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
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 environment.