Most evolutionary biologists agree that tools were the driving force that led ancient primates down from the trees and onto flat land, where they eventually started walking upright and evolved into modern day humans. According to a press release from Eurekalert, a new study from scientists at UC San Francisco presents new fossil evidence for this transition and may even point towards the identification of a more recent common ancestor.

The split between humans and our closest cousins occurred roughly 7 million years ago. As we left chimpanzees and bonobos swinging from tree branches, an important change happened.

Nathan Young, PhD and assistant professor at the UC San Francisco School of Medicine and lead author of the recent study explains, “Humans are unique in many ways. We have features that clearly link us with African apes, but we also have features that appear more primitive, leading to uncertainty about what our common ancestor looked like.”

Young’s study suggests that certain parts of the ancestor probably looked similar to a modern day chimp or gorilla. The shape of the apes’ shoulders reveals a steady change in function and therefore behavior over time, mirroring the adoption of tools that helped humans find new ways to acquire shelter and food.

By examining 3-D measurements of fossilized shoulder blades from early hominids and African apes, Young found that the shape of humans’ shoulder blades are somewhere in between orangutans and African apes. They are primitive in many lights, but they also differ significantly from most apes around today.

Australopithecus fossils reveal that there was a distinct diversion from tree-dwelling apes to land-based creatures. The evidence provided by shoulder blades reinforces the evolutionary history humans have written so far, and scientists hope to uncover more fossils that will further illuminate the question of how our species developed over time.