Scientists have identified the first humans to recolonise Britain after the last ice age. The country was taken over in a couple of years by individuals who practised cannibalism, they say - a discovery that revolutionises our understanding of the peopling of Britain and the manner in which men and women reached these shores.
Research has shown that tribes of hunter-gatherers moved into Britain from Spain and France with extraordinary rapidity when global warming brought an end to the ice age 14,700 years ago and settled in a cavern – known as Gough's Cave – in the Cheddar Gorge in what is now Somerset.
From the bones they left behind, scientists have also discovered these people were using sophisticated butchering techniques to strip flesh from the bones of men, women and children.
"These people were processing the flesh of humans with exactly the same expertise that they used to process the flesh of animals," said Professor Chris Stringer of the Natural History Museum in London. "They stripped every bit of food they could get from those bones."
The discovery of the speed of Britain's recolonisation after the last ice age, and the disquieting fate of some of those first settlers, is the result of two major technological breakthroughs. The first involves the development of a technique known as ultra-filtration carbon dating. Perfected by scientists based at Oxford University's radiocarbon accelerator unit, it allows researchers to pinpoint the ages of ancient bones and other organic material with unprecedented accuracy.
The second breakthrough involves the use of a machine known as the Alicona 3D microscope. Using this device, Dr Silvia Bello of the Natural History Museum has studied the cut marks left on bones of humans and animals in Gough's Cave. Scientists already knew cannibalism had been practised in the cavern, but were unclear if it was a ritual process or involved the deliberate killing of humans. However, Bello has found humans had been butchered with the same stone tools that had been used to cut up animals. In other words, animal and human flesh was treated the same way by these early Britons.
In addition to these findings, the discovery – by Danish scientists a few years ago – that the last ice age ended with astonishing rapidity has also played a key role in reappraising the recolonisation of Britain. Far from being a gradual process, in which men and women slowly reoccupied territory that had been taken from them by spreading glaciers, the resettling of Britain now appears to have been rapid, dramatic and bloody.
For around 60,000 years the planet had shivered as ice sheets fluctuated over large parts of the northern and southern hemisphere – including Britain, then a peninsula of northern Europe, which supported a small population of humans for much of this time. However, around 24,000 years ago, the weather worsened drastically. Britain's last inhabitants either died out or headed southwards for some continental warmth in refuges in northern Spain and central France.
Britain's icy desolation ended abruptly 14,700 years ago when there was a dramatic leap in temperatures across the globe according to ice-cores found in Greenland and lake sediments in Germany. In less than three years, temperatures had soared by around 6 to 7 degrees Celsius and ice sheets began a rapid retreat throughout the world.
Such a jump in temperature brought about an astonishing change in the world's weather patterns – though the underlying cause remains unclear, scientists admit. Suggestions include the proposal that variations in the orbit of the Earth around the Sun allowed more solar radiation to bathe the planet and so warm it up. It has also been proposed that there may have been a sudden eruption of carbon dioxide from the oceans. This helped trap heat from the sun in the atmosphere and so heat up the world.
"Whatever the reason, it was good news in those days, because the world was so cold and so it heated up nicely. However, if a rise like that happened today it would be devastating," said Dr Tom Higham, deputy director of the Oxford radiocarbon unit. "The world would be scorched. That is one of the most important aspects of the story of the resettling of Britain."
Higham's work, in collaboration with his late colleague Roger Jacobi, has involved studying the ages of the bones found at Gough's Cave in the Somerset Mendips, the earliest post-ice age site at which modern human remains have been found. The bones of half a dozen people – including children, adolescents and adults – were found in the cave in the 1980s, a discovery that made national headlines when it was revealed that these remains bore patterns of cut marks that suggested they had been the victims of cannibalism.
Other sites of this antiquity, in Germany and France, have also supplied evidence that human bones had been butchered. But the Gough's Cave finds were puzzling because radiocarbon dates indicated that humans had used the cave for more than 2,000 years, including several centuries in which the country would have been covered in ice sheets.
"The problem with radiocarbon dates of this antiquity is that it only takes a tiny trace of contamination from modern organic material to distort results," said Higham. "That is why we kept getting such a range of ages from the Gough's Cave bones."
To get round this problem, Jacobi and Higham worked on a technique – known as ultra-filtration – which involves using a series of complex chemical treatments to destroy any modern contamination in samples taken from the cave. First results of dates supplied using this technique were published by the scientists in a paper in Quaternary Science Reviews last year and were based on their re-analysis of the bones of Gough's Cave. These revealed a very different picture for the ages for the bones than had previously been calculated.
Instead of dates being spread over a couple of thousand years, the new ones clustered tightly round an age of 14,700 years before present – the exact moment that the world had begun its dramatic defrosting. Within a year or two, humans had left their southern refuges and were heading north into Britain, it was revealed. In other words, the end of the ice age was almost instantaneous – and so was the manner in which we exploited it.
In those days, humans were nomadic hunter-gatherers: strong, relatively well-nourished individuals who followed the herds of wild horses that then roamed Europe. These animals provided men, women and children with their main source of protein. "The weather suddenly got warm, the horses headed north and men and women followed them," said Higham. "It would have been a very rapid business."
