More than three million years ago, a distant cousin of ours called Australopithecus afarensis was walking around on two legs—making the species a key chapter in the human story. But a new study of a rare A. afarensis toddler suggests that the species' feet retained some apelike traits, perhaps helping them climb trees.
The study, published in Science Advances on Wednesday, takes a close look at the foot of Selam, a 3.3-million-year-old female A. afarensis that died before the age of four. The fossil helps scientists see how A. afarensis feet changed from birth to adulthood, which in turn lets us glean some details about how they grew up.
“We can understand what was going on in the young individuals versus the adults, and whether there was a shift in the way they moved around,” says American Museum of Natural History paleoanthropologist Will Harcourt-Smith, who reviewed the study before publication. “That itself is really cool.”
A. afarensis is best known from the fossil called Lucy, a 3.2-million-year-old hominin found in 1974 in Ethiopia. In the years that followed, researchers found a scattering of other fossils from this species, allowing them to piece together details of its behavior. (Also see fossils from 'Little Foot,' an enigmatic skeleton that may rival Lucy in age.)
Crucially, the species' hips and legs closely resemble ours in many ways, leaving little doubt that they walked on two legs. But some apelike parts of the skeleton imply a knack for climbing beyond what we see in anatomically modern humans. A. afarensis's finger and toe bones are curved, helpful for gripping, and its arm bones suggest that it was a strong climber.
Researchers have long debated what these features mean. Did A. afarensis readily climb in addition to walking, or were these traits just an evolutionary hangover? But the discussion largely centered on adult fossils. Examining a juvenile A. afarensis would help—to live to reproductive age, a species must survive childhood, which places intense evolutionary pressure on the youngsters' traits.
The dream of a “baby Lucy” became a reality in 2006, when University of Chicago paleontologist Zeresenay Alemseged announced his team's discovery of Selam in Ethiopia's Dikika area, not far from where Lucy was found.
“Every fossil gives us some bit of our past, [but] when you have a child skeleton, you can ask questions about growth and development—and what the life of a kid was like three million years ago,” says lead study author Jeremy DeSilva, a paleoanthropologist at Dartmouth College. “It's a magnificent find.”
DeSilva first saw Selam's remains in 2009, and Alemseged and DeSilva decided a few years later to focus on her foot.
Humans' big toes aren't offset like our thumbs; instead, they're in line with our other toes, which boosts our walking efficiency. The big toe of A. afarensis is similarly aligned, but its base joint is more curved than ours. In short, A. afarensis could wiggle its big toes side to side more than we can.
Selam's big-toe joint is even more curved than it is in adults of the species, which suggests that she had especially limber big toes well suited to grasping. DeSilva sees this as a sign that young A. afarensis needed grippier feet.
As DeSilva pictures it, groups of A. afarensis were on foot during the day, but they climbed trees at night to sleep safe from predators. Young A. afarensis may have climbed trees more often to avoid predators, or perhaps their feet let them better hold onto their mothers, making themselves easier to carry.
Future work may help resolve any lingering debate over tree-climbing tendencies in adults. For instance, scans of Selam's foot bones will show how her feet distributed her weight. But these fossils' extreme rarity mean that some answers will remain elusive. To really know developmental details, says DeSilva, you need to see a species at a series of ages, stepping from two to four to six and beyond.
“We're talking about a fossil record that will either never exist or is hundreds of years off,” he says. “I'd be shocked to see that kind of thing in my lifetime.”