But during warm interglacial periods, sea levels rose and the chalk ridge was the only link.
At some point the ridge crumbled. Theories as to why have included river or glacial erosion, tidal scraping, and—most controversial of all—a megaflood.
Now Sanjeev Gupta of Imperial College London and colleagues say they have found the first concrete evidence to support the megaflood theory.
A 3-D map of part of the English Channel reveals features that could only have been created by a massive flood, the team says.
"We have identified huge scours on the seafloor and streamlined islands," said Gupta, whose results will appear in tomorrow's issue of the journal Nature.
These features are very unusual, he said, and have previously been found only in regions where megafloods are known to have occurred.
Similar features exist, for example, in the Channeled Scabland in eastern Washington State, which was deluged when the glacial Lake Missoula burst its banks about 12,000 years ago.
Based on their analysis, Gupta and colleagues say the most likely source of all this water was a huge glacial lake sitting in what is now the southern North Sea off the east coast of Britain.
The water was probably held back by the chalk ridge, and a small earthquake could have caused the first few cracks to appear.
"Chalk is not very strong, and eventually the water probably just started to over-spill," Gupta said.
Determining exactly when the megaflood took place is difficult.
But the divergence of plant and animal species between Britain and mainland Europe suggest that the event must have occurred sometime between 450,000 and 200,000 years ago.
"We now need to drill into the sediments to get an accurate date," Gupta said.
The great flood could help explain why Britain remained an uninhabited region for a large chunk of the archaeological record.
"There seems to be a large gap in the evidence for human occupation [of Britain] during cold and warm phases from about 180,000 until about 60,000 years ago," said Nicholas Ashton, an archaeologist at the British Museum in London.
(Related: "Humans Sped to U.K. After Ice Age, Study Says" [November 3, 2003].)
When the climate was warm, sea level between the island and the mainland was too high for humans to cross, Ashton said.
And during the much colder ice ages, humans could have crossed, but seem to have preferred to live in sunnier regions such as modern-day Italy and Spain.
"It wasn't until 60,000 years ago," Ashton said, "that humans—late Neanderthals—had the technological capabilities, such as more effective clothing and shelter, to survive the cold conditions."
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