Lost in the myths of time

Recent evolutionary theories based on DNA research have grabbed the headlines but, asks Chris Stringer, do they even come near the truth?

A few weeks ago, I had a rather surprising experience. My colleagues and I published what we thought was a significant note about ancient human DNA in the journal Science, and the Natural History Museum sent out a press release. But with one honourable exception (this newspaper), it made about as much impact as the Conservatives managed at the last election. Why?

The fact that an election was about to happen certainly didn't help; nor did Science publishing a paper in the same issue describing a huge dinosaur find from Egypt. But the most relevant factor was probably the nature of the note itself.

In January there had been widespread publicity over research claiming that ancient DNA had been recovered from the 60,000-year-old Mungo skeleton from Australia. It was further claimed that the results challenged the widely held theory known as "Out of Africa" – that our species had a recent African origin.

Our note, based on experience with European fossils, raised the uncomfortable possibility that the published DNA resulted from contamination, and that even if the reported DNA was authentic, the results raised no serious challenge to Out of Africa at all. The original authors maintained that they had probably recovered genuine ancient human DNA, but agreed that their results did not really challenge Out of Africa. This situation, where a sensational challenge to orthodoxy generates extensive publicity, but the ensuing return to reality is virtually ignored, is a familiar one to many scientists – and one that I have witnessed several times recently.

To examine some of these cases in more detail, I need to outline the debate about modern human origins. The idea that we all have an African origin appears in Darwin's writings, some 50 years before evidence began to emerge to confirm it.

But when was our common African origin? Over the past 20 years, two evolutionary models have dominated the debate. The first, Multiregional Evolution, argues that the common ancestral population lived in Africa about two million years ago. Soon after that, early humans spread from Africa for the first time, and so began the evolution of our species. The descendant populations evolved in each inhabited area of the world and exchanged their genes through interbreeding, so that the features of modern humans gradually accumulated in each of the populations. Under Multiregional evolution, non-modern people such as the Neanderthals, who lived in Europe about 50,000 years ago, could have been ancestral to the Cro-Magnons, the succeeding modern humans, and thus ancestral to recent Europeans.

In contrast, the Out of Africa model argues that, while there was indeed a spread of early humans from Africa nearly two million years ago, followed by further evolution, there was only one region where evolution produced modern humans – Africa. About 100,000 years ago, these early moderns began to spread to the rest of the world, replacing the older lineages elsewhere, such as the Neanderthals. So Out of Africa is also known as the Replacement Theory. As those early moderns spread out, they carried with them modern behaviour, such as complex technologies, language, symbolism and art.

Multiregional Evolution and Out of Africa were first developed some 20 years ago from the fossil evidence, but in 1987 a new factor entered the debate – DNA. Data from our genes, initially just a trickle of pioneering studies, has become a flood that has threatened to overwhelm fossil studies, based as they are on fragmentary and disputed evidence. In particular, the Multiregional Model has suffered from the attention of the geneticists, since most of the DNA studies have tended to support the idea of a recent African origin for our species. DNA has been extracted from three Neanderthal fossils, and its structure confirms that they represent a separate evolutionary lineage from our own, one that apparently began to diverge from our African ancestors about 500,000 years ago. Thus over the last 15 years, Out of Africa has moved from a peripheral to a central position in the debate about our origins.

But in the last few years several seemingly serious challenges to the new Out of Africa orthodoxy have been thrown up. In 1996 it was claimed that an Australian site called Jinmium showed that non-modern humans were the first to arrive in Australia, perhaps 150,000 years ago, and so could have been ancestral to the Aborigines. Three years later, the Mungo 3 burial in south-east Australia was dated to 60,000 years ago, suggesting that the symbolic burial of a modern human had taken place there long before any were known from Africa or Europe.

That news was featured in Scientific American under the headline "Is Out of Africa going out the door?". This burial was reported to contain DNA that contradicted Out of Africa. All of these claims have since been re-evaluated, with minimal publicity.

In the case of Jinmium, new studies showed that the site had not been dated properly, and the evidence of human occupation was actually less than 10,000 years old. In the case of Mungo 3, scientists have suggested that this burial was also wrongly dated. I prefer to argue that it is evidence of one of the earliest dispersals of modern humans and modern behaviour from Africa. The form of the skeleton is quite unlike that of its local non-modern predecessors, butcan be linked to early modern predecessors in Africa and the Middle East. As for the Mungo DNA, our Science note showed the need for the results to be independently replicated, when their significance might become clearer.

Europe has also thrown up highly publicised challenges to Out of Africa. In 1995 it was claimed that a bone flute made by Neanderthals had been discovered in Slovenia, suggesting that they, rather than modern humans, were the first to make music. In 1999, it was announced that a child's skeleton with mixed Neanderthal-Cro-Magnon features had been discovered in Portugal, demonstrating that Neanderthals were part of the ancestry of modern Europeans. And in the last few months it was claimed that the gene for ginger hair in Europeans had been inherited from Neanderthals – leading Simon Heffer to write an article in the Daily Mail headlined "Why I'm proud to be a Neanderthal".

To no publicity at all, two separate studies have shown that the shape and holes of the Neanderthal "flute" were probably the result of chewing by cave bears – there was no evidence that a Neanderthal had ever touched it. On further examination it emerged that the gene for ginger hair probably appeared about 50,000 years ago, when modern humans were establishing themselves outside Africa. But since DNA extracted from Neanderthals suggests that they began to diverge from our lineage about 500,000 years ago, the fact that the ginger gene is much younger is an indication that it emerged within modern humans, rather than arriving via Neanderthal admixture. As for the hybrid, until detailed studies of the skeleton are published, the jury is out on whether this was an unusually stocky Cro-Magnon child, or one showing the results of Neanderthal genes. If it is a hybrid, will this prove Multiregional evolution and disprove Out of Africa? It would certainly show that a 100 per cent replacement model must be wrong, but other versions of Out of Africa do allow for a little interbreeding with the natives as early moderns spread from Africa. But if it turns out not to be a hybrid, I will wait with interest to see how much media coverage that story gets.

The writer is head of Human Origins at the Natural History Museum in London