Last September, scientists announced the discovery of a never-before-seen human relative (hominin), now known as Homo naledi, deep in a South African cave. The site yielded more than 1,500 bone fragments, an astonishing number in a field that often celebrates the identification of a single tooth. That rich fossil cache revealed much about the creatures, yet it left one glaring question unanswered: when did Homo naledi live? The scientists had no evidence for how old the fossils were. Without that information, it was very hard to know where the new species fits on the tangled human family tree, and to figure out its true meaning.

Difficulties in dating fossils have plagued anthropology since its inception. In 1856, a fossilised skeleton discovered in a small cave in the Neander Valley in Germany became the first hominin ever recognised by science. Quarry workers uncovered the fossils while clearing out a limestone cave, but before the bones were flagged as important, the workers had shovelled them out of the cave mouth. The fossils tumbled to the valley floor 20 metres below, obscuring contextual information that could have provided clues to their age – for example, how deep the skeleton was buried, and whether any fossilised animals had been found nearby…

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