The following correction was printed in the Guardian's Corrections and clarifications column, Saturday August 9 2008

Some confusion has arisen over our account of Neanderthals, which said at one point that they are thought to have died out 30,000 years ago, but at another that this happened 40,000 years ago. Our correspondent's best judgment now is that they disappeared somewhere between these two dates. As to their height, which has also been disputed, that seems usually to have been between 5ft 4in and 5ft 7in.

Strands of DNA recovered from the fossilised leg bone of a Neanderthal have shed light on the fragility of the ancient population 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 since become part of a landmark project to read the entire genetic sequence of an ancient human ancestor, 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 sequence of DNA 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, which suggests that the Neanderthals may only ever have existed in relatively small numbers, with less than 10,000 alive at any one time.

"This has implications for our picture of Neanderthals and perhaps the reasons for their extinction. If the population was teetering on the brink for hundreds of thousands of years, it maybe changes our impression of what it would have taken to make them go extinct rather than if there were millions of them," said Adrian Briggs, a molecular biologist who co-authored a report on the work in the journal Cell.

Theories of what drove the Neanderthals to extinction range from an inability to adapt to a quickly changing environment, to genocide by early humans. The species is though to have died out in Europe around 30,000 years ago, shortly after the arrival of early humans.

Neanderthals were short and stocky and well adapted to a cold climate. The tallest male, found in a cave in France, was only 5ft 5in (1.71m). Despite having barrel chests, strong ridges above their eyes and a lack of chins, their brains were on average larger than those of modern humans. Some fossil evidence suggests that they were occasional cannibals, though more commonly hunted large animals including horses and mammoths. Remains of Neanderthals dating back 400,000 years suggest they crafted tools and weapons and buried their dead. The last Neanderthals died out nearly 40,000 years ago, as Homo sapiens migrated to, and eventually settled throughout, Europe.

The Leipzig team has read 4% of the Neanderthal's entire genetic code and hopes to complete the full sequence by the end of the year. Comparing the Neanderthal genome with the human genetic sequence should highlight subtle genetic differences, such as genes for improved brain capacity and other traits that underpin what it means to be human.