Every so often, a curious thing happens to the Earth’s magnetic field. We don’t really know why it happens, or even when it is likely to happen next, but every several hundred thousand years or so, the Earth’s magnetic field reverses. North becomes south; south becomes north. We know this, because when rocks are formed, they are indelibly marked with the normal or reverse polarity of their birth time, or chron.
Apart from being a geological curiosity, the constant toing-and-froing of the Earth’s polarity has proven extraordinarily useful in pin-pointing the geological ages of rocks. This is perhaps no better illustrated than in the recent – and incredibly precise – dating of the Australopithecus sediba remains from South Africa.
Last month, many around the world read about the unveiling of the remarkably intact remains of two Australopithecus sediba individuals from the Malapa cave site in South Africa. What these remains mean for the way we draw our family tree – are they or aren’t they our direct ancestors, for instance – is still being debated.
One thing that is certain, however, is that the remains fit into a previously rather barren period in the fossil record of early hominids. Before their discovery by paleoanthropologist Lee Berger and his son in 2008, there were fossils of Homo erectus, the earliest known representative of our own genus Homo, which were dated to around 1.9 million years old. Then there was Lucy, a fossil remain from the pre-Homo hominid Austraopithecus afarensis. Lucy was found in Ethiopia and dated to 3.2 million years ago.
At the 2 million year mark, the crucial transition point when Australopithecus became Homo, few fossil remains existed. It turns out that the two ill-fated Au. sediba individuals found at the Malapa cave site some 60 km northwest of Johannesburg met their demise at almost precisely this time. In fact, the age estimate for the skeletons is 1.977 years old, give or take 2000 years – a remarkably precise approximation given their antiquity.
Dr Robyn Pickering, a geochemist working at the School of Earth Sciences at the University of Melbourne, is the lead author on a paper in the journal Science that describes how Australopithecus sediba was dated so accurately.
The first step, and the step that Dr Pickering was most intimately involved in, was uranium-lead dating of the limestone deposits, or flowstones, which lay above and below the Au. sediba fossils. Uranium-lead dating is similar to carbon-14 dating, in that it measures the gradual decay of a chemical element’s radioactive variants, or isotopes, over time. In the case of radiocarbon dating, carbon-14 (14C) that is trapped into a biological material upon death decays over time, and predictions can therefore be made about when an organism lived according to the amount of carbon-14 remaining in the sample. In uranium-lead dating, the principal is the same: uranium-238 trapped in rocks during their formation decays to lead-206 (and uranium-235 to lead-207), and measuring these isotopes gives us an idea of the age of the rocks.
Unlike carbon-14 dating, which measures ages in the tens of thousands of years, uranium-lead dating measures in the millions to billions of years. In fact, uranium-lead dating was responsible for providing us with one of the first accurate measures of the age of the Earth at over 4.5 billion years.
When Lee Berger found the Au. sediba remains at the Malapa site, Robyn Pickering was fine-tuning a technique for her PhD that would enable uranium-lead dating of much younger rocks than it had previously been used to date. “It’s only been recently that the technology that we use, the laboratory techniques and technologies, have become sophisticated enough to be able to measure uranium in relatively young rocks,” Dr Pickering says.
Using her methodology, which was perfectly suited to dating Berger’s remains, Dr Pickering measured the age of the flowstone from underneath the fossils at the Malapa site. “What we need to do is extract the uranium and the lead out of the rocks,” explains Dr Pickering, “then we can measure the amount and the various isotopes of uranium and lead and then put all that back together and calculate the ages.”
The uranium-lead dating of the underlying flowstone in 2010 provided an upper age limit of 2 million years. A lower age limit of 1.5 million years was deduced by the presence of particular species of animals at the site. Meanwhile, Dr Andy Herries, from La Trobe University measured the magnetic polarity of the flowstone as being reversed, and the cement-like sediment within which the skeletons were buried as being normal. This narrowed the fossil age approximation to 1.95–1.78 million years old, within what’s known as the Olduvai Subchron.
As excavations continued, however, an overlaying flowstone was identified, and uranium-lead dating indicated that the upper and lower flowstones were of similar age and polarity. Given these findings, the only explanation for the normal-polarity fossil-bearing sediment sandwiched in-between two reverse-polarity flowstones, was for the sediment to have formed during a short period known as the Pre-Olduvai event.
The Pre-Olduvai event was not a complete reversal of the Earth’s polarity, but merely an excursion – a brief, and incomplete change in the magnetic field. The signature of this shift remains, nonetheless, and in the case of Au. sediba provides an impressively narrow window of geological time within which we now know the individuals perished in the Malapa cave.
As for whether Au. sediba are our linear descendents, Dr Pickering thinks so. “We believe that this fossil is the best candidate to be the ancestor of our genus Homo.”
Others are not so sure, and as excavations of the Malapa site progress, debate with undoubtedly continue about whether or not Au. sediba is the immediate predecessor to the very first species named Homo. But in determining what Au. sediba means for the way in which we draw our family tree, precisely dating the fossil remains was a crucial first step.
The Up Close episode featuring Dr Robyn Pickering can be accessed here.
Pickering R, Dirks PHGM, Jinnah Z, et al. 2011. Australopithecus sediba at 1.977 Ma and Implications for the Origins of the Genus Homo. Science, 333: 1421-1423. [DOI:10.1126/science.1203697] [link to abstract: http://www.sciencemag.org/content/333/6048/1421.abstract]