Stepping back 3.6m years: footprints yield new clues to humans’ ancestors

Tracks found by accident on proposed museum site in Tanzania were preserved in volcanic ash dampened by ancient African rains

The footprints of five ancestors of humans who walked the Earth more than 3.6m years ago have been found preserved in volcanic ash that was dampened by ancient African rains. Researchers unearthed the tracks by accident when they began to excavate test pits that had been called for as part of an assessment of the impact of building a proposed museum on the site in Tanzania.

The markings reveal that the ancient human relatives walked side by side for at least 30 metres. The footprints were laid down in a layer of ash that was subsequently buried, but which when moistened retained the tracks like clay.

A first analysis of the footprints suggests that they were made when a male, three females and a child passed through what is now Laetoli in the African country. The individuals almost certainly belong to a species of hairy bipedal ape called Australopithecus afarensis which is known to have lived in the region.

Initial analysis of the footprints suggests that they were made by a male, three females and a child.
Initial analysis of the footprints suggests that they were made by a male, three females and a child. Photograph: Raffaello Pellizzon

The most famous member of the species, known as Lucy, lived in the Hadar area of Ethiopia 3.2m years ago. A mere 1.1 metre tall, she was tiny in comparison to those who left their marks in Tanzania. The male stood more than half a metre taller, at 165cm (5ft 5in), making him the largest Australopithecus yet recorded. His impressive stature – for his species – led researchers to nickname him Chewie after the towering hairy Wookiee in Star Wars.

Marco Cherin, a palaeontologist at the University of Perugia in Italy, helped to excavate the tracks after the first prints were discovered by a team in Tanzania. “When we reached the footprint layer and started to clean it with a soft brush and saw the footprints for the first time, it was really one of the most exciting times of my life,” he said.

tanzania graphic

Having uncovered the footprints and measured them, the scientists used a number of mathematical models to calculate the heights of the different individuals. If the scientists are right that the group consisted of a tall male with three adult females and a child, it would bolster the theory that Australopithecus afarensis was polygynous, meaning that, like gorillas, males would have had several female partners at once. The adult females stood about 140cm tall.

Measurements of the length and width of the footprints, the angle of the gait and the stride lengths allowed the scientists to calculate rough weights for the five. The tall male came in at the heaviest, weighing 48.1kg, with the lightest only 28.5kg.

The layer of ash that preserved the tracks has been dated to 3.66m years old, the same age as a similar sequence of hominin, or human ancestor, footprints found nearby by famed palaeontologist Mary Leakey in the 1970s.

“These footprints enrich our knowledge about the most ancient hominin footprints in the world,” Cherin told the Guardian. “But they tell us something about the makers too, in this case that we think there were significant differences between the males and females. This is the most striking thing.”

“A tentative conclusion is that the group consisted of one male, two or three females, and one or two juveniles, which leads us to believe that the male – and therefore other males in the species – had more than one female mate,” Cherin added. Details of the tracks are published in the journal, eLife.

Researchers now want to return to the site to dig a trench that links the excavation pits and then work outwards, in the hope of revealing more tracks. “We are pretty sure that at least one more individual is waiting for us, and possibly more. Our goal is to discover new individuals,” Cherin said.

Giorgio Manzi, director of the archaeological project in Tanzania, said the evidence portrayed several human ancestors moving through the landscape after a volcanic eruption that was followed by rain. “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. In fact, the 165cm stature indicated by his footprints makes him the largest Australopithecus specimen identified to date.” In being so short, it seems that Lucy was an outlier.

Preliminary digging and cleaning operations at the Laetoli site.
Preliminary digging and cleaning operations at the Laetoli site. Photograph: Sofia Menconero

Earlier this year, scientists at the University of Texas in Austin declared that Lucy might have died after falling out of a tree. John Kappelman, an anthropologist, conducted a fresh analysis of the bones and concluded that a number of cracks matched the traumatic fractures seen in humans that suffer serious injuries from high falls. But the suggestion was met with scepticism by many researchers, who pointed out that a lot can happen to a skeleton in 3.2m years. Lucy’s body might have been crushed by stampeding animals, for example, before her bones were covered in sediment and encased in rock.

Last month, the same team sought to boost their argument with a study that found that Lucy’s arms were heavily built, implying she was an accomplished tree climber – though not accomplished enough, it seems, to have prevented her falling to her death.

William Jungers of Stony Brook University in New York said: “To judge by the profound scientific impact of the first set of Laetoli footprints, we can expect the new ones to figure prominently in future narratives of the origins of humans. They will likely stimulate new research and debate for years to come.”


Ian Sample Science editor

The GuardianTramp

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