The fossilized shoulder joint of an early hominin named “Little Foot” suggests the upright-walking species was also proficient at swinging through trees, at a skill long gone among modern humans.
Some 3.7 million years ago in what is now South Africa, a human relative straddled two evolutionary moments: our propensity for spending time in trees and our emerging preference for walking on the ground. That specimen, called StW 573 or Little Foot, was an Australopithecus prometheus. The fossil was finally fully excavated in 2018, over 20 years after its discovery, when paleontologists finished extricating the fossil from the breccia it was encased in. Immediately, Little Foot offered a remarkable glimpse into human origins.
Research describing the shoulder joint’s morphology was published this week in the Journal of Human Evolution. The research team inspected Little Foot’s pectoral girdle: literally, the specimen’s shoulder blade and collar bone. By comparing the girdle’s formation to that in other human relatives, including some of the great apes, the team sussed out how Little Foot and others in its species got around.
“By understanding how the shoulder joints of early hominins are structured, and more broadly how their shoulder blades are capable of moving on their torsos, we can understand how they used their upper limbs while interacting with the environment,” said Kristian Carlson, a biological anthropologist at the University of Southern California and lead author of the new paper, in an email. “This is a crucial question during this period of our evolutionary history.”
In its subtle shape, the pectoral girdle of Little Foot indicated to the researchers that the hominin did exploit trees for its survival, perhaps for acquiring a meal or to avoid becoming one. That lines up with research last year on the specimen’s vertebrae, which suggested Little Foot was capable of head movements (useful for climbing) that go beyond modern human capacities. That said, Little Foot was still bipedal, featuring the upright gait associated with humans. The new finding brings up an interesting comparison with Ardi (a specimen of Ardipithecus ramidus), a lesser-known ancient relative from 4.4 million years ago. Paleoanthropologists recently suggested that Ardi’s hands were built for swinging in trees, though some experts disagreed, saying Ardi was more human-like than ape-like. Though the fossil record is as ossified as can be, the conclusions drawn from the bones we pull from the ground remain fickle. It’ll take some time to see whether the interpretations of Little Foot’s lifestyle, drawn from these shoulder bones, stick.
Little Foot’s pectoral girdle is the earliest evidence of such a skeletal structure so close to when hominins split off from ape and bonobo ancestors. That upper limb is a crucial piece of the puzzle, though Carlson said it can only tell us so much.
“As special as Little Foot is, it is only one individual,” he explained. “While we are still intensely investigating other anatomical regions of the Little Foot skeleton, we also must continue to appreciate the growing morphological variability that appears to exist within the early hominin fossil record, for example in Australopithecus.”
Based on their comparisons, Carlson’s team determined that Little Foot’s shoulder structure may be a good indicator for what that structure looked like in even older human relatives, in the 7- to 8-million-year-old time frame. Such a discovery would make Little Foot look like a spring chicken. But until that happens, looks like we’re stuck with one of the most complete Australopithecine fossils ever found, the continued analysis of which reveals new details and theories with each pass. Woe is us!