Footprints made by an early human ancestor more than 3.6 million years ago have been uncovered in Tanzania, Africa.
The tracks are the oldest prints of their kind ever found, and were made when some of our distant relatives, known as Australopithecus afarensis, walked together across wet volcanic ash.
The footprints vary in size and are thought to have been made by a male walking with smaller females.
"The footprints of one of the new individuals are astonishingly larger than anyone else's in the group, suggesting that he was a large male member of the species," said lead researcher Prof Giorgio Manzi, director of the archaeological project in Tanzania.
"In fact, the 165cm stature indicated by his footprints makes him the largest Australopithecus specimen identified to date."
Australopithecus afarensis is one of the best-known early human species. The fossil of "Lucy", a young adult female who lived in Ethiopia 3.2 million years ago, is perhaps the most famous example.
The discovery of the tracks comes 40 years after similar tracks were discovered just 150m north, at a site in Laetoli, Tanzania.
The researchers, based in Italy and Tanzania, now think the two sets could belong together, giving clues to the lifestyle of Australopithecus.
As well as providing crucial evidence that walking on two legs was picked up early in the human lineage, the footprints also suggests that males may have had more than one sexual partner.
"A tentative conclusion is that the group consisted of one male, two or three females, and one or two juveniles," said Dr Marco Cherin, director of the school of paleoanthropology at the University of Perugia.
This led the researchers to believe that the male - and therefore other males in the species - had more than one female mate, suggesting their social structure was "closer to a gorilla-like model than to chimpanzees or modern humans".
The report, entitled "New footprints from Laetoli (Tanzania) provide evidence for marked body size variation in early hominins," was published today in the journal eLife .