Footprints left by ancient humans 800,000 years ago have been found in Britain, the earliest evidence of such markings outside Africa.
Researchers discovered the footprints, left by both adults and children, in ancient estuary mud at Happisburgh in Norfolk, eastern England.
The only older footprints found so far are at Laetoli in Tanzania, at about 3.5 million years old, and at Ileret and Koobi Fora in Kenya at about 1.5 million years, scientists said.
"This is an extraordinarily rare discovery," said Nick Ashton of the British Museum, who led the research team.
The discovery came at an archaeological site that has yielded several previous discoveries of stone tools and fossil bones, including mammoth remains. The researchers found the prints at low tide when waves washed away much of the beach sand to expose the silt below.
"At first we weren't sure what we were seeing, but as we removed any remaining beach sand and sponged off the seawater, it was clear that the hollows resembled prints, perhaps human footprints, and that we needed to record the surface as quickly as possible before the sea eroded it away," Ashton said.
The group of early humans that left the footprints appeared to have consisted of at least one male and several smaller people, believed to be females and youngsters. "They are clearly a family group rather than a hunting party," said Ashton.
Analysis of the prints found they were from a "range of adult and juvenile foot sizes" equating to modern shoe sizes of up to British 7 or 8 (US 8 or 9). The researchers estimated that the height of the ancient humans who left the prints varied from about 0.9 metres to over 1.7 metres, not far off the height of modern humans.
They were dated at 800,000 years old partly on the basis of the site's geological position beneath glacial deposits, but also because the fossils there come from nowextinct types of mammoth and horse and early forms of vole that were alive at that time.
But the question of exactly what type of ancient humans left their footprints in the sands of time remains a mystery. They may have been related to people of a similar period in history found in Atapuerca in Spain, assigned to the species Homo antecessor, or Pioneer Man, said Chris Stringer of the Natural History Museum.
"These people were of a similar height to ourselves and were fully bipedal," he said.
Stringer, who is part of the project, said that 800,000 or 900,000 years ago Britain was "the edge of the inhabited world". He said: "This makes us rethink our feelings about the capacity of these early people, that they were coping with conditions colder than the present day.
"Maybe they had cultural adaptations to the cold we hadn't even thought were possible 900,000 years ago. Did they wear clothing? Could they have they have the use of fire that far back?"
Additional reporting by Associated Press