February 2008

The question of whether we, as a species, are still evolving, sometimes inspires visions of a new-and-improved Homo sapiens, complete with super-sized brain, disease-resistance, and the ability to withstand the pollutants and toxins common in a techno-centric future. While science fiction writers have come up with imaginative and entertaining answers to the question of how humans might be evolving, the responses of the scientific community have been more staid. Perhaps, they’ve suggested, some genes for withstanding epidemic disease are currently on the rise. However, with the improved genetic sequencing technologies that have come online in the last decade, many biologists are now prepared to offer more specific hypotheses as to how species are changing. Recently, a team of researchers led by scientists at the University of Utah announced that they’d scanned the genomes of 270 people and found evidence that humans are not only evolving — but that we’ve been adapting at an unusually rapid pace — at least on evolutionary timescales. The group has also made the controversial suggestion that human populations on different continents are evolving away from one another. However, a look at the evolutionary biology behind the headlines highlights some limits of this research.

Where's the evolution?

How do scientists look at the DNA of people alive today and figure out how recently natural selection acted on their ancestors? The answer relies on an evolutionary phenomenon called genetic hitchhiking (or a selective sweep). To understand, imagine that a new advantageous mutation (X) occurs on Chromosome 4, in the middle of gene versions P, Q, and R. In genetic terms, we would say that the mutation and those genes are linked — that is, they are close together on the same chromosome. The new mutation is so beneficial that its carrier leaves lots of offspring — many of whom also carry the mutation and the other linked genes. Over many generations, natural selection increases the frequency of mutation X, and because they are physically attached to X, gene versions P, Q, and R come along for the ride (i.e., “hitchhike” to high frequency). Of course, as X spreads, recombination occasionally occurs between it and its neighboring genes, breaking down this tight association somewhat. We begin to see X in association with different combinations of gene versions (e.g., with r instead of R). If we examine the population at the end of this process of natural selection, we will see mutation X at high frequency, often occurring alongside the same set of gene versions (P, Q, and R), and less frequently alongside other gene versions (p, q, and r).

When geneticists observe this — a stretch of DNA that often shows up in the same genetic background (e.g., X usually appearing alongside P, Q, and R) — they begin to suspect that somewhere in that DNA segment lies a favorable mutation. And by studying how long the hitchhiking genetic sequence is — for example, whether X usually drags along just P and Q or P, Q, R, S, T, and U — among other factors, we can estimate how long ago the favorable mutation arose and began to spread. Younger favorable mutations tend to retain more hitchhikers than older ones, since the longer a mutant and its hitchhikers have been around, the more opportunities recombination and mutation have had to break them up.

When Henry Harpending of the University of Utah and his colleagues applied this technique to the genomes of people who trace their ancestry to different geographic regions (Europe, Africa, China, and Japan), what they found surprised them — lots of evidence for favorable mutations! Natural selection seems to have acted on these mutants in many different areas of our genome. In fact, the team identified more than 10,000 selection events (i.e., stretches of DNA bearing the marks of natural selection) that seem to have taken place in the past 80,000 years of human history. Interestingly, the researchers found that most of these selection events traced to the recent past, with the largest numbers having arisen in the last 10,000 years. Judging by these results, human evolution seems to have sped up: small numbers of beneficial mutations spread through human populations for most of our history, but since the end of the last ice age, we’ve experienced a renaissance of evolutionary innovation in which many new advantageous mutations arose and began to spread. This ratcheting up of our evolution seems to correspond with the timing of major lifestyle changes — a period when many groups of humans began relying on agriculture, as opposed to hunting and gathering, and started living in denser populations. Furthermore, according to the Harpending team’s evidence, people from different geographic regions seem to have experienced different selection events. Only a small percentage of these positively selected genome regions were found in more than one of the four populations studied. This could mean that people on different continents were evolving in different directions.

These results are intriguing (and controversial — they’ve already generated much discussion within the scientific community), but they do have limitations. The technique that the researchers used (looking for genomic evidence of past hitchhiking events) is reliable, but it is not particularly good at detecting very old or very recent episodes of selection. That’s because old advantageous mutants have been around for so long that recombination and mutation may have already wiped out the evidence of their selective sweep. Recent advantageous mutants, on the other hand, may not have spread enough to be identified at all by this technique. And this highlights a problem with the portrayal of these results in some media outlets. The scientists’ techniques focused on the time period between 5000 and 80,000 years ago, but some media sources extend this window to today — suggesting that rates of human evolution are now peaking and that human racial groups are currently evolving away from one another. In fact, this sort of genomic evidence of hitchhiking can’t tell us much about how humans are evolving right now — or even how our evolution might have shifted in the past 200 years as the result of the industrial revolution or more frequent global travel. Sure, over the past 10,000 years, we’ve come along way — but figuring out the current trajectory of human evolution is a topic that this technique can’t directly address.

Nevertheless, the new results are suggestive. Humans are now able to mediate our environments with technology — to keep ourselves warm, to treat diabetes with insulin, and to provide food for those without farming, hunting, or gathering skills, amongst a myriad of other cultural innovations. So, for example, in many developed countries, the gene versions that contribute to juvenile diabetes are no longer strongly selected against. Some have argued that such technological advances mean that we’ve opted out of the evolutionary game and set ourselves beyond the reach of natural selection — essentially, that we’ve stopped evolving. However, if they are correct, the Harpending teams’ results imply that technological and cultural advancement does not necessarily halt natural selection, but may rather change its direction. After all, if the cultural innovations of the late Pleistocene spurred human evolution in new ways 10,000 years ago, perhaps the technological innovations of the past 200 years are simply changing the rules of the evolutionary game we humans are playing today.