What is a human? Why the split from our ancestors is so hard to define

Is it in the way we live, laugh and love? Or maybe it is our dislike of cheesy clichés? Deep within each of us, there must be something that makes us distinctly human. The trouble is, after centuries of searching, we still haven’t found it. Perhaps that’s because we have been looking in the wrong place.

Ever since researchers began unearthing ancient hominin bones and stone artefacts, their work has held the tantalising promise of identifying the moment long ago when our ancestors made the transition to become human. Two of the most important fossil discoveries in this quest celebrate significant milestones this year. It is 100 years since the very first “almost human” Australopithecus fossil came to light in South Africa, overturning established thinking about our place of origin. And it is 50 years since the most famous Australopithecus of them all – Lucy, also known as “the grandmother of humanity” – emerged from a dusty hillside in Ethiopia. Both fossils led researchers to believe we really could identify humanity’s big bang: the time when a dramatic pulse of evolution saw the emergence of our human genus, Homo.

But today, the story of humanity’s birth has become far more complicated. A string of discoveries over the past two decades suggests the dawn of our genus is harder to pin down than we had thought. So why did it once seem like Lucy and her ilk allowed us to define humanity and pinpoint its emergence? Why do we now find ourselves as far as ever from establishing what, exactly, a human is?…