Are humans still evolving?

The goal of much of modern medicine and culture is effectively to stop evolution. Is that happening?

Many biologists say that the question they most often receive from students and the public is "Are humans still evolving?"

To many researchers, the answer is obvious: Human biology, like that of all other living organisms on Earth, is the result of natural selection and other evolutionary mechanisms. Some say the question itself betrays a misunderstanding of how evolution works. "The very notion that ... we might not be evolving derives from a belief that all other life forms were merely stages on the way to the appearance of humans as the intended end point," says primatologist Mary Pavelka of the University of Calgary in Canada.

But other scientists point out that in developed countries, culture, technology, and especially medical advances have changed the evolutionary rules, from survival of the fittest to the survival of nearly everyone. The result, they say, is a "relaxation" of the selective pressures that might have operated 50 or 100 years ago. "Biologically, human beings are going nowhere," says anthropologist Ian Tattersall of the American Museum of Natural History in New York City. University College London geneticist Steven Jones agrees. "The central issue is what one means by 'evolving,' " Jones says. "Most people when they think of evolution mean natural selection, a change to a different or better adapted state. In that sense, in the developed world, human evolution has stopped."

Yet millions of people in developing countries continue to live under the combined stresses of poverty and disease. Under these conditions, even skeptics of ongoing human evolution agree that natural selection may be favoring genes that confer resistance to disease or enhance reproductive fitness in other ways. Indeed, researchers are now tracking how deadly maladies such as AIDS and malaria exert selective pressure on people today. "As long as some people die before reproducing or reaching reproductive age, selection is likely to be acting," says geneticist Chris Tyler-Smith of the Sanger Institute near Cambridge, United Kingdom.

Even in developed countries, where survival tends to be prolonged for almost all, recent studies suggest that there are still genetic differences among people in fertility and reproductive fitness, an indication that natural selection is operating. "The question 'Are humans still evolving?' should be rephrased as 'Do all people have the same number of children?' " says Pavelka. "The answer is that we do not make equal contributions to the next generation, and thus we are still evolving."

Over the past few years, a wealth of new data has begun to illuminate how natural selection has shaped--and may still be shaping--humanity. The human genome project and genetic data from people around the world have powered an explosion of research seeking signs of natural selection in human DNA. "A lot of the tools we are now using to search for selection were developed by people working on flies and other organisms," says evolutionary geneticist Bruce Lahn of the University of Chicago. "But once researchers began to discover examples of ongoing selection in humans, it opened the door and gave them confidence that they could find even more."

Drifting toward modernity?

To science-fiction fans, the future of human evolution conjures up visions of dramatic changes in our bodies, such as huge brains and skulls. "Many people see us continuing on the righteous path of increasing intelligence," says Pavelka. "But we will not head in the direction of larger brains and crania as long as infants are required to pass through a woman's pelvis to get into the world."

Whatever lies in our evolutionary future, scientists agree that the modern human body form is largely the result of evolutionary changes that can be traced back millions of years. The uniquely human lineage dates from about 6 million years ago, and many studies have demonstrated that our divergence from chimpanzees was accompanied by strong selective pressure, for example on the human brain. Yet researchers caution that not all morphological changes--the ones we can see in body shape and size--are the result of natural selection; some may not be due to genetic evolution at all. For example, the increase in average height seen in many developed nations over the past 150 years or so is probably due mostly to better diets rather than natural selection.

Even very early evolutionary changes in the hominid line were not necessarily due to natural selection. Take the hominid face, which has changed dramatically in the past 3 million years from the heavy-jawed mugs of the australopithecines to the relatively small and gracile skulls of modern humans. Anthropologist Rebecca Ackermann of the University of Cape Town in South Africa and anatomist James Cheverud of the Washington University School of Medicine in St. Louis, Missouri, analyzed hominid faces over time, using formulas that model natural selection as well as random genetic drift, in which some traits or alleles become more common simply through chance. They concluded last December in the Proceedings of the National Academy of Sciences (PNAS) that natural selection probably drove the evolution of facial form up to the birth of early Homo. But they also found that genetic drift could explain most of the changes in the human face after the birth of Homo about 2.5 million years ago. "Selective pressures on the face may have been released" when humans began using tools and so did less biting and chewing, says Ackermann.

