Portions of the human spine structure that enable efficient walking motions were established millions of years earlier than previously thought, according to a new study of a 3.3 million year-old fossil skeleton.
The fossil is known as “Selam,” and is a nearly complete skeleton of a 2½ year-old child discovered in Dikika, Ethiopia in 2000 by Zeresenay (Zeray) Alemseged, Professor of Organismal Biology and Anatomy at the University of Chicago and senior author of the new study.
Analysis revealed the most complete spinal column of any early human relative, including vertebrae, neck and rib cage, the university said.
“Continued and painstaking research on Selam shows that the general structure of the human spinal column emerged over 3.3 million years ago, shedding light on one of the hallmarks of human evolution,” Alemseged said. “This type of preservation is unprecedented, particularly in a young individual whose vertebrae are not yet fully fused.”
Primates share many features of the human spinal column and rib cage, but the human spine also reflects our distinctive mode of walking upright on two feet. Humans have fewer bones of the back than those of our closest primate relatives, and more vertebrae in the lower back, which allows us to walk effectively. When and how this pattern evolved has been unknown until now as complete sets of vertebrae are rarely preserved.
“Selam has provided us the first glimpse into how our early ancestors’ spines were organized,” said Carol Ward, Professor of Pathology and Anatomical Sciences at the University of Missouri School of Medicine, and lead author of the study.
The fossil was taken to the European Synchrotron Radiation Facility in Grenoble, France, where Alemseged and the research team used high-resolution imaging technology to visualize the bones.
“This technology provides the opportunity to virtually examine aspects of the vertebrae otherwise unattainable from the original specimen,” added Fred Spoor, a Professor of Evolutionary Anatomy in the Department of Biosciences at the University College London and a coauthor of the study.
The scans indicated that Selam had the distinctive thoracic-to-lumbar joint transition found in other fossils of human relatives, but the specimen is the first to show that, like modern humans, human’s earliest ancestors had only 12e thoracic vertebrae and twelve pairs of ribs. That is fewer than in most apes.
“This unusual early human configuration may be a key in developing more accurate scenarios concerning the evolution of bipedality and modern human body shape,” said Thierra Nalley, an Assistant Professor of Anatomy at Western University of Health Sciences in Pomona, California, who is also an author on the paper.
The configuration found in Selam marks a transition toward the type of spinal column that allows humans to be efficient, athletic walkers and runners, the university said.
“We are documenting for the first time in the fossil record the emergence of the number of the vertebrae in our history, when the transition happened from the rib-bearing vertebrae to lower back vertebrae, and when we started to extend the waist,” Alemseged said. “This structure and its modification through time is one of the key events in the history of human evolution.”
The study is published in the Proceedings of the National Academy of Sciences.
Source:.University of Chicago