Why it’s so hard to tell when Homo sapiens became a distinct species

For the vast majority of our planet’s history, there were no humans. Today, there are more than eight billion of us. Logically, then, there must have been a moment when Homo sapiens became a distinct species. Yet that moment is surprisingly hard to pin down. The problem, for once, isn’t a lack of fossils. Instead, disagreement about when to mark the origin of humanity comes down to the speciation process itself.

We often imagine the human evolutionary tree as a grander version of a personal family tree – indeed, researchers tend to talk about parent, daughter and sister species. In…