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A 3.3-million-year-old fossil with a missing rib tells us a lot about human evolution

Our spinal column is different from that of apes because — yes, you guessed that right — we walk upright on two feet. But when did our distinctive back structure first evolve? The skeleton of an early human ancestor found in Africa shows that some features were already established at least 3.3 million years ago, earlier than previously thought.

The fossil was found to have only 12 rib-bearing vertebrae, the same number as modern humans — and one fewer than most apes. That feature had previously been observed in early humans dated to only 60,000 years ago or later. The findings, published today in the journal Proceedings of the National Academy of Sciences, show that the general structure of our spinal column was already emerging 3.3 million years ago.

 Photo by Zeray Alemseged, University of Chicago
Vertebrae of the Selam skeleton

The fossil described in the study belongs to a 2-and-a-half-year-old child from the early human species Australopithecus afarensisthe same species as the famous Lucy skeleton. The fossil, called Selam, was discovered in Dikika, Ethiopia in 2000, and has been cleaned and studied ever since. It represents the most complete spinal column of any early human relative — including vertebrae, neck, and rib cage.

 Photo by Zeray Alemseged, University of Chicago
Selam’s full skeleton

“This type of preservation is unprecedented, particularly in a young individual whose vertebrae are not yet fully fused,” study co-author Zeresenay Alemseged, a professor of organismal biology and anatomy at the University of Chicago who discovered Selam, said in a statement.

Selam was sent from the National Museum of Ethiopia to the European Synchrotron Radiation Facility in Grenoble, France, so that its bones could be analyzed using high-resolution imaging technology. The ancient specimen was shown to have only 12 rib-bearing vertebrae and 12 pairs of ribs. That is fewer than most apes and the same as modern humans. It also represents the only known evidence for the presence of 12 rib-bearing vertebrae in early humans prior to 60,000 years ago, the study says.

Researchers also saw that Selam had the distinctive joint transition from rib-bearing vertebrae to lower-back vertebrae found in other early human ancestors. Selam, however, represents the earliest and most complete example, the study says.

The findings suggest that 3.3 million years ago our early ancestors were already on their way to develop a back structure that eventually allowed modern humans to walk straight on two legs. “This structure and its modification through time is one of the key events in the history of human evolution,” Alemseged said.

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