New research from the Arizona State University Institute of Human Origins appears to have found evidence that a 4.4 million year old bipedal species is distantly related to modern humans. The family tree of the species Ardipithecus ramidus, also known as “Ardi”, has been contested in scientific circles for many years, but the latest findings seem to have answered at least some of the outstanding questions.

Pieces of the Ardipithecus ramidus skeleton were first found in 1992 in the Afar Depression in the Middle Awash river valley of Ethiopia. It had a grasping big toe which suggested bipedalism, the ability to walk upright. The remains also showed reduced canine teeth, similar to those of man. Whether or not Ardi was an actual human ancestor or whether it was simply another offshoot of ape has been a matter of considerable debate.

Now it seems that the debate may have been settled, at least partially. New research published in the January 6, 2014, online edition of Proceedings of the National Academy of Science reveals the close evolutionary relationship between Ardipithecus ramidus and humans. It was published by ASU paleoanthropologist William Kimbel and co-authors by Gen Suwa (University of Tokyo Museum), Berhane Asfaw (Rift Valley Research Service, Addis Ababa), Yoel Rak (Tel Aviv University), and Tim White (University of California at Berkeley).

The new study shows similarities between the cranial base of Ardi’s skull and that of Australopithecus and modern humans but not to apes. Australopithecus is another species of pre-historic bipeds widely believed to be ancestors of modern humans. The best known example of Australopithecus is “Lucy,” found in 1974 in Hadar, Ethiopia. “Given the very tiny size of the Ardi skull, the similarity of its cranial base to a human’s is astonishing,” says Kimbel.

The cranial base is extremely useful in studying adaptive evolution and relationships because of what it reveals about brain size, posture and chewing systems. Although humans and apes share common ancestors, the human cranial base differs from that of other primates, including apes. The new work adds to the evidence linking humans to Australopithecus, and Ardipithecus.

It is hoped that, as a more complete human family tree emerges it will help scientists to understand how the human species evolved the way it did and help better explain the process of evolution in general. “The Ardi cranial base fills some important gaps in our understanding of human evolution above the neck,” says William Kimbel. “But it opens up a host of new questions…just as it should!”