Main techniques dating hominids
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.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.Rather, most are reworked by stream action and are redeposited into the sedimentary environment.In the process of reworking, these ashes can pick up pre-existing detrital grains that, by definition, are older than the juvenile ash.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. 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.
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.
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.
The ages assigned to these fossils have been obtained through radiometric dating of volcanic rocks interbedded with the fossiliferous sediments.
Such numerical calibrations are crucial to understanding rates and timing of evolutionary change.
Yet even this seminal K-Ar dating study was plagued by the seemingly insurmountable problem of contamination.