New hominin species joins Lucy on the human family tree

The discovery of another species of early hominin adds further evidence that the human ancestral family tree is more diverse than previously thought.

Lower jaw fossil of the new hominin species.
Image credit: Yohannes Haile-Selassie,
Cleveland Museum of Natural History

Writing in the journal Nature, the researchers and the curator of physical anthropology at the Cleveland Museum of Natural History in Ohio, describe how they recovered the upper and lower jaw fossils of a new human ancestral species believed to have lived 3.3 million to 3.5 million years ago.

The discovery was made in the Woranso-Mille area of the Afar region of Ethiopia, the region that gives its name to the famous Lucy species, Australopithecus afarensis. 

"The new species is yet another confirmation that Lucy's species, Australopithecus afarensis, was not the only potential human ancestor species that roamed in what is now the Afar region of Ethiopia during the middle Pliocene."

The newly discovered hominin has been given the name Australopithecus deyiremeda - "deyiremeda" means close relative in the language of the Afar people.

Hominins include modern humans, extinct human species and all our direct ancestors.

Lucy's species lived some 2.9 million to 3.8 million years ago, and until recently was thought to be our only hominin ancestor, but it is becoming clearer that this is not the case.

Evidence of more than one human ancestral species living over 3 million years ago

The newly discovered species differs from Lucy's in terms of the shape and size of its teeth - which are more thickly enameled, and its lower jaw - which has a more robust structure. The teeth nearer the front are also smaller than Lucy's, suggesting A. deyiremeda probably had a different diet.

The researchers say the fossils also show evidence of tooth and jaw traits that were thought to have appeared much later in the human family tree.

The research team says the discovery offers the most conclusive evidence for the existence of more than one closely related early human ancestor species living more than 3 million years ago.

For a long time, it was thought that humans descended in a straight line from one pre-human species living 3 million to 4 million years ago. This theory was backed up by the fossil record - including the discovery of Lucy - until the end of the 20th century.

But then, the new century brought some surprises - researchers discovered Kenyanthropus platyops in Kenya andAustralopithecus bahrelghazali in Chad. Both these species date from Lucy's period - challenging the idea that humans descended from a single hominin species.

Researchers have not assigned the 'Burtele foot' to the new species

At first, scientists were highly sceptical following the discoveries in Kenya and Chad. But some started to change their view when in 2012, the researchers announced the discovery of the 3.4 million-year-old Burtele partial foot fossil, confirming the likelihood of multiple hominin species living at the same time 3 million to 4 million years ago.

A team of anthropologists at Harvard University said at the time the researchers announced the discovery that the Burtele foot is "very much like the Ardipithecus foot, which they believe had many hominin features, so it's likely to be a hominin."

While several features of the Burtele foot confirm that it is truly a hominin, and despite the similarity in geological age and close geographic proximity, the researchers have not assigned the partial foot to the new species due to lack of clear association - there are no skull or dental elements to go with the foot.

Nevertheless, the new discovery adds to the fossil evidence from the Woranso-Mille study area that at least two, if not three, early human species were around at the time of Lucy, the researchers note.

In fact, the new species takes the ongoing debate on early hominin diversity to another level.

"Some scientists are going to be skeptical about this new species, which is not unusual. However, the researchers think it is time that we look into the earlier phases of our evolution with an open mind and carefully examine the currently available fossil evidence rather than immediately dismissing the fossils that do not fit our long-held hypotheses."

The researchers say the growing evidence of multiple species raises important questions about how our early ancestors coexisted, shared their environment and resources.

The following video gives an account of the find and its implications, and includes comments from Dr. Haile-Selassie, the antropologist curator at the Cleveland Museum of Natural History in Ohio:

References:

1.  New species from Ethiopia further expands Middle Pliocene hominin diversity, Yohannes Haile-Selassie et al., Nature, doi:10.1038/nature14448, published online 27 May 2015, abstract.
2.  Cleveland Museum of Natural History news release, accessed 28 May 2015.