Strands of DNA recovered from the fossilised leg bone of a Neanderthal have shed light on the fragility of this ancient hominid species and pinpointed when they first split from what were to become modern humans.

The 38 000-year-old bone was unearthed in a cave in Vindija in Croatia and has become part of a landmark project to read the entire genetic sequence of Neanderthals, a feat scientists believe will help reveal how modern humans evolved into the world's dominant species.

Researchers at the Max-Planck Institute for Evolutionary Anthropology in Leipzig, Germany, read the complete DNA sequence held in tiny biological powerhouses called mitochondria, which provide energy for cells. The mitochondria are only passed down the female line, so can be used to trace the species back to an ancestral "Eve", the mother of all Neanderthals.

The team analysed the DNA of 13 genes from the Neanderthal mitochondria and found they were distinctly different to modern humans, suggesting Neanderthals never, or rarely, interbred with early humans. The genetic material shows that a Neanderthal "Eve" lived around 660 000 years ago, when the species last shared a common ancestor with humans.

Further tests on the DNA revealed surprisingly few evolutionary changes, suggesting that the Neanderthals may only ever have existed in relatively small numbers with less than 10 000 alive at any one time.