Bill Bryson - A short history of nearly everything

Thorne emphatically (and I believe sincerely) dismisses the idea that his theory is in any measure racist and accounts for the uniformity of human evolution by suggesting that there was a lot of movement back and forth between cultures and regions. “There’s no reason to suppose that people only went in one direction,” he says. “People were moving all over the place, and where they met they almost certainly shared genetic material through interbreeding. New arrivals didn’t replace the indigenous populations, they joined them. They became them.” He likens the situation to when explorers like Cook or Magellan encountered remote peoples for the first time. “They weren’t meetings of different species, but of the same species with some physical differences.”

What you actually see in the fossil record, Thorne insists, is a smooth, continuous transition. “There’s a famous skull from Petralona in Greece, dating from about 300,000 years ago, that has been a matter of contention among traditionalists because it seems in some ways Homo erectus but in other ways Homo sapiens . Well, what we say is that this is just what you would expect to find in species that were evolving rather than being displaced.”

One thing that would help to resolve matters would be evidence of interbreeding, but that is not at all easy to prove, or disprove, from fossils. In 1999, archeologists in Portugal found the skeleton of a child about four years old that died 24,500 years ago. The skeleton was modern overall, but with certain archaic, possibly Neandertal, characteristics: unusually sturdy leg bones, teeth bearing a distinctive “shoveling” pattern, and (though not everyone agrees on it) an indentation at the back of the skull called a suprainiac fossa, a feature exclusive to Neandertals. Erik Trinkaus of Washington University in St. Louis, the leading authority on Neandertals, announced the child to be a hybrid: proof that modern humans and Neandertals interbred. Others, however, were troubled that the Neandertal and modern features weren’t more blended. As one critic put it: “If you look at a mule, you don’t have the front end looking like a donkey and the back end looking like a horse.”

Ian Tattersall declared it to be nothing more than “a chunky modern child.” He accepts that there may well have been some “hanky-panky” between Neandertals and moderns, but doesn’t believe it could have resulted in reproductively successful offspring. [49]“I don’t know of any two organisms from any realm of biology that are that different and still in the same species,” he says.

With the fossil record so unhelpful, scientists have turned increasingly to genetic studies, in particular the part known as mitochondrial DNA. Mitochondrial DNA was only discovered in 1964, but by the 1980s some ingenious souls at the University of California at Berkeley had realized that it has two features that lend it a particular convenience as a kind of molecular clock: it is passed on only through the female line, so it doesn’t become scrambled with paternal DNA with each new generation, and it mutates about twenty times faster than normal nuclear DNA, making it easier to detect and follow genetic patterns over time. By tracking the rates of mutation they could work out the genetic history and relationships of whole groups of people.

In 1987, the Berkeley team, led by the late Allan Wilson, did an analysis of mitochondrial DNA from 147 individuals and declared that the rise of anatomically modern humans occurred in Africa within the last 140,000 years and that “all present-day humans are descended from that population.” It was a serious blow to the multiregionalists. But then people began to look a little more closely at the data. One of the most extraordinary points-almost too extraordinary to credit really-was that the “Africans” used in the study were actually African-Americans, whose genes had obviously been subjected to considerable mediation in the past few hundred years. Doubts also soon emerged about the assumed rates of mutations.

By 1992, the study was largely discredited. But the techniques of genetic analysis continued to be refined, and in 1997 scientists from the University of Munich managed to extract and analyze some DNA from the arm bone of the original Neandertal man, and this time the evidence stood up. The Munich study found that the Neandertal DNA was unlike any DNA found on Earth now, strongly indicating that there was no genetic connection between Neandertals and modern humans. Now this really was a blow to multiregionalism.

Then in late 2000 Nature and other publications reported on a Swedish study of the mitochondrial DNA of fifty-three people, which suggested that all modern humans emerged from Africa within the past 100,000 years and came from a breeding stock of no more than 10,000 individuals. Soon afterward, Eric Lander, director of the Whitehead Institute/Massachusetts Institute of Technology Center for Genome Research, announced that modern Europeans, and perhaps people farther afield, are descended from “no more than a few hundred Africans who left their homeland as recently as 25,000 years ago.”

As we have noted elsewhere in the book, modern human beings show remarkably little genetic variability-“there’s more diversity in one social group of fifty-five chimps than in the entire human population,” as one authority has put it-and this would explain why. Because we are recently descended from a small founding population, there hasn’t been time enough or people enough to provide a source of great variability. It seemed a pretty severe blow to multiregionalism. “After this,” a Penn State academic told the Washington Post , “people won’t be too concerned about the multiregional theory, which has very little evidence.”

But all of this overlooked the more or less infinite capacity for surprise offered by the ancient Mungo people of western New South Wales. In early 2001, Thorne and his colleagues at the Australian National University reported that they had recovered DNA from the oldest of the Mungo specimens-now dated at 62,000 years-and that this DNA proved to be “genetically distinct.”

The Mungo Man, according to these findings, was anatomically modern-just like you and me-but carried an extinct genetic lineage. His mitochondrial DNA is no longer found in living humans, as it should be if, like all other modern people, he was descended from people who left Africa in the recent past.

“It turned everything upside down again,” says Thorne with undisguised delight.

Then other even more curious anomalies began to turn up. Rosalind Harding, a population geneticist at the Institute of Biological Anthropology in Oxford, while studying betaglobin genes in modern people, found two variants that are common among Asians and the indigenous people of Australia, but hardly exist in Africa. The variant genes, she is certain, arose more than 200,000 years ago not in Africa, but in east Asia-long before modern Homo sapiens reached the region. The only way to account for them is to say that ancestors of people now living in Asia included archaic hominids-Java Man and the like. Interestingly, this same variant gene-the Java Man gene, so to speak-turns up in modern populations in Oxfordshire.

Confused, I went to see Harding at the institute, which inhabits an old brick villa on Banbury Road in Oxford, in more or less the neighborhood where Bill Clinton spent his student days. Harding is a small and chirpy Australian, from Brisbane originally, with the rare knack for being amused and earnest at the same time.

“Don’t know,” she said at once, grinning, when I asked her how people in Oxfordshire harbored sequences of betaglobin that shouldn’t be there. “On the whole,” she went on more somberly, “the genetic record supports the out-of-Africa hypothesis. But then you find these anomalous clusters, which most geneticists prefer not to talk about. There’s huge amounts of information that would be available to us if only we could understand it, but we don’t yet. We’ve barely begun.” She refused to be drawn out on what the existence of Asian-origin genes in Oxfordshire tells us other than that the situation is clearly complicated. “All we can say at this stage is that it is very untidy and we don’t really know why.”