The pelvis plays a pivotal role in the way our species moves and gives birth. But the details of when and how the pelvis changed shape in relation to other human-defining traits, such as walking upright on two feet and large brain size at birth, are hotly debated. Now, a new study led by Museum paleoanthropologists provides insight about a critical period in the evolution of the human family based on a description of a partial pelvis from a 1-million-year-old human relative, Homo erectus, from Eritrea.
“The unique pelvic shape of humans is a defining part of our anatomy, but the pelvis is delicate and does not often preserve. Any fossil pelvis that we find is critical to unraveling the story of human evolution,” says Ashley Hammond, an assistant curator in the Museum’s Division of Anthropology and lead author of the study. “What is interesting about this pelvis is that the anatomy does not look exactly like other pelves attributed to Homo erectus, and quantitatively, it cannot be distinguished from modern human pelves.”
In hominins—the group that includes modern humans, our immediate ancestors, and other extinct human species—the pelvis forms a sturdy base that supports and balances the weight of the upper body for walking or standing on two legs. But the modern human pelvis is thought to be distinct from other hominins in several ways.
The earliest known pelvis of a fossil Homo sapiens—known as Omo 1, which lived 195,000 years ago—is within the range of modern human variation. But when researchers look a bit further back in evolutionary history to Homo erectus, which has modern human-like body proportions and stature, the evolutionary state of the pelvis is less clear. Only two hipbones have been clearly linked to H. erectus, and neither has the hallmark features of modern human pelvic bones. However, these comparisons are problematic because the two individuals, one of whom was a juvenile, suffered from illnesses, and the bones were recovered in many fragmented pieces.
There are other hipbones from the Early Pleistocene—the period of time between about 2.5 million years and 780,000 years ago—that have been assumed to be H. erectus despite a substantial amount of diversity in shape. For this reason, researchers have assumed that there may have been considerable variation in the pelvis bones of Early Pleistocene hominins, perhaps even within a single species, but that no hominin from this period had a modern human-like pelvis.
The pelvis from Eritrea studied by Hammond and her colleagues provides compelling new data, with researchers finding that the bones likely represent H. erectus but is not discernably different from a modern human pelvis, contradicting long-held assumptions about how the human evolutionary story unfolded.
“This fossil suggests that aspects of modern human pelvic anatomy could have evolved earlier than we thought, perhaps in a later Homo erectus population from East Africa,” says Hammond. “It is also particularly exciting that this early Homo pelvis was found in Eritrea because this is part of East Africa that is relatively unexplored paleontologically, expanding our view of human evolution to include other regions in Africa.”