Destiny of species: Have we sidetracked evolution?

Several weeks ago I got a call from a reader who wanted me to investigate whether humans are still evolving, and if so, into what.

Will we become superbrainiacs, or will we mentally degenerate, as in the film Idiocracy, or Planet of the Apes? Or, will we split into two species, as H.G. Wells imagined in The Time Machine?

In the last few years, scientists have come up with some clever ways of getting at this question, and they're picking up signs that people are evolving. We aren't necessarily all evolving together as a species, however. What counts as "fit" in a high-altitude place may be a liability in another. Several genetic mutations protect people against malaria, but they can also cause diseases such as sickle cell anemia.

Scientists have two ways to approach the question of current evolution, said Yale biologist Stephen Stearns. One is to compare DNA among lots of people.

DNA can show "signatures" of recent evolution, because at first, new advantageous mutations take big chunks of other DNA with them as they pass from parents to children. Over time, those chunks of DNA break apart and the advantageous genes can pass through generations less encumbered. The size of that genetic entourage works as a kind of molecular clock - the bigger it is, the more recently a new mutation appeared.

Comparing DNA samples from diverse ethnic groups has shown that new genetic mutations spreading in the last few thousand years are changing the way some people digest food, grow hair, store fat, and fight disease.

But these studies examine only a long time frame - evolution over the last few thousand years.

Another way to detect current evolution is to examine inheritance patterns of specific traits, a technique that's being used to study evolution over the last 150 years. That's when modern medicine and global travel became widespread, and, some say, drastically altered the course of human evolution. Some scientists have even argued that these factors are halting evolution's course for our species.

One such study, led by Case Western Reserve University anthropologist Cynthia Beall, showed that mountain dwellers in Tibet are being selected for better tolerance to their thin-air environment.

She found some Tibetans had much greater oxygen-carrying capacity in their blood than others. This trait ran in some families, she said, in a way that suggested it was tied to a genetic variant.

She also found that the women with the better oxygen-carrying red blood cells had significantly more surviving children, suggesting that natural selection is at work.

Stearns looked for signs of evolution in a 5,000-person health survey known as the Framingham Heart Study, which has been running since 1949. He found that the women with the most children were, on average, shorter, had lower cholesterol, and bore their first child earlier.

One explanation he offers for these changes is that, in the past, having kids too early was disadvantageous, since babies of teens were less healthy and more likely to die.

But over the last 150 years, modern medicine kept more babies alive, so the evolutionary advantage shifted to those who started reproducing earlier. The shorter stature could be explained by the fact that women who mature earlier wouldn't grow as tall, Stearns said.

If this scenario is true, he said, medicine isn't halting evolution, but it's changing its course, emphasizing fertility more and survival less.

Another study, published just last week, used church records from 1799 to 1940 to seek signs of recent evolution among the inhabitants of the Canadian island of L'Isle-aux-Coudres. The authors found that the average age of first childbirth fell from 26 to 22.

Such a change might easily be attributed to a cultural shift - some change in attitude, custom, or even economy, since poverty can push people to delay childbirth. In this case, the researchers used statistics to infer that they were seeing at least some biological change.

That's plausible, said Stearns, though again it's hard to detangle cultural changes from genetic ones, since both can run in families.

These studies don't tell us whether the human race is evolving as a whole, since they address only individual populations.

What might happen to us as a species? For one thing, said Stearns, we may become more mixed up, with traits such as skin color becoming less geographically localized as people increasingly move around the globe. Some speculate that the present diversity of human skin colors will eventually blend into brown.

Biological evolution for humans is tied up with cultural changes such as vaccines, antibiotics, and farming. Culture changes fast, and we don't have a great track record in predicting what's next, leaving plenty of leeway for the speculations of science-fiction writers.