This article was updated Monday, Nov. 7 at 10:20 a.m.
LAS VEGAS — A famous trail of footprints once thought to have been left behind by a family of three human ancestors may have actually been made by four individuals traveling at different times.
In a new examination of Laetoli in northern Tanzania, where a 3.6-million-year-old track of footprints of the bipedal human ancestor Australopithecus is preserved, researchers now argue that the classic understanding of this site is mistaken. The footprints have been buried since the mid-1990s for preservation, but a section recently opened for study as Tanzanian officials make plans for a museum on the site.
Preserved at Laetoli are two lines of hominid prints, along the crisscrossing tracks of early rabbits and other animals. The site is the earliest example of an upright, humanlike gait in our ancestors. Early analysis had suggested the tracks were laid down by three individuals, evolutionary relatives of the famous Australopithecus aferensis "Lucy," discovered in Ethiopia. One Australopithecus walked next to another, while a third, smaller individual trailed behind, stepping in the tracks of one of the larger individuals.
Researchers speculated that the three Australopithecus walkers were a male, female and juvenile — a "first family" of upright walkers. But new high-resolution photographs reveal a different story, said study researcher Brent Breithaupt, a paleontologist at the U.S. Bureau of Land Management (BLM) in Cheyenne, Wyo. [10 Things That Make Humans Special]
The multiple-footprint impressions appear to contain not two sets of toe-prints, but three, Breithaupt said. And all of the individuals who walked through the plain had the same-sized feet.
"So instead of having three individuals of different sizes, with the sizes related to different ages, there are probably four individuals of the same size moving through this area, perhaps not traveling as a group," Breithaupt told LiveScience.
New look at old footprints
The Australopithecus tracks were first uncovered in the 1970s by researchers led by anthropologist Mary Leakey. While taking a break, the scientists struck up a rowdy elephant-dung fight on the Serengeti Plain. In the hilarity, one researcher practically tripped over the Laetoli footprints.
But because the footprints are fragile, they were re-buried to keep them conserved, and re-opened only once in the 1990s. That means that much of the study of the footprints has been secondhand, said study researcher Neffra Matthews, a scientist at the BLM in Denver.
"Any secondary interpretations had to be made from the first interpretations or from a cast or reproduction," Matthews told LiveScience. "So not having access to that primary set of data kind of channeled the way the interpretations would go from then on."
In February 2011, however, a section of the track area was re-excavated for evaluation as a future museum site. Using a photographic technique that provides high-resolution three-dimensional views of the footprints, Matthews, Breithaupt and Charles Musiba, an anthropologist at the University of Colorado at Denver, and their colleagues from the U.S., Tanzania, Korea and Spain were able to get a new view of the footprints. The research was funded by Tanzania's Ministry of Tourism and Natural Resources.
Party of four
The three-dimensional data provided by the new technique revealed an extra set of toe-prints in the multiple-footprint impressions. Careful measurements revealed that each individual had a footprint that was about 7.5 inches (19 centimeters) long, suggesting that each Australopithecus was the same size.
In addition, the lone set of tracks neighboring the multi-footprint trail revealed that the individual's right foot was angled strangely compared with the left foot, Matthews reported Nov. 4 here at the annual meeting of the Society of Vertebrate Paleontologists.
"Not only do we notice there is a change in the angle, but we also notice there is more weight being placed on the left foot, possibly indicating that there is a problem with the right foot," Matthews said.
The findings are preliminary, and future plans to re-excavate more of the tracks could provide more information on how these ancient human ancestors walked and how many there were, she said.
The new findings raise questions of why Australopithecus might have been walking in each other's footprints, said Martin Lockley, a paleontologist at the University of Colorado at Denver who was not involved in the current study. They also highlight the importance of taking a hard look at the evidence so that modern biases don't creep into ancient stories, he said.
"We kind of idealized the interpretation of the evidence to have a family, a mother, father and a youngster," Lockley told LiveScience, adding that there is a temptation to interpret fossil finds through a modern lens.
"There's a temptation to say, 'Well, we've got this data and it must mean something,'" he said. "And where do we get our meaning? We get our meaning from our cultural experience. So we say, 'We've got happy families today, here's a family 3.5 million years ago.'"
The new evidence torpedoes that nuclear-family ideal, he said.
"The whole concept of the family, possibly two adults and a baby, it's kind of like the three bears: mama, papa and baby," Lockley said. "We've brought in an uncle or a sister or a brother or something."
You can follow LiveScience senior writer Stephanie Pappas on Twitter @sipappas. Follow LiveScience for the latest in science news and discoveries on Twitter @livescience and on Facebook.