A new study reveals that the latest common ancestor of chimpanzees and humans had shoulders that were similar in shape to the shoulders of modern African apes. According to a report from Live Science, the findings of the study suggest that humans gradually came down from the trees and began living on flat land.

Humans diverged from chimpanzees roughly 7 million years ago. The latest common ancestor was the hominid that split off into two different species, one that remained living in the forests, and one that developed and adapted to a life on the ground.

Biologists have a good idea of what early hominids looked like, but there are still two opposing theories for which was truly an ancestor to both modern species. One scenario suggests that the similarities between humans and modern apes were inherited from the same ancient species, which would explain the genes that are present in both.

The other theory explains that the similarities between the primates evolved independently, but through similar processes. The last common ancestor may have had traits that were more primitive than those observed in modern apes, suggesting that there are still minute differences in the traits we believe to be similar.

The shoulder is key to understanding the differences between modern apes and humans, and at which point the common ancestor may have split off into two distinct lineages. As early humans began using tools, they had less of a use for being able to grab and swing from tree branches.

The model described in the study suggests that the transition from trees to flat land was slow and gradual. The shoulder adapted from needing to support weight from below, with a wide range of motion, to a function that supported working with tools and building things. The evolution of the shoulder may be instrumental in describing the transition from apes to humans over time.