The fossilized shoulder joint of an early hominin called the “Little Leg” suggests that the upright species were also swaying through trees, to a skill long passed among modern humans.
About 3.7 million years ago in what is now South Africa, a human relative stepped into two evolutionary moments: our tendency to spend time in trees and our emerging preference for walking. This specimen, called StW 573 or Little Foot, was a Australopithecus prometheus. The fossil was finally completely excavated in 2018, more than 20 years after its discovery, when paleontologists finished extracting the fossil from the breccia in which it was enclosed. Immediately, the Little Foot offered a remarkable look at human origins.
Research describing the morphology of the shoulder joint has been 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 formation of the belt with that of other human relatives, including large monkeys, the team analyzed how the Little Leg and others of its species came around.
“By understanding how hominin early shoulder joints are structured and, more broadly, how shoulder blades are able to move on the torso, we can understand how they used their upper limbs while interacting with the environment.” said Kristian Carlson, an anthropologist at the University of Southern California and lead author of the new paper, in an email. “This is a crucial question in this period of our evolutionary history.”
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In its subtle form, the pectoral girdle of the Little Leg indicated to researchers that the hominin exploited trees for its survival, perhaps to obtain a mass or to avoid becoming one. That aligns with research last year on the vertebrae of the specimen, which suggested that the Little Leg was capable of head movements (useful for climbing) that go beyond modern human capabilities. That being said, Little Foot was still bipedal, showing the vertical gait associated with humans. The new discovery brings an interesting comparison with Ardi (a copy of Ardipithecus ramidus), a lesser-known old relative 4.4 million years ago. Paleoanthropologists recently suggested that Ardi’s hands were built to swing in the trees, although some experts disagreed, saying that Ardi was more human than a monkey. Although the fossil record is very ossified, the conclusions drawn from the bones we draw from the ground remain volatile. It will take some time to see if the interpretations of Little Foot’s lifestyle, drawn from these shoulder bones, stick together.
Little Foot’s pectoral girdle is the oldest evidence of such a skeletal structure so close to the time when the hominids parted ways with their monkey and bonobo ancestors. That upper limb is a crucial piece of the puzzle, although Carlson said he can tell us so much.
“No matter how special the Little Leg is, there is only one individual,” he explained. “While still intensively researching other anatomical regions of the Small Leg skeleton, we must continue to appreciate the increasing morphological variability that appears to exist in the earliest fossil records of hominin, for example in Australopithecus.”
Based on their comparisons, Carlson’s team determined that Little Foot’s shoulder structure could be a good indicator of what that structure looked like to even older human relatives in the 7- to 8-million-year-old age range. Such a discovery would make the Little Leg look like a spring chick. But until this happens, it seems that we are stuck with one of the most complete australopithecine fossils ever found, whose continuous analysis reveals new details and theories with each pass. Woe to us!
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