Three becomes four. The extraordinarily sparse fossil record of the Denisovans – an ancient form of human – has gained one more specimen: a tiny, worn tooth belonging to a young girl. It adds to evidence that the Denisovan population in what is now Siberia remained small for tens of thousands of years. The Denisovans are perhaps the most mysterious of all ancient humans. They were discovered in 2010 when geneticists were sequencing DNA from ancient bones they had assumed belonged to Neanderthals. The DNA from a 50,000-year-old fragment of finger bone was so different from any known Neanderthal genetic sequence that the researchers concluded it represented a separate group of humans. Later genetic studies suggested that these distinct ancient humans split away from the Neanderthals sometime between 470,000 and 190,000 years ago. They were named the Denisovans – a reference to the Denisova cave in Siberia, where the finger bone fragment was found. Later, Denisovan DNA turned up in two