Armed with this combination of approaches, researchers have begun to excavate our speciesвЂ™ early evolution, hinting at a far more complex past than was previously appreciated-one rich in diversity, migration, and possibly even interbreeding with other hominin species in Africa. In recent years, a field that has traditionally relied on fossil discoveries has acquired helpful new tools: genomics and ancient DNA techniques. If Homo heidelbergensisВ wasnвЂ™t one of our recent ancestors, then who was? If our species really did overlap in time with Homo heidelbergensis, what role did they play in our evolutionary history? As a result, finds such as the Kabwe skull continue to raise more questions than answers. The story in Africa remains murky, however, as researchers have not been able to reconstruct human history in vivid detail, in part because hominin fossils informative about our speciesвЂ™ emergence and coexistence with other species are rare in Africa.

вЂњNow we know that there were probably at least three different kinds of hominins around.вЂќ ThatвЂ™s akin to the situation that unfolded in Eurasia, where Neanderthals and Denisovans thrived for hundreds of thousands of years before Homo sapiensВ migrated out of Africa and at times even interbred with the other hominin groups. вЂњTen years ago, I think most of us wouldвЂ™ve thought, well, Africa in the last 300,000 years is just going to show you the evolution of Homo sapiens, and thatвЂ™s really all-the other species would have disappeared, gone extinct,вЂќ notes Stringer. 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 sapiensвЂ“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, GrГјn 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. GrГјn is one of very few geochronologists proficient in a laser technique that extracts and reduces a barely visible grain of bone-smaller than the boneвЂ™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 GrГјn was determined to find a solution. For decades, no dating method existed that could identify the fossilвЂ™s age without the destructive process of grinding up bits of bone for analysis. But nobody knew exactly how old the skull was. 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. 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.

Human evolution timeline