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Together with yet another hominin, Homo naledi, known to have existed in southern Africa at that time, Africa may have been a crowded place. Along with other archaeological evidence, the findings suggest that perhaps Homo heidelbergensis was not our ancestor, but a neighbor. That meant that the Kabwe individual had lived not before, but around the same time as the first Homo sapiens–like people dwelled in North Africa. As he, Stringer, and others reported in Nature this past April, their best estimate was 299,000 years, give or take 25,000. Upon evaluating the mass spec data, he could tell that the fragment was much younger than previously believed. Having returned from his trip to procure the Homo heidelbergensis sample, Grün watched as the laser poked two tiny holes into the bone fragment and the particles disappeared into the mass spectrometer. The laser is coupled with a mass spectrometer, which measures the concentrations of uranium isotopes that undergo radioactive decay at a specific rate over time. Grün is one of very few geochronologists proficient in a laser technique that extracts and reduces a barely visible grain of bone-smaller than the bone’s natural pores-to atoms, he says. With few sediments, let alone fossils, left behind from that time, the birth of our genus is one of the most poorly understood periods in our evolution. But Grün was determined to find a solution. For decades, no dating method existed that could identify the fossil’s age without the destructive process of grinding up bits of bone for analysis. But nobody knew exactly how old the skull was.
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Based on its primitiveness, Stringer says, most researchers guessed it was an early individual who lived around half a million years ago, some 200,000 years before the earliest Homo sapiens were starting to emerge.
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In the 1980s, however, museum paleoanthropologist Chris Stringer took another look at the skull and classified it as belonging to the species Homo heidelbergensis, an ancient hominin thought to be a human ancestor. But later that year, museum paleontologist Arthur Smith Woodward noticed what he interpreted as typically human features, such as the skull’s thin and relatively large braincase, that motivated him to designate the specimen as its own hominin species. It looked so primitive that the miner who had discovered it in 1921 at a lead mine in the Zambian town of Kabwe, then in the British territory of Rhodesia, first thought it had belonged to a gorilla. The bone fragment had come from a skull-still stored at the Natural History Museum in London-with a heavy brow ridge and a large face. In 2016, he carried with him a particularly precious piece of cargo: a tiny sliver of fossilized bone covered in bubble wrap inside a box. Grün specializes in developing methods to discern the age of such specimens. Jawbones from extinct hominins in Indonesia, Neanderthal teeth from Israel, and ancient human finger bones unearthed in Saudi Arabia have all at one point spent time in his lab at Australian National University before being returned home. It’s not unusual for geochronologist Rainer Grün to bring human bones back with him when he returns home to Australia from excursions in Europe or Asia. ABOVE: MODIFIED FROM © MARK GARLICK, SCIENCE PHOTO LIBRARY