26 March 2012

Could the ‘Red Deer Cave’ people be a new species?

By
The fossilised remains of an ancient human-like species have been found in southwest China. Do these Red Deer Cave people belong to our species, or a previously unknown species? Co-lead researcher Darren Curnoe explains.
red deer cave skull-hominin-species

A skull unearthed from the Red Deer Cave in China.
Credit: Darren Curnoe

Recently, the world media was abuzz with talk of a new human-like species found in China and dated to between 14,500 and 11,500 years old. With Ji Xueping from the Yunnan University in China, I co-led the team that made the discovery of the ‘Red Deer Cave’ people and brought their significance to the attention of the scientific and broader community.

One of the main questions surrounding the find, and a major source of speculation in the media, has been the scientific classification of the remains. Do the Red Deer Cave people belong to our species, Homo sapiens, or some other, perhaps a new species?

Right up front I need to say that despite many media reports to the contrary, we did not announce a new species.

We were careful in both our scientific publication in PLoS One and media comments to emphasise the complex nature of our find. We outlined two possibilities in our research, and didn’t choose between them. To the media, I did state that I thought the evidence was finely weighted towards the Red Deer Cave people possibly representing a new evolutionary line, and at the very least, this would be an important line of enquiry for us to explore in the future.

But, many media outlets were not capable of dealing with these scientific complexities within the context of a 24-hour news cycle and the fast-paced world of Internet news. Some quotes attributed to me were not even my words! Of the more than 50 journalists who contacted us directly for comment in the first week or so after the publication of our work, I’m pleased to say that many of them did our work justice.

So, how did our colleagues react? The views published in the media covered every possible interpretation of the remains: they are “an unfortunate overinterpretation and misinterpretation of robust early modern humans, probably with affinities to modern Melanesians” (E. Trinkaus, Washington University in St. Louis, quoted by National Geographic News) to “these humans were modern humans that spread into east Asia and interbred with archaic humans that were around at the time…(or) evolved these archaic or more primitive features independently, because of isolation” (Isabelle de Groote, Natural History Museum London, quoted by the ABC’s PM Program). Colin Groves at the Australian National University in Canberra was quoted saying, “I think it is potentially very important, telling us something about species close to us but not quite ‘us’” (Sydney Morning Herald).

Of course, behind each of these comments, like my own, lies a set of assumptions and biases about what counts as reasonable evidence, how species should be defined conceptually and how they might be identified in the fossil record, as well as the sometimes-unshakeable belief in a particular scenario about how recent human evolution might have occurred.

I suspect that had the remains of the Red Deer Cave people been 300,000 years old, the reactions of some my colleagues would have been quite different. So, the young age was a big surprise, and elicited some strong reactions.

Of course, the question of which species they belong to is a biological one.

I must say that despite the negative tone surrounding some comments, they do point to a major conundrum in our work bearing on a deeper debate within anthropology. Why would we find ‘early modern humans’ – ones who look like those alive perhaps 100,000 years ago or more in Africa – living 11,500 years ago in China, when all around them were people who looked very modern (i.e. a lot like contemporary East Asians)?

When we look at how living people and other widely distributed animals vary physically around the world we find that populations are not separated by large ‘gaps’ or discontinuities in their anatomy. When people live where their ancestors have for millennia, populations blend into each other or overlap both physically and genetically across space. They form a continuum, known technically as a ‘cline’.

If the Red Deer Cave people were simply part of the modern human population alive in East Asia towards the end of the Ice Age, why don’t they lie within a continuum?

There are modern skulls from sites in Indonesia, Malaysia, Thailand, Laos, Vietnam and China from around the same time. Yet, all of them are unmistakably modern in appearance, despite some of them being ‘rugged’. Remains of these types were even recovered from sites only a few hundred kilometres away from where the Red Deer Cave people were found.

What stands out in our study is a ‘gap’ between the anatomical features of the Red Deer Cave people and the modern humans around them that points towards a genetic and historical break. The bones and teeth of the Red Deer Cave people possess a range of features that are either rare in, or absent from, modern humans.

There is also a set of features of our bones and teeth that are thought to set sapiens apart from other two-footed apes like our Neandertal cousins. In my experience, and that of many of my colleagues, modern humans have large and vertical chins, with several finer features that make our chin triangular in shape; in the rare cases where a modern human has a brow ridge, it is usually divided into two halves about mid-way; our braincases are tall with long and steep foreheads and long and arched posterior parts (the parietal bones); our faces are narrow and tucked well beneath the front part of our brains; our noses and eye sockets are narrow; and our jaws are flat with small molar teeth that have branching tooth roots.

This ‘package’ of features is missing from the Red Deer Cave people. They possess a mosaic of ancestral (or ‘primitive’ in an evolutionary sense) features, modern-human like and Neandertal-like traits, and some features that are unique to them. Modern human skulls don’t possess this set of characters, whether they are 150 or 150,000 years old, and no matter where they lived.

All of this hints at the possibility that the Red Deer Cave people might indeed represent a new line, perhaps even a new species.

As we go forward, it will be necessary to debate our findings further in the scientific literature, just as much as we need to research these and other materials to test our current conclusions. Our colleagues will scrutinise every last feature and data point in our article. After all, significant claims must be backed up by significant evidence.

Finally, if we’re successful in obtaining DNA from the Red Deer Cave people’s remains, the exact nature of their relationship to us should be revealed, opening the door to new and exciting debates.

Darren Curnoe is an evolutionary biologist from the University of New South Wales in Sydney.
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