankind, have now been shown to be 400,000 years too young to be a part of man’s family tree.

Which, I think, is somewhat misleading in that all we know about STW 573 is that it is an Australopithecine…
At any rate, previous attempts to date STW 573 include:
1) Biostratigraphy – Chasmaporthetes nitidula (a type of hyaena), for example has been found at Sterkfontein. Other faunal studies, based on Makapania (a type of bovid) seemed to support the date. However, Berger et al’s (linked to above) article calls the faunal dating into question. Finding Equus in Member 4 (implying Member 4 is about 2.3 million years old. Member 4 is above Member 2 where STW 573 was found) means that the 3.0-3.5 MYA date will have to be revised down.
2) Magnetostratigraphy – The original dating matched the top of the sequence with the Gauss Normal epoch which provides a date of about 3.3 MYA for STW 573. Other attempts just as convincingly match the top of the sequence with the Olduvai epoch giving a date of about 3.0-2.5 MYA for STW 573.
3) Other – cosmogenic 26Al and 10Be dating of quartz found in Member 2 suggested a date of 4.0 MYA.
As mentioned above, this new paper used the Uranium to lead technique on specimens collected in the vincity of the skeleton. This yielded dates of 2.2 MYA for STW 573, which gives with the magnetostratigraphy if one starts with the Olduvai epoch.
What does this mean for our interpretation of the fossil record? I’ll let Berger et al explain it:

If, as we propose, Australopithecus africanus* from Sterkfontein dates to between 1.5 Ma-ca. 2.5 Ma, then the paleontological range of A. africanus probably does not overlap the temporal ranges of early East African hominins such as Australopithecus afarensis, A. bahrelghazali, and Kenyanthropus platyops. Rather, it is broadly contemporaneous with more derived species such as Australopithecus garhi, A. aethiopicus, A. boisei, and early members of the genus Homo. Our results may explain the well-recognized enigma posed by the derived cranial and dental morphology of A. africanus compared to the more primitive cranial and dental morphology of hominins dated to older than ca. 3.0 Ma.

* Although I agree with Berger et al and others that STW 573 is A. africanus, as far as I know Clarke – the excavator – hasn’t identified it as to species.
Added Later: Paul Wren also mentions the redating…

2 Responses

  1. I am amazed at how many contemporaneous hominins there were in the late Pliocene.

  2. Even more interesting is that the three main South Arican sites have yielded over 1,000 specimens…

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