Three has become five. Laetoli in northern Tanzania is the site of iconic ancient footprints, capturing the moment – 3.66 million years ago – when three members of Lucy’s species (Australopithecus afarensis) strode out across the landscape.
Now something quite unexpected has come to light: the footprints of two other individuals.
“Our discovery left us without words,” says Marco Cherin at the University of Perugia, Italy.
The find looks set to transform our understanding of the Laetoli site and the social dynamics of australopiths, as well as their style of walking.
The original Laetoli footprints were discovered in 1976. Nothing quite like them had ever been found before. They remain by far the oldest hominin footprints we know, fortuitously preserved because a group of australopiths walked across damp volcanic ash during the brief window of time before it turned from soft powder into hard rock.
“Geologists say this hardening process must have occurred in just a few hours,” says Cherin.
The new discovery came about by chance. Keen to build a museum at Laetoli to attract tourists, the authorities asked Fidelis Masao, a researcher at the University of Dar es Salaam, Tanzania, to investigate the impact that building work would have on the site’s valuable geology, says Cherin. Masao and his colleagues excavated about 65 pits to get a better sense of the extent of the ash layer in which the footprints were found. One of the pits contained the new footprints – and more archaeologists, including Cherin, began to study them.
So far the researchers have uncovered 13 prints belonging to a large individual – dubbed S1 – and a single print belonging to a smaller S2 australopith. Once the whole area has been excavated, there could be as many as 50 prints belonging to S1, they say.
S1 seems to have been walking in the same direction, at the same speed – and in all probability at the same time – as the australopiths whose footprints were uncovered in the 1970s.
It has been all too tempting to interpret the original trackways – often reconstructed as belonging to two adults and one juvenile – as evidence of a prehistoric “nuclear family”.
The new footprints show more adults were present, including one who was much larger, the S1 individual. That has spawned a new hypothesis about australopith social groups.
“They were probably similar in certain respects to those of our cousins, the gorillas, with a single dominant big male accompanied by his females and their offspring,” says Giorgio Manzi at the Sapienza University of Rome, Italy, who was also involved in the excavations.
However, in other respects australopiths might have been different from gorillas. A 2011 study of isotopes in australopith teeth – which can reveal where an individual grew up – suggests it was the smaller (probably female) australopiths that left their family groups to wander and join a new social group. This is unlike gorillas, where it’s larger males that leave their family to establish a new social group.
The new footprints find could also help us determine if the australopiths walked like us, a controversial issue. Some researchers, like Robin Crompton at the University of Liverpool, UK, have studied the Laetoli prints found in the 1970s. They say the depth profiles of the prints show clearly that australopiths walked in a broadly modern way: the hominins seem to have had a well-developed arch in the foot, and they used their big toe to push off the ground.
Other researchers, including Kevin Hatala at Chatham University in Pittsburgh, Pennsylvania, have interpreted the prints differently. Earlier this year, Hatala and his colleagues concluded that the australopith gait might have looked a little strange to modern eyes, with the knees slightly bent as each foot struck the ground.
“A substantially long trackway could prove very informative,” says Hatala.
For Cherin, one of the most exciting aspects of the new finds is the size of the S1 australopith. Its feet were 26 centimetres in length – 3.5 cm longer than the other Laetoli prints. The scientists estimate S1 might have stood 1.65 metres tall, comparable with modern humans.
However, William Jungers at Stony Brook University School of Medicine, New York, says we should treat that figure with caution. Hominin foot bones are exceptionally rare in the fossil record, and it’s not at all clear that australopiths had feet in the same proportion to their bodies as we do. “Australopiths aren’t modern people in many respects,” he says.
We can expect to hear more when the researchers return to Laetoli to continue excavations, potentially as early as mid-2017.
Journal reference: eLife, DOI: 10.7554/eLife.19568