3.3-Million-Year-Old Fossil Reveals Origin of Human Spine

The vertebrae of the Selam skeleton, scientists’ first complete look at the evolution of the human spine. (Zeray Alemsegad/University of Chicago)

(CN) – Analysis of a 3.3 million-year-old fossil skeleton shows that portions of the human spinal structure which enable walking motions developed millions of years earlier than previously thought.

The fossil, known as “Selam,” was found in Dikika, Ethiopia, in 2000 by Zeresenay Alemseged at the University of Chicago. Alemseged analyzed the specimen’s largely intact spinal column, which includes its neck, vertebrae and ribcage. The findings were presented Monday in the journal the Proceedings of the National Academy of Sciences.

Selam, which means “peace” in the Ethiopian Amharic language, was an early human relative from the same species as the famous Lucy skeleton – the Australopithecus afarensis.

Alemseged and his team slowly chipped away at the sandstone surrounding Selam, then used advanced imagery tools to further analyze the fossil.

“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.”

Several aspects of the human ribcage and spinal column are shared among primates. However, the human spine also reflects our distinctive form of walking upright on two feet – such as having more vertebrae in the lower back. How and when our distinct features evolved has remained unknown until now due to a lack of preserved, complete sets of vertebrae in the fossil record.

“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,” said lead author Carol Ward, a professor at the University of Missouri School of Medicine. “Selam has provided us the first glimpse into how our early ancestors’ spines were organized.”

The team took Selam to the European Synchrotron Radiation Facility in Grenoble, France, where they used high-resolution imaging technology to visualize its bones.

“This technology provides the opportunity to virtually examine aspects of the vertebrae otherwise unattainable from the original specimen,” said study co-author Fred Spoor, a professor at the University College London.

The analysis showed that Selam had the distinguishing thoracic-to-lumbar joint transition found in other fossils of human relatives. However, it is the first specimen to show that like modern humans, our earliest ancestors had only twelve thoracic vertebrate and twelve pairs of ribs – 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 of Western University of Health Sciences in Pomona, California, a co-author of Monday’s paper.

The physical layout represents a transition toward the type of spinal column that allows humans to be efficient, athletic walkers.

“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.”

 

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