By Paul Rincon
BBC News Online science staff
The remains of 28 early humans found buried at the bottom of a cave shaft in northern Spain may belong to a group that died suddenly in a "catastrophe".
Homo heidelbergensis appears about 600,000 years ago
Experts conducted an analysis to determine whether it was likely the bodies accumulated in the shaft over years or were dumped at the same time.
They concluded the 400,000-year-old death chamber may have held the victims of a disease outbreak or a massacre.
The study details are published in the
Journal of Anthropological Research.
"We still don't know how they died. But what does seem increasingly clear is that the death of these people could have been simultaneous," Jose Bermudez de Castro, co-director of the Atapuerca excavation, told BBC News Online.
The remains were recovered from a 14m-long shaft called Sima de los Huesos (the pit of bones) in the caves of Atapuerca, near the town of Burgos.
Atapuerca contains one of the richest records of prehistoric human occupation in Europe.
The bodies in the pit belong to a hominid species called Homo heidelbergensis. Some people think this species could have been the common ancestor of the Neanderthals and modern humans (Homo sapiens), although Professor Bermudez de Castro believes it was just ancestral to the Neanderthals.
Death and disasters
However, exactly how the bodies came to be deposited at the bottom of the shaft has always perplexed and intrigued those researchers that have excavated the site.
Professor Bermudez de Castro and his colleagues considered whether the remains in the pit fitted an attrition profile, in which individuals die one by one over long periods; or a catastrophic one, in which the dead cover the age spectrum of a population.
Natural disasters, violence, epidemics of diseases such as influenza or bubonic plague and occasionally famine can be responsible for catastrophic mortality profiles.
A single, pinkish-red stone axe was found in the pit of bones
The team compared the mortality profile of the Atapuerca remains with 26 other Homo heidelbergensis hominid remains from across Europe.
The proportion of individuals between the ages of 11 and 20 was 64% compared with just 39% for the other European sample.
"This corresponds best with a group of people who all died at the same time - a catastrophic profile," said Professor Bermudez de Castro.
"The only problem with this model is that we are missing the infants. But it is certainly a more rational model than the attritional one."
Victims of conflict?
Other scientists have suggested that carnivores could have removed the bones of infants, although this is difficult to prove.
Dr Andrew Chamberlain, a biological anthropologist at the University of Sheffield, UK, agreed with this broad assessment of the mortality profile.
"This profile is very similar to the kind of one you get in conflicts, with people who fight and die in battle; you get this peak of teenagers and young adults.
"But in that young age group - there are more females than males. With combat mortality, the deaths are nearly all males.
More than 7,000 Bosnian Muslims were killed at Srebrenica in 1995
"The other way you can get this age profile is deaths of non-combatants. If you look at the 7,500 civilians that were killed at Srebrenica in the former Yugoslavia, you get the same age profile.
"You could have one group of hominids attacking the other, with very large numbers killed. The problem is that we just don't know enough about the social behaviour of these early humans."
Professor Bermudez de Castro predicts that it will be difficult to progress further with efforts to understand how the Atapuerca hominids died.
"Skull 5 has a serious infection in his face. It's possible that he could have died from it, but the truth is we don't know," he explained.
"There are also signs of trauma in other individuals and it is possible that these traumas caused their deaths. But it is very difficult to give these people a death certificate."