Fossil lower jaw

Fossil lower jaw from Ethiopia claims to push back Homo to 2.8 million years ago © Kaye Reed

More complexity in early human evolution in East Africa

Two new studies show the evolution of early humans to be more complex, and the Natural History Museum’s human origins expert Chris Stringer says, makes us question what it is to be human.

A study on a fossil lower jaw with teeth, found at Ledi-Geraru in the Afar region of Ethiopia in 2013, claims to push back Homo to 2.8 million years ago. ‘This is only 200,000 years after the last evidence of Lucy’s species Australopithecus afarensis’, said Prof Stringer.

A afarensis, a small-brained upright-walking species, may have been the direct ancestor to our genus Homo. And being so near to the date of the fossil in this study suggests the jawbone, known as LD 350-1, could have belonged to the earliest species of human.

‘The fossil is primitive like afarensis at the front, but distinguished from afarensis by the shape of its premolars and molars and the reduced robusticity of the jaw further back.’ Although it is in the genus Homo, the authors say the specimen is too incomplete to classify to a species.

This study was led by Dr Villmoare of University of Nevada Las Vegas and published in the journal Science.

Another study reveals a virtual reconstruction of a Homo habilis jawbone fossil from 1.8 million years ago. The fossil, known as OH7, is distorted from preservation so this reconstruction will make it easier to compare it with other early Homo fossils, and will help clear up some controversy about the species.

The jawbone, along with partial braincase and hand bones from Olduvai Gorge, Tanzania, was published in 1964 and the species H. habilis was named from them.

The controversy has been about whether H. habilis should be a new species or not, and where it sits in the human family tree. Also, some of the H. habilis fossils show quite different traits from each other, some with a small-sized body and brain, and others larger-sized and sometimes given a separate species name Homo rudolfensis. This suggests the H. habilis species has a lot of variation in form, or that the fossils actually belong to different species.

The new research helps clear the confusion. It reveals the jaw is much more primitive in shape and tooth form than previously thought. While its brain size is larger than previously thought, it is clearly different from some other key H. habilis fossils.

So, in addition to Homo erectus that also lived in the area at this time, if habilis is distinct from rudolfensis, Chris Stringer says ‘this means there could already have been three early Homo lineages in East Africa by 1.8 million years ago: habilisrudolfensis anderectus.

This study was led by Dr Spoor of the Max Planck Institute for Evolutionary Anthropology and is published in Nature.

What it is to be human

Stringer comments on the research, ‘These new studies leave us with an even more complex picture of early humans than we thought, and they challenge us to consider the very definition of what it is to be human.

‘Are we defined by our small jaws and teeth, our large brain, our long legs, habitual tool-making and meat-eating, or some combination of these or other traits?

‘If we require the combined presence of several traits to recognise a fossil as Homo, many of these specimens are simply too incomplete to make a confident diagnosis, and that is true overall until we arrive at the more complete remains and behavioural evidence of Homo erectus.

Parallel origins?

Stringer concludes, ‘Is the diversity we already see by 1.8 million years ago the result of evolution from a single primeval species of Homo, or could these different ‘Homo' lineages have evolved in parallel from separate australopithecine-like ancestors? In which case they cannot all justifiably be assigned to the one genus Homo.’

‘The human-like features shown by Australopithecus sediba in South Africa at ~1.95Ma are likely to have developed independently of the processes which produced Homo in East Africa, showing that parallel origins are a distinct possibility.’  

Diagram showing Homo fossils from East Africa

Diagram showing Homo fossils from East Africa. The 2.8 million-year-old Ethiopian fossil jawbone LD350-1 has not been designated a species yet (along with another fossil 5431) but does have human characteristics.

Related information

Villmoare B, Kimbel WH, Seyoum C, Campisano CJ, DiMaggio EN, Rowan J, Braun DR, Arrowsmith, JR and Reed KE (2015) Early Homo at 2.8 Ma from Ledi-Geraru, Afar, EthiopiaScience (347) 1352-1355.