As for the route of this migration, it probably took these ancient hunter-gatherers across Doggerland – a now submerged stretch of land in the North Sea that is known as Dogger Bank today – and into eastern England. Within a couple of years, they had reached Gough's Cave, though the cavern would not have formed a permanent residence but would most likely have served as a refuge to which they could return on a regular basis.
Previously it had been thought that the cave had been occupied, on or off, for around 2,000 years. However, the new set of dates generated by Higham shows that these not only cluster round the date of 14,700 years before the present, but that they cover only a very narrow range of about a hundred years or less. In other words, the cave was occupied for only a few generations at that time.
However, it is the behaviour of those few generations that has perplexed scientists for the past 20 years and which led to the new investigation by Bello. "The bone fragments we have found suggest we are looking at the remains of five individuals," she said. "These remains include one young child, aged between three and four, two adolescents, a young adult and an older adult. So we have every kind of age group represented in the Gough's Cave remains."
Bello has found that each of these sets of remains is covered with marks that show they had been the subject of comprehensive butchery, with all muscle and tissue being stripped from them. But why de-flesh those bones in the first place? What triggered such an extreme act? To provide answers, scientists have put forward a number of different theories. These include suggestions that it was a form of ritual which involved the eating of small pieces of a relative's flesh, not as a source of nutrition, but as an act of homage.
Others have argued that it involved a form of crisis cannibalism in which people ate the flesh of others because all other sources of food had disappeared. "An example of that sort of cannibalism was provided by the Andes air crash in 1972 when survivors ate the flesh of those who had been killed in the accident," said Stringer.
And finally there is straightforward cannibalism in which humans hunt, kill and eat other humans because they have a preference for human flesh. This is sometimes known as homicidal cannibalism.
The new evidence that is emerging from Bello's work does not resolve the issue, though some significant pointers have been uncovered. "These people were breaking up bones to get at the marrow inside," she said. "They were stripping off all of the muscle mass. Brains seemed to have been removed. Tongues seemed to have been removed. And it is also possible that eyes were being removed. It was very systematic work." In addition, human remains appear to have been disposed of in the same way as animal bones, by being dumped in a single pit.
Such evidence suggests straightforward cannibalism was carried out in Gough's Cave. However, there are other factors to note, said Bello. "These were very difficult times and it is still quite possible people ate each other because there simply wasn't anything else to eat." The landscape – although rapidly recovering – would still have been pretty barren, particularly in winter.
In addition, Bello also pointed out that the remains of only a few individuals had been found at Gough's Cave. In other words, there is no evidence that large-scale human butchery had been practised there. "That means we cannot completely rule out the possibility that this was some form of ritual cannibalism, although I think it is unlikely," said Bello.
At present, most evidence indicates that humans were probably using the skills that they had acquired in butchering animal flesh, in particular the meat of horses as well as reindeer, another stone age favourite, in order to cut up humans who had died of natural causes.
"We don't see any traumatic wounds in these remains which would suggest violence was being inflicted on living people. This was some kind of cultural process that they brought with them from Europe," she said.
Whatever the nature of the cannibalism that was carried out by these early settlers, it did them little good in the end. Two thousand years after the ice age ended, Europe was plunged into a new, catastrophic freeze. A massive lake of glacial meltwater built up over northern America. Then it burst its banks and billions of gallons of icy water poured into the north Atlantic, deflecting the Gulf Stream. Temperatures in Britain plunged back to their ice age levels and the country was once again completely depopulated.
"This new period of intense cold lasted for more than a thousand years," said Stringer. "Only by 11,500 years did conditions start to return to their present level – and Britain was colonised by humans for the last time."
• This article was amended on 23 June 2010 to correct the spelling of Dr Silvia Bello's name.
SOLVING THE RADIOCARBON PROBLEM
Radiocarbon dating was developed in 1949 and used to give dates for ancient Egyptian sites. These fitted well with previous age estimates, earning its developer – Willard Libby, of Chicago University – a Nobel prize. Since then the technique has played a pivotal role in archaeology.
The technique has some unworldly roots, however, and exploits the fact that cosmic rays strike atoms of nitrogen in the upper atmosphere, transforming them into an isotope of carbon called carbon-14. This is radiocarbon and chemically identical to normal carbon. Both are absorbed into the bodies of living beings, a process that stops when an organism dies. Its store of carbon-14 begins to decay back into nitrogen. After about 5,700 years, half is left. Then after 11,400 years, a quarter is left, and after about 17,000 years an eighth remains. By measuring a piece of organic material's radiocarbon content, its age can then be calculated.
There are drawbacks, however. After 35,000 years, only 2% of a sample's radiocarbon will remain. Not only is this tricky to measure, it puts a sample at risk of contamination. If an impurity of only 1% of new carbon – for example, from plant material - is added, it would appear 4,000 years younger. This has bedevilled research into human origins in Europe and explains why radiocarbon dates for the Gough's Cave bones from the Cheddar Gorge have produced such variable results.
To get round this problem, Tom Higham uses the ultra-filtration technique. Long strands of chemicals making up collagen in bone samples are isolated while shorter sections – found in plants and other sources of contamination - are removed. Combined with new mathematical techniques, this technology is giving scientists new precision in pinpointing ages of bones and skulls.