spine
May 24, 2017

3.3-million-year-old fossil shows us how the spine evolved

Thanks to the discovery of an astonishingly complete fossil of an ancient toddler in Ethiopia, a team of researchers has now discovered that key elements of the human spinal structure already were in place 3.3 million years ago – far earlier than experts had previously believed.

Writing in the journal Proceedings of the National Academy of Science (PNAS), Carol Ward, a pathology and anatomical sciences professor at the University of Missouri School of Medicine, and her colleagues revealed that they discovered the well-preserved remains back in 2000.

The child, identified as Selam (which means “peace” in the Ethiopian Amharic language) was a 2.5-year-old female belonging to the species of early human relatives known as Australopithecus afarensis, the study authors explained in a statement. Her spinal column was said to be the most complete of its kind discovered to date and included the neck, rib cage and vertebrae.

Since Selam’s spine was so complete and so well preserved, Ward’s team was able to determine that she had only 12 rib-bearing vertebrae – the same number as modern humans and one fewer than is typically found in most apes, according to The Verge. Previously, this characteristic had only been found in ancient humans dating back roughly 60,000 years, the website added.

The discovery established that “the human type of segmentation and numbering of our backbone emerged 3.3 million years ago, and this fossil provides us for the first time the hard evidence, the fossil evidence, to confirm that indeed the structure is as ancient as we're claiming it now to be,” senior author Zeray Alemseged told NPR.

Findings confirm that human ancestors were upright walkers

Alemseged, a professor of organismal biology and anatomy at the University of Chicago, found Selam while working at Dikika, Ethiopia, 17 years ago. In the years that followed, he and his lab assistant Christopher Kiarie worked to clear away the sandstone surrounding the bones, and used imaging technology to further analyze the structure of the early human predecessor.

In a statement, he explained that “continued and painstaking research on Selam” confirmed 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.” He added that this degree of preservation was “unprecedented, particularly in a young individual.”

Selam likely died unexpectedly, and then drifted into nearby water, where her body was covered in sediment that eventually hardened into sandstone, said NPR. While her vertebrae were still in the process of forming and fusing together, she was clearly found to possess the same number of neck and mid-back vertebrae as modern humans (seven and 12, respectively), they added.

The analysis revealed “unique” information that has helped provide new insight into “one of the key milestone events in human evolution... the transition from the more ape-like arrangement of the backbone to the more humanlike arrangement of the backbone,” Alemseged told NPR. While the research confirms that Australopithecus afarensis could walk upright on two legs, he pointed out that there were “some minor differences” between their spines and those of modern humans.

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Image credit: Zeray Alemseged, University of Chicago