By Paul Rincon
Science reporter, BBC News, Gibraltar
A study of prehistoric animals has revealed the crucial role of the English Channel in shaping the course of Britain's natural history.
The remains we find today tell a story of Britain's ancient past
The Channel acted as a filter, letting some animals in from mainland Europe, but not others.
Even at times of low sea level, when Britain was not an island, the Channel posed a major barrier to colonisation.
This was because a massive river system flowed along its bed, UK researchers told a palaeo-conference in Gibraltar.
Today the English Channel is 520km long, 30-160km wide, about 30-100m in depth and slopes to the south-west.
Even now, the bed of the Channel is incised by a network of valleys, the remains of the river system, which may have been cut by catastrophic drainage of meltwater from further north.
"It would have been an incredible barrier at times of high sea level, but it would also have been a formidable barrier at times of low sea level for populations trying to move south to north," said Chris Stringer of London's Natural History Museum.
Professor Stringer presented the results here at the Calpe conference, a meeting of pre-history experts from all over the world.
The big flood
The evidence comes from the Ancient Human Occupation of Britain Project (AHOB). This five-year undertaking by some of the UK's leading palaeo-scientists has reassessed a mass of scientific data and filled in big knowledge gaps with new discoveries.
Chris Stringer's co-researchers Andy Currant, Danielle Shreve and Roger Jacobi have been studying how the mammal fauna of Britain has changed over the last 500,000 years.
During that period, animals have colonised, abandoned and re-colonised Britain many times as the climate shifted from warm to cold and back to warm.
The Channel is thought to have formed during a cold period 200,000 years ago or more.
Meltwater from an ice sheet formed a lake, which then overflowed in a catastrophic flood - cutting through a chalk ridge that previously connected Britain to France.
Changes in climate were accompanied by changing sea levels. At the height of an ice age, these would have been low. During interglacial periods, when the climate was warm, sea levels rose.
But even when water was locked up in the ice sheets and sea levels plummeted, the Rhine and the Thames rivers dumped meltwater into a major river system that flowed along the floor of the Channel.
This means that once the Channel formed, there was never again a simple land crossing to be made from northern France to Britain.
"We find we're getting only a selection of the mammals during the British interglacials that there are in mainland Europe," said Professor Stringer.
For example, at one pre-historic site, researchers found hippopotamus and fallow deer; but unlike mainland Europe at the time, there were no horses and no humans.
"This suggests that the Channel, or the Channel river system, is acting as a filter to prevent the movement of some of these [mammal] forms into Britain," Professor Stringer added.
Once sea levels rose high enough for Britain to be an island, the select fauna that had made it across from mainland Europe could develop in extraordinary ways.
During one warm stage, about 80,000 years ago, fossils from Banwell Cave in Somerset show Britain was populated by some very unusual animals. These included reindeer, bison, and a giant bear similar to a polar bear.
Interestingly, there are no hyena fossils at Banwell Cave, as there were in mainland Europe. Instead, it appears, their role in the food chain may have been taken up by wolves.
"The wolves were developing much larger jaws. Their teeth show incredible signs of breakage and wear as if they're chomping bones like hyenas," said Professor Stringer.
The mammals at Banwell seem to be the kinds of animals normally found today in cold regions. But they lived in Britain during a warm stage and seemed to be adapting to their new environment.
The team thinks the antecedents of these animals must have arrived in Britain when the climate was cold. But when conditions warmed up, sea levels rose and isolated Britain, marooning this cold-adapted fauna in a warm land.
THE HISTORY OF HUMANS AND OTHER ANIMALS IN BRITAIN
Major incursions were possible during periods of warmth
A number of important palaeo-sites mark the periods of influx
Extreme cold made Britain uninhabitable for thousands of years