New fossils in the lower part of the back are the “missing link” that resolves decades of debate, proving that the earliest hominins used their upper limbs to climb like monkeys and their lower limbs to walk like humans.
An international team of scientists from New York University, the University of Witwatersrand, and 15 other institutions announced in the journal e-Life the discovery of fossil vertebrae dating to two million years ago of an extinct species of ancient human relatives.
The recovery of new lumbar vertebrae from the lower back of a single individual of the human relative Australopithecus sediba, and portions of other vertebrae from the same female from Malapa, South Africa, together with vertebrae previously discovered, form one of the most complete lower-back vertebrae ever found in the fossil record of early hominins and provide a picture of how this ancient human relative walked and climbed.
The fossils were discovered in 2015 during excavations of a trackway beside the Malapa site, northwest of Johannesburg, South Africa.
Malapa is the site where, in 2008, Professor Lee Berger of the University of Witwatersrand and his then nine-year-old son, Matthew, discovered the first remains of what would be a new species of an ancient human relative called Australopithecus sediba.
The fossils from the site have been dated to approximately two million years old. The vertebrae described in the present study were recovered from rock.
Rather than risk damaging the fossils, they were prepared virtually after a scan at the University of the Witwatersrand, thus eliminating the risk of damaging the delicate bones positioned close together during manual preparation. After virtual preparation, the vertebrae were reassembled with fossils recovered during earlier work at the site and articulate perfectly with the spine of the fossil skeleton, part of the original Australopithecus sediba specimens described for the first time in 2010.
The skeleton’s catalog number is MH 2, but scientists nicknamed the female skeleton “Issa,” which means “protector” in Swahili. The discovery also established that, like humans, Australopithecus sediba had only five lumbar vertebrae.
“The lumbar region is critical to understanding the nature of bipedalism in our earliest ancestors and to understanding how they were well adapted to walking on two legs,” said Professor Scott Williams of New York University and the study’s lead author. “The associated series of lumbar vertebrae are extraordinarily rare in the fossil record of hominins, with only three comparable lower spines known from the entire early African record.”
The discovery of the new specimens means that Issa now becomes one of the first two hominin skeletons to preserve both the lower part of the spine and the dentition of the same individual, allowing certainty as to which species the spine belongs to.
“Although Issa was already one of the most complete skeletons of an ancient hominin ever discovered, these vertebrae virtually complete the lower back and make Issa’s lumbar region not only the most well-preserved hominin lumbar region yet discovered, but also probably the best-preserved,” says Berger, who is the study’s author and leader of the Malapa project.
The scientist adds that this combination of integrity and preservation gave the team an unprecedented look at the anatomy of the species’ lower back.
Earlier studies of the incomplete lower spine by authors not involved in the present study raised the hypothesis that sediba would have a relatively straight spine, without the curvature or lordosis typically observed in modern humans. They also proposed that Issa’s vertebral column was more like that of extinct Neanderthals and other more primitive ancient hominins over two million years old.
However, with the more complete spine and excellent preservation of the fossils, the present study found that sediba’s lordosis was indeed more extreme than any other australopithecine ever discovered, and the amount of curvature observed was only exceeded by that seen in the Turkana Boy’s spine (1.6 million-year-old Homo erectus) from Kenya and some modern humans.
“Although the presence of lordosis and other features of the spine represent clear adaptations for walking on two legs, there are other features, such as the large upward-facing transverse processes, that suggest a powerful trunk musculature, perhaps for arboreal behaviors,” concludes Gabrielle Russo of Stony Brook University and the study’s author.
Strong upward-facing transverse spines are indicative of powerful trunk muscles, as observed in monkeys.
Earlier studies of this ancient species highlighted mixed adaptations across sediba’s skeleton that indicated its transitional nature between walking like a human and climbing adaptations. This includes features studied in the upper limbs, pelvis, and lower limbs.
The study concludes that sediba is a transitional form of an ancient human relative and its spine is clearly intermediate in shape between modern humans (and Neanderthals) and great apes.
“Issa walked like a human, but could climb like a monkey,” says Berger.
