Our ancient relative who “walked like a human, but climbed like an ape”

An international team of scientists from New York University, the University of the Witwatersrand, Liverpool John Moores University and others announced today the discovery of two-million-year-old fossil vertebrae from an extinct species of ancient human relative. 

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 of the same female from Malapa, South Africa, together with previously discovered vertebrae, form one of the most complete lower backs ever discovered in the early hominid record and give insight into how this ancient relative walked and climbed.

The fossils were discovered in 2015 during excavations of a mining trackway near Johannesburg.

Malapa is the site where, in 2008, Professor Lee Berger from the University of the Witwatersrand and his nine-year old son discovered the first remains of a new species of ancient human relative named Australopithecus sediba.

Bipedalism

Rather than risking damaging the fossils, they were prepared virtually after scanning with a Micro-CT scanner and reunited with fossils recovered during earlier work and found to articulate perfectly with the spine of the fossil skeleton, part of the original Type specimens of Australopithecus sediba first described in 2010.  

Researchers have nicknamed the female skeleton “Issa,” meaning protector in Swahili. The discovery also established that like humans, 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 well adapted they were to walking on two legs,” says Professor Scott Williams of New York University and Wits University and lead author on the paper published today in the journal e-Life, which includes data collection from Dr Mark Grabowski, programme leader of the MSc in Human Evolution at LJMU. 

    "From the fossil we can see that this species walked in a way that was somewhat similar to modern humans" - Dr Mark       Grabowski, paleoanthropologist, LJMU

Previous studies of the incomplete lower spine hypothesised that sediba would have had a relatively straight spine, without the curvature, or lordosis, typically seen in modern humans. They further hypothesised Issa’s spine was more like that of the extinct species Neandertals and other more primitive species of ancient hominins older than two million years. Lordosis is the inward curve of the lumbar spine and is typically used to demonstrate strong adaptations to bipedalism.

However, with the more complete spine, and excellent preservation of the fossils, the present study found the lordosis of sediba was in fact more extreme than any other australopithecines yet discovered, and the amount of curvature of the spine observed was only exceeded by that seen in the spine of the 1.6-million-year-old Turkana boy (Homo erectus) from Kenya, and some modern humans.

“While the presence of lordosis and other features of the spine represent clear adaptations to walking on two legs, there are other features, such as the large and upward oriented transverse processes, that suggest powerful trunk musculature, perhaps for arboreal behaviors,” says Professor Gabrielle Russo of Stony Brook University and an author on the study.

Physiology 'in transition'

Strong upward oriented transverse spines are typically indicative of powerful trunk muscles, as observed in apes. Professor Shahed Nalla of the University of Johannesburg and Wits who is an expert on ribs and a researcher on the present study says: “When combined with other parts of torso anatomy, this indicates that sediba retained clear adaptations to climbing”.

Previous studies of this ancient species have highlighted the mixed adaptations across the skeleton in sediba that have indicated its transitional nature between walking like a human and climbing adaptations. These include features studied in the upper limbs, pelvis and lower limbs.

“The spine ties this all together,” says Professor Cody Prang of Texas A&M, who studies how ancient hominins walked and climbed. “In what manner these combinations of traits persisted in our ancient ancestors, including potential adaptations to both walking on the ground on two legs and climbing trees effectively, is perhaps one of the major outstanding questions in human origins.”

The study concludes that sediba is a transitional form of ancient human relative and its spine is clearly intermediate in shape between those of modern humans (and Neandertals) and great apes.

"This is an amazing fossil discovery, a nearly complete lower back from a single individual of a species that lived around 2 million years ago. From the fossil we can see that this species likely retained the ability to climb trees like our more distant ancestors and other great apes, but also walked in a way that was somewhat similar to modern humans," said Dr Mark Grabowski of LJMU.

Note: The virtual fossils published in the new study are free to download on Morphosource.org