In our ancestors' footsteps

Two new sets of footprints, left by an ancient human relative over three and a half million years ago, have been unearthed in Tanzania.

The new findings indicate that these early human relatives were a lot larger than scientists thought originally.

It took serendipity to unearth the footprints. The Tanzanian government had commissioned an impact survey to assess the feasibility of constructing a tourist facility at the world-renowed Laetoli site. This is the place where Mary Leakey famously unveiled in 1978 the first set of hominid footprints, three track ways left by what may have been a family unit comprising one larger, one smaller and one diminutive individual.

Fossil remains of Australopithecus afarensis, the species believed to have made the tracks, have been recovered at the same site. Keen to capitalise on this legacy the Tanzanians had dispatched an academic team to dig some test pits ahead of any construction.

In one of these pits, one of the study authors, Fidelis Masao, from the University of Dar es Salaam, uncovered a new set of tracks, which were subsequently uncovered and explored in more detail. The analysis has been published this week in the journal eLife.

According to University of Perugia palaeontologist and co-author Marco Cherin, the footprints have been preserved in a layer of volcanic ash which fell over the Laetoli area over a short period about 3.66 million years ago.

Fortuitously, the eruption coincided with the onset of the rainy season, which dampened the ash and turned it into the ideal blank canvas for capturing the footsteps of a range of game - and importantly this group of early human ancestors - who wandered across the area. A subsequent eruption then quickly covered the area with a protective cap of ash, preserving the indentations for millions of years.

The new results follow the movements of two individuals. By looking at the depths of the footprints, and their relative spacing, the team have been able to deduce the likely stature of the walkers.

"We were shocked," Cherin says. "One of these individuals was over 165cm tall. That's equivalent with modern day humans. It means they [Australopithecus afarensis] were potentially much larger than we thought."

Prevailing theories suggest that ancient human relatives remained relatively small until the time that the exodus out of Africa began, closer to 100,000 years ago.

"This shows that there was considerable body size variation, and probably sexal dimorphism [meaning size differences between male and female] as early as 3.66 million years ago," says Cherin. "We didn't expect that."