Were the Neanderthals our ancestors? Or was it the Cro-Magnons? Or both? The answer depends on which anthropologist you talk to, and the question goes back to the birth of paleoanthropology, heralded by the first scientific description of a fossil skeleton found in the Neander Valley - hence the name Neanderthal - in Germany. The year was 1857, just two years before Darwin's The Origin of Species shook the world, and the skeleton ignited an intense debate - which continues today - about the relationship of these extinct people to modern man.
At the time, scientists believed that the distinctive features of the first Neanderthal skeleton were due to disease, but as other similar finds were made across Europe, it became clear that these fossils represented an ancient population of humans who inhabited the Continent during the Ice Age. We now know that the Neanderthals originated in Europe over 200,000 years ago and it was about 30,000 years ago that they disappeared. But the situation is complicated by the additional presence in Europe, from about 35,000 years ago, of much more modern-looking humans known as the Cro-Magnons, who seem to have originated in Africa.
Experts remain fiercely divided over whether the Neanderthals were replaced by the invading Cro-Magnons to become our ancestors, or whether they mixed and mated with each other - in which case modern Europeans might be partly descended from the Neanderthals.
To get an idea of the physique of the Neanderthal, you could do worse than conjure an image of Fred Flintstone: muscular and thickset, with a short, wide body reflecting adaptation to the cold of the Ice Age and a skeleton built for strength and endurance. Their brains were as large as ours, but enclosed in a longer, lower skull, with a strong browridge over the eyes. Their faces were large and long, and dominated by an enormous nose. The front teeth were larger than ours and heavily worn, and the lower jaw lacked a prominent chin.
Recent research using three-dimensional X-ray machines on Neanderthal skulls has revealed further details of their anatomy, even down to the fact that their inner ear structure was slightly different from ours.
The Neanderthals were mobile hunter-gatherers who lived off the land, collecting plants and hunting or scavenging large and small game. They made effective but relatively simple tools, and although they practised the very human activity of burying their dead, many archaeologists have questioned whether they had other modern human attributes, such as complex language or art.
In seeming contrast, the Cro-Magnons, also hunter-gatherers, produced sophisticated tools and art, including the spectacular paintings of the Lascaux and Chauvet Caves in France. Most palaeoanthropologists believe they were very similar to us in both anatomy and behaviour.
In 1997, fragments of ancient DNA were recovered from the original Neanderthal skeleton discovered in 1857, and they provided our first glimpse into the genetic make-up of an extinct form of human. Comparisons of this DNA with that of modern humans and chimpanzees have implied that the evolutionary differences between modern humans and Neanderthals might have begun about half a million years ago - although this estimate is based on data from only a fraction of the whole DNA of one Neanderthal individual. Analyses have failed to prove any particularly close relationship between modern Europeans and the Neanderthal; recent European DNA is as different to Neanderthal DNA as to that of African, Australian or Asian DNA.
Late in 1998, at a site called Lagar Velho in Portugal, a 25,000-year-old fossil skeleton of a child was discovered, reputedly bearing combined Neanderthal and modern features. It is described as combining a modern-looking lower jaw and teeth with the robust, cold-adapted body shape characteristic of Neanderthals. However, the paper describing the material awaits publication, and so we can't yet evaluate in detail the claim that it is a hybrid.
But if we accept for the moment the interpretation that it is a hybrid, what would such a find tell us? The child was buried with the kind of materials normally found with early modern human burials, and this could be the latest definite record of Neanderthal features yet discovered.
But claims in the press that this discovery completely revolutionises our view of the Neanderthals are wide of the mark. We can trace the evolution of the Neanderthals for at least 200,000 years in Europe, as we can trace the evolution of our own separate line in Africa over a similar time scale. Most anthropologists now accept that modern humans originated in Africa and dispersed from there during the past 100,000 years - the "out of Africa" theory of modern human origins.
A new twist to the most recent debate has centred on the question of whether there might have been interbreeding with Neanderthals during the dispersal phase, and whether such hybrid genes could be found today. If the Lagar Velho skeleton really is that of a hybrid, it would mean that such interbreeding did take place. But it can't answer the question of how common such matings were, whether the hybrids were fertile, whether their genes penetrated into Cro-Magnon populations, and what was the eventual fate of such genes after some 1,500 generations. The existence of hybrids would not disprove the belief of some anthropologists, myself included, that the Neanderthals were probably a different species from us.
The real question would be whether the species merged, and we have no evidence of that for Neanderthals and Cro-Magnons, since their core populations seem to have remained quite distinct. Fossil anatomy and the recent DNA studies still support the view that Neanderthals were not our ancestors.
However, recent research on Neanderthal living sites certainly forces us to upgrade our view of their capabilities. At the Arcy site in France, finds indicate that Neanderthals made jewellery from mammoth ivory and animal teeth, and a fierce debate has ensued about whether they could have done this independently or only under the influence of Cro-Magnons. And in contrast to the prevailing view that Neanderthals did not use marine resources, evidence has been found that 50,000 years ago, at Vanguard Cave in Gibraltar, their diet included baked mussels harvested at least a mile away. And so even if the Neanderthals were not our ancestors, we do know that ours was not the only path that led to humanity.
Professor Chris Stringer is principle researcher in human origins at the Natural History Museum, London.