The oldest human footprints have been found in volcanic ash in Italy.
By Dr David Whitehouse
BBC News Online science editor
They were made by individuals scrambling down the flanks of an active volcano about 350,000 years ago.
Italian scientists, who identified three separate fossilised trackways, say the people that made them walked on two feet using their hands only to steady themselves on a difficult descent.
"They're the oldest footprints to be found of the genus Homo, the group that we belong to," the researchers told the BBC.
Commentators say the prints were probably made by Homo heidelbergensis, a forerunner of Neanderthals, that dominated Europe at this time.
'A shocking experience'
Paolo Mietto and colleagues from the University of Padua studied the trackways, known to locals as "devil's trails", and say they are particularly detailed.
The prints were found in the western margin of the Roccamonfina volcanic complex in southern Italy, in a pyroclastic flow dated between 385,000 and 325,000 years ago.
"We found three sets of footprints. One set came down in a zig-zag, while another showed that the person didn't run but walked normally," Dr Mietto told BBC News Online.
HUMAN FAMILY TREE
Scientists are trying to piece together the species relationships
Occasional handprints are also visible, suggesting that the hands were used for steadying the individual during the steep descent, but otherwise each person walked upright.
On one of the trackways, the footprints make two sharp turns, presumably made in order to also negotiate the descent more easily.
"Finding the footprints was a shocking experience - an astounding experience because I wasn't expecting to find something like that. It was astounding because the footprints were so well preserved," said Dr Mietto.
Commenting on the discovery, Professor Clive Gamble, director of the Centre for Palaeolithic Archaeology at Southampton University, UK, told BBC News Online: "By 385,000 years ago, hominids are hunting and making really quite elaborate stone tools. But we still don't have much evidence of campsites. These were very well adapted and successful hominids."
The earliest hominid footprints of any kind are roughly 3.75 million years old. They are the famous Laetoli footsteps in Tanzania, discovered in 1973 by Mary Leakey.
They were made by the human ancestor known as Australopithecus afarensis. These footprints demonstrated unequivocally that our distant relatives were bipedal.
Dr Mietto said: "[The Italian footprints] are the oldest footprints to be found of the genus Homo; not as old as those found in Tanzania - but the genus Homo. The footprints in question have one unique aspect: the ones found up to now have been on flat ground and this is on a slope."
Professor Gamble points out that our particular species, Homo sapiens or "wise man", was not around at the time the Italian footprints were made.
One trackway takes a zig-zag route
"We did not emerge until about 200,000 years ago from an African version of heidelbergensis," he said.
The team that found the Homo footprints do not want to debate who exactly made them. "It's outside our competence [to say whether they're our direct ancestors]," they say. "We prefer to say they're pre-Neanderthals."
The footprints - 20 centimetres in length - also suggest that the ancient travellers were diminutive in stature, with heights of less than 1.5 metres.
According to Professor Gamble, this "is interesting because the bone evidence of heidelbergensis is of a big, strapping lad 1.8 metres tall and weighing 100 kg.
"Sexual dimorphism could be an interesting factor - whether there are big differences between males and females, which you don't get with the Neanderthals that came later."
The research is published in the journal Nature.