Double act at Dmanisi: evidence for two lineages?

Despite being a subscriber to the likes of Science Direct and Wiley Online Library for my Palaeo-related new release alerts, sometimes one still has to rely on more informal contacts for the necessary heads-up. And on this occasion I have my newly forged Google+ contacts Palaeontology Rocks and the wider Human Evolution community to thank for this latest blog posting.

Not a year has passed since the revelatory unearthing of Skull 5 at the famous Georgian fossil site of Dmanisi. The discovery received much coverage in the blogosphere, therefore please refer to the following sources (1, 2 and 3) for your better informed background information.

Jaw-droppingly different? Two of the mandibles recovered from the Dmanisi assemblage.

Prior to the finding, interpretations of the Dmanisi collection were at best as flaky as a Stone Age tool. Dated close to 1.8 million years ago (Ma), the original remains sported an odd “mosaic” of features. But were they suggestive of Homo erectus (“standing man”, the first to cook meat), Homo habilis (“handy man”, the first to make tools), or an altogether new species dubbed Homo georgicus (“Georgian man”, western Asia not southeast USA)? 

The discovery of Skull 5 (catalogued as D4500) was major in that it apparently unified the Dmanisi fossils with those of Homo habilis in Africa and Homo erectus in Asia. Rather than being separate species, all the fossils were most likely representative of a single evolving lineage. Our family tree it seemed was in need of a pruning…

Now to the new development. Published in PLoS One last week, a study of the Dmanisi jawbones (see photo) contests the above interpretations of Skull 5. Marked shape differences in the jaws, unrelated to the size or sex of the individual who chompsed with it, are present early in growth. This, according to the authors, is evidence of difference not similarity.

What’s more, the argument is supposedly strengthened by the uncertain dating of the earth at Dmanisi. Therefore, on the grounds of the uncertain ground and the developmental differences in mandibular (jawbone) morphology (shape), more than one lineage could well be represented at Dmanisi.

Source: Bermudez de Castro, JM. et al. 2014. On the variability of Dmanisi mandibles. PLoS One 9(2): e88212

Sediba fossils may yet yield mummified skin

Australopithecus sediba - the 1.97 million-year-old extinct human relative causing contention at the foot of the human family tree - may yet have more surprises in store.

Researchers studying the fossils of a woman and young boy are optimistic that a thin layer coating the bones may be mummified skin. If so, this would be the first evidence of any soft tissue preserved in an extinct human relative.

Such a find could have monumental implications for the study of human evolution, as skin contains the vital ingredient of life - DNA. Tests could then be performed to detect signatures of interbreeding for instance, between Sediba and other hominins.

What’s more, stone tools found in the surrounding area may also have been fashioned by A. sediba, but caution must be shown when making the latter assumption.

According to discoverer Lee Berger from the University of Witwatersrand in Johannesburg, “what we have never found is direct association of an early hominid species with lithics [stone tools].”

Sediba was discovered in 2008 at South Africa’s notoriously rich Cradle of Humankind site and later described in detail in a series of Science papers. Remnants of the pair’s last meal is embedded in their teeth and their diet is also being investigated by examining their enamel.

Translating as “spring” or “fountain” in a local dialect, Sediba’s position in the story of human evolution remains largely unanswered. Donald Johansson, discoverer of Lucy – the 3.2 million-year-old Australopithecus afarensis female – even went as far as to suggest that Sediba belonged to our genus, Homo.

Read the New Scientist news story here.

Nature news article: Calloway, E. 2011. Fossils raise questions about human ancestry. Nature. doi:10.1038/news.2011.527

A fountain of questions offered by our latest ancestor

Despite the assertions made and many matters resolved by the new research on Australopithecus sediba, questions remain unanswered. How, for example, does Sediba compare to other extinct human species such as “handy man” Homo habilis? Sediba’s brain volume is well short of handy man’s 600cc, who also appears at earlier dates of 2.3 million years ago. Following the news that stone tools predate the genus Homo, what characteristics actually define our genus? Where to draw the line between Australopithecus and Homo? How can Sediba give rise directly to H. erectus when the latter’s brain capacity averages of 850cc? Where do the contemporary Dmanisi fossils fit into this updated story of human evolution?

From the Cradle: Sediba springs into picture of human evolution

They lived 2 million years ago and perished by falling into a crevice. They could walk upright but were equally efficient climbers. Their brains, hips and limbs were tantalisingly between ape and human. Their name: Australopithecus sediba.

The fossilised skull of a young, male Australopithecus sediba. His brain capacity of  420 cubic centimetres is roughly a third the volume of a typical adult male human brain.

In 2008, a bone pit discovered at the Cradle of Humankind, South Africa yielded bones of sabre-toothed cat, hyena and wild-dog. Alongside these were the partial remains of an ape-like human, later crowned a new species Australopithecus sediba. Yesterday in Science a new collection of papers examining the A. sediba fossils were published with fascinating new insights. The latest analysis of sediba, meaning “fountain”, has certainly sprung a few surprises.

Sediba stood at just over 4 feet and had a brain volume of 420 cubic centimetres, more chimpanzee than human. Despite this Sediba’s brain shows signs of reorganisation into a more human shape and level of sophistication, which could indicate the ability to plan. Sediba also has strikingly human hand bones albeit with a longer thumb adapted to an arboreal lifestyle in the trees.

The team of researchers found all this by conducting a computerised 3D scan of sediba’s brain, a hip reconstruction and a close examination of the hands and feet. “What is remarkable about Australopithecus sediba is that, as a field, it is a discovery we never thought would be made: a bona fide transitional species,” says Lee Berger, Palaeontologist at the University of the Witwatersrand, South Africa.

The age of the fossils at 2 million years and their mixture of human and ape features leads the authors to propose a direct role in our ancestry. Sediba, is thought by some, to be ancestral to Homo erectus in which case, also to our own species Homo sapiens, thus diverting the focus of human evolution to southern Africa.

Sediba was announced to the world in April 2010 and was discussed in the following Science podcast – Australopithecus sediba, interview Lee Berger. “Sediba may very well be the Rosetta stone that unlocks our understanding of the genus Homo“, Berger commented back then. Donald Johanson, co-discoverer of the famous 3.2 million year old Lucy skeleton, was so convinced of its keystone importance that he bullishly proclaimed, “it’s Homo“. Where to draw the line between Australopithecus and Homo is the key question and surely the object of future research.

As Berger neatly professes, “these skeletons are going to be studied by humans for as long as humans study themselves”.

Paper: Carlson, AJ., et al. 2011. The endocast of MH1, Australopithecus sediba. Science 333, 1402-1407.