The take-home lesson, she says, is that "genetic drift has played an important role in shaping human diversity. This is evolution, too." Drift has continued to shape modern human faces and skulls in the more recent past, according to other studies.

Signs of selection

Even if random drift and other nongenetic forces have helped shape modern humans, there is growing evidence that natural selection has also played an important role, even if its effects have been more subtle. Human evolution researchers are now mining the riches of genomic data to spot genes subject to recent selective pressures (Science, 15 November 2002, p. 1324). Geneticists have a large arsenal of "tests of selection" at their disposal, all of which exploit the genetic diversity of human populations to determine whether individual alleles or larger blocks of the genome--called haplotypes--are behaving as would be expected if they were only subject to random drift and were not under selection.

Some tests look for evidence that mutations in an allele that alter the protein it codes for have been favored over those that cause no change; others examine whether certain alleles are more common than expected. A fairly new and powerful approach compares the frequency of an allele in a population with the genetic diversity within a haplotype to which it belongs. If the allele is common due to random drift over a long time, the adjacent region of the genome should show considerable variation due to genetic recombination, the exchange of DNA between chromosomes during meiotic cell divisions. But if the variation is less than expected, the allele may have risen to high frequency in a much shorter period of time--a telltale sign of selection. "These tools are powerful," says Lahn. "Where we are lagging behind is in good data."

AIDS and malaria are arguably the worst scourges of humankind today, and they may both be exerting selective pressure on African genomes. Several genes have alleles that provide resistance to malaria, including those that code for hemoglobin C and an allele of the so-called Duffy blood group found only in sub-Saharan Africa; accumulating evidence suggests that they have both been under recent selective pressure. Four years ago, Tishkoff and colleagues showed that two different alleles of a gene called glucose-6-phosphate dehydrogenase (G6PD) have also been favored by strong selective pressure. The mutant alleles, A- and Med, are found only where malaria is or recently was a problem and offer resistance against malaria, although they can cause blood diseases.

Tishkoff and her co-workers used the known geographical variations in the G6PD gene to estimate that the A- allele probably arose in Africa about 6300 years ago and then spread rapidly across the continent; the Med allele, found in southern Europe, the Middle East, and India, is estimated to be only about 3300 years old (Science, 20 July 2001, pp. 442 and 455). These estimates are consistent with archaeological evidence that malaria only became a major health problem after the invention of farming, when the clearing of forests left standing pools of water in which the vector for the disease, the Anopheles mosquito, could breed. Thus a cultural change again led to a genetic one.

Our evolutionary future

To many researchers, the limited but growing evidence that natural selection is currently acting on the human genome means that humans are still evolving, even if in subtle ways. But can we actually predict the course of future evolution, à la Sykes's disappearing males or the vanishing blonds? Most researchers' predictions are considerably more narrow and cautious and are tied to known selective pressures.

Evolutionary predictions are tied to speculation about just what kind of environment we may face. Some researchers suggest that changing climate conditions may diminish the benefits of culture and medicine, creating a new era of natural selection. "There has been a relaxation in selective pressures in industrialized societies," says evolutionary geneticist Peter Keightley of the University of Edinburgh, U.K. "But our ability to sustain that relaxation is probably temporary. We are using up our energy resources, our population is growing, and the climate is changing. All this is bound to lead to greater difficulties and renewed selective pressures."

Despite such concerns, however, most scientists remain leery of long-term forecasts, in part because of the way evolution works. "Evolution is not directed towards a goal," says Tyler-Smith. "It always takes the short-term view, operating just on what allows us to survive and reproduce better in this generation." For now, predicting humanity's evolutionary future may be little more than crystal ball gazing--better suited to science fiction than scientific research.

•	Are humans still evolving? EVOLUTIONARY GENETICS Science Magazine, Friday, July 8, 2005 Byline: Michael Balter