There's no such thing as race: geneticist

Modern genetics is finally laying to rest some of the arguments about the expansion of humans out of Africa and across the world, says Washington University professor Alan Templeton, who presented data from his work on human genetic evolution at this week's Genetics Society of Australia conference in Melbourne.

Evolutionary geneticists use DNA sequence differences from populations around the world to construct "haplotype trees" -- essentially, evolutionary trees illustrating the history of polymorphic changes to a gene sequence.

"DNA is the ultimate fossil because it makes a copy of itself that is passed on to the next generation -- it's a living fossil," Templeton said.

Since the first mitochondrial DNA haplotype tree was constructed in the late 1980s, researchers have constructed many others for a variety of genes and DNA regions including male-specific Y-chromosomal DNA, regions of the X-chromosome and various genes on other chromosomes. But drawing conclusions about human evolution based on the haplotype tree of one genetic region is dangerous territory.

"The mitochondrial DNA haplotype tree only represents the evolutionary history of mitochondrial DNA, not the totality of evolution," Templeton said. "The haplotype tree is not the whole story -- it represents part of the story -- only that of the piece of DNA being studied."

Templeton has designed a statistical technique -- called nested clade phylogeographic analysis -- to examine haplotype trees in the context of geographical data. Using this method, statistical tests are performed to look for significant associations between the genetic data and the other data, and anything that doesn't meet with a high level of statistical significance is thrown out of the analysis.

"One of the big misconceptions on past events is that you can't test your hypothesis. [But] you can do a rigorous statistical analysis ... we now have various kinds of statistical tools that allow us to extract information on the past," Templeton said.

With this approach, he has performed meta-analyses on ten genes and regions including the mitochondrial DNA, Y chromosome, two X-linked genes and half a dozen genes from the other chromosomes.

What his analysis essentially shows is that there were three expansions of humans out of Africa, the first occurring approximately 1.7 million years ago and the most recent around 80-150,000 years ago. Overlaid on the three expansion events is continual gene flow, evident for at least 600,000 years in this analysis, demonstrating that populations of humans in Africa, Europe and Asia, and later on in other parts of world, were evolving together, not in isolation as some models supposed. More recently there has been expansion from Asia back into Europe and Africa, and expansion from Asia into the Pacific region and the Americas.

Eight of the ten genes and regions studied by Templeton, those passed down from both maternal and paternal lineages, were able to provide information on replacement -- the controversial idea that the most recent expansion out of Africa completely wiped out earlier populations of humans in Europe and Asia (see breakout) -- and all eight rejected the replacement model.

"When you look at all three classes of DNA together, you get a clear picture of [human] evolution," he said.

Interestingly, the second out of Africa expansion indicated by Templeton's data, which occurred approximately 420-840,000 years ago, had never been identified by evolutionary geneticists but was backed up by archaeological data showing cultural expansions out of Africa as well as fossil evidence.

"You get more insight by contrasting the genetic data with archaeological data ... which is more precise and detailed than genetic data," Templeton said. "I think the real hope for advances in human evolution lies in combining genetics, archaeology and paleoanthropology [the study of human fossils]."

There are a number of implications emerging from this data, according to Templeton. Firstly, all living humans come from a single lineage that has evolved as a cohesive unit for at least the last 600,000 years. There is also no evidence for biologically distinct races in humans -- unlike chimpanzees for example, which can be divided into three distinct races.

"Current human populations show genetic differences but these are small compared with other species of large-bodied mammals, and primarily reflect geographical distance," he said. "Our standard ways of defining people on characteristics such as skin colour simply don't work.

"Humanity consists of only one type. We do have high levels of diversity between individuals but only modest differences between geographical populations."

Templeton plans to perform another set of analyses in the next year or so to take advantage of genetic data sets and haplotype trees generated in the three years since his first set, which will double or treble the amount of genetic information in his database. He expects this will result in an increased level of detail about more recent expansion events and gene flow, as well as increase confidence in older events.

"I think that in future we can push back throughout all of human evolution and back into the common ancestor of humans and other primates," he said.

Discredited theories and mitochondrial DNA

Templeton said the evolution of modern humans over the last two million years -- starting when Homo erectus first expanded out of Africa into Europe and Asia -- is an era characterised by extreme controversy and multiple models. Over the last 15 years or so, the controversy has increased, particularly as the role of genetics in the study of evolution has blossomed.

One model, first suggested in the 1950s and long since discredited, suggests that after the initial expansion out of Africa, populations in Europe and Asia evolved independently, giving rise to separate races. A second model suggests that gene flow between populations in Africa, Europe and Asia was maintained after the initial expansion event, although isolation by distance allowed populations to differentiate.

Prominent among the controversial models of human evolution is the replacement model, which postulates that a first wave of human expansion from Africa into Europe and Asia around two million years ago was replaced by a more recent wave of expansion from Africa that occurred some 100,000 years ago, driving earlier European and Asian populations into genetic extinction.

With the introduction of DNA analysis techniques, researchers have started to look at the evolution of specific sequences. The first of these, which looked at maternally inherited mitochondrial DNA, postulated that all humans shared a single female ancestor in Africa -- the so-called "Eve" hypothesis. Similar analyses of Y-chromosome sequences, specific to the male lineage, have supported an expansion from Asia back into Europe and Africa, as well as to other regions.