A new study has found the human spinal structure that enables efficient walking motions was established millions of years earlier than previously thought. This 3.3 million-year-old fossil skeleton is the most complete spinal column of any early human relative; it includes the vertebrae, neck, and rib cage.
The fossil is a near complete skeleton of a 2½-year-old child known as “Selam,” which means “peace” in the Ethiopian Amharic language. The skeleton was found 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.
“Selam” is an early human relative from the species Australopithecus afarensis — the same species as the famous Lucy skeleton. After the discovery of “Selam,” Alemseged began slowly chipping away at the sandstone that surrounded the skeleton. Alemseged said in a statement:
“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.”
Now, with the sandstone removed, they have been able to use advanced imaging tools to further analyze its structure. Co-author of the study Fred Spoor, a professor of evolutionary anatomy in the Department of Biosciences at the University College London, said:
“This technology provides the opportunity to virtually examine aspects of the vertebrae otherwise unattainable from the original specimen.”
There are many features of the human spinal column and the rib cage that are shared among primates; however, the human spine also reveals our distinctive mode of walking upright. For example, the human has fewer rib-bearing vertebrae (bones of the back) than that of our closest primate relatives.
Humans also have more vertebrae in the lower back (allows us to walk effectively). It has been unknown until now when and how this pattern evolved. This has been because complete sets of vertebrae are very rarely preserved in the fossil record. Alemseged explained:
“This type of preservation is unprecedented, particularly in a young individual whose vertebrae are not yet fully fused.”
Carol Ward, a Curator’s Distinguished Professor of Pathology and Anatomical Sciences in the University of Missouri School of Medicine and lead author on the study, said in a statement:
“For many years we have known of fragmentary remains of early fossil species that suggest that the shift from rib-bearing, or thoracic, vertebrae to lumbar, or lower back, vertebrae was positioned higher in the spinal column than in living humans.
“But we have not been able to determine how many vertebrae our early ancestors had.
“Selam has provided us the first glimpse into how our early ancestors’ spines were organized.”
The researchers found that “Selam” had the unique thoracic-to-lumbar joint transition found in other human relatives. However, it was also the first to show that, like modern humans, our earliest ancestors had only 12 thoracic vertebrae and 12 pairs of ribs. Thierra Nalley, an assistant professor of anatomy at Western University of Health Sciences in Pomona, California, and an author on the paper, said:
“This unusual early human configuration may be a key in developing more accurate scenarios concerning the evolution of bipedality and modern human body shape.”
The configuration marks a shift toward the type of spinal column that allows humans to be the efficient walkers and runners we are today. Alemseged went on to say:
“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.
“This structure and its modification through time is one of the key events in the history of human evolution.”
Either way you look at this, it clearly shows how much we don’t know about ourselves.
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