In this Spotlight, we ask whether modern humans are still evolving or whether we have stepped out of the path of natural selection.
Charles Darwin published his totemic work on evolution — On the Origin of Species — in 1859.
Based on the concept of natural selection, Darwin's book provided scientists with a new toolkit for understanding the place that humans and animals occupy in the natural world.
The tome also gave clues as to where their earthly origins may lie.
According to Darwin's thesis, evolution is slow and incremental with tiny genetic changes spread tens of thousands of years apart gently pushing forward changes in species.
In 2000, the paleontologist Stephen Jay Gould famously declared that "there's been no biological change in humans in 40,000 or 50,000 years," suggesting that evolution in humans is imperceptibly slow or has perhaps stopped altogether.
"We stopped natural selection as soon as we started being able to rear 90–95 percent of our babies that are born. We are the only species to have put a halt to natural selection, of its own free will, as it were," he told the British magazine The Radio Times in 2013, adding that our species has instead ensured our continued survival through accelerating cultural evolution:
"Stopping natural selection is not as important, or depressing, as it might sound — because our evolution is now cultural [...] We can inherit a knowledge of computers or television, electronics, airplanes, and so on."
Natural selection requires variation
Both positions have been hotly contested. For instance, Dr. Ian Rickard — from Durham University in the United Kingdom — responded to Attenborough's claims by pointing out that while abortion and birth control may mean that some people are having children while others are not, natural selection does not end here.
Rather, it places a renewed focus on the genetic material that is being passed along by those who are having children. Writing in The Guardian, Dr. Rickard explains, "Natural selection requires variation. It needs some individuals to thrive more than others."
"So the improved survival prospects around the world over recent decades and centuries drastically decreases the potential for natural selection to work in those populations. But that is not the end of the argument. Even if everyone survives to the same age, there is still variation for natural selection to work with. Natural selection doesn't really care about survival."
And, a 2010
Templeton gives the example of how technological advances in transportation have facilitated a rapid mixing of the human gene pool across the globe, resulting in the waning of differences between different populations with overall beneficial effects to human health.
Human evolution is now '100 times faster'
In their 2009 book The 10,000 Year Explosion: How Civilization Accelerated Human Evolution, Gregory Cochran and Henry Harpending calculate that — rather than there having been no biological change in humans over the past 50,000 years — human evolution has accelerated in the past 10,000 years.
Rather than slowing or stopping, the authors argue that evolution is now happening approximately "100 times faster than its long-term average over the 6 million years of our existence."
Modern technology also presents us with opportunities to observe changes in humans at a molecular level. Scott Solomon, a biologist from the University of Texas in Austin, highlights in his book Future Humans: Inside the Science of Our Continuing Evolution that since 2000 — when Gould declared human evolution to have slowed or stopped — it has been possible to sequence the human genome.
In the 18 years since then, it has become much faster and cheaper to do so, providing scientists with an unprecedented insight into our recent evolutionary past.
From these data, Solomon explains, researchers have found evidence of natural selection altering genes responsible for our:
- tolerance of dietary changes
- protection from infectious changes
- ability to withstand ultraviolet radiation from sunlight
- ability to thrive in mountainous regions with decreased oxygen
The milk revolution
One easy-to-understand example of how humans have evolved over recent centuries is how, on some continents, our bodies have adapted to tolerate the most abundant food sources common to that region.
As humans in some regions began to rely on dairy farming as a source of nourishment, our bodies adjusted over time to be more able to digest this food, which, previously, was only tolerated by infants and toddlers.
We can see evidence of this evolution today because humans in areas with a long tradition of dairy farming — such as Europe — are much more tolerant of lactose in their diet than people in regions that do not have a heritage of dairy farming — such as Asia. Around 5 percent of people descended from Northern Europeans are lactose intolerant, compared with more than 90 percent of people of East Asian descent.
The Framingham Heart Study
Another source of evidence for recent human evolution cited by biologists is the Framingham Heart Study — the longest-running multigenerational medical study in the world.
Framingham is a small town in Massachusetts, and in 1948, a study of the town's female population began; scientists wanted to understand what causes heart disease. The Framingham Heart Study is ongoing, and it has become an important repository for scientific data, not only relating to heart disease but also on changing trends in human health overall.
Scientists say that the Framingham data demonstrate that natural selection influenced the Framingham population — reducing height, increasing weight, lowering cholesterol levels, and lowering systolic blood pressures.
Importantly, the data do not show that average weight is increasing in Framingham because the women in the study are eating more. Instead, people with genes that affect these traits tend to have more children, meaning that these traits will become more common with subsequent generations.
"We see rapid evolution when there's rapid environmental change, and the biggest part of our environment is culture, and culture is exploding," Dr. Pardis Sabeti, a geneticist at Harvard University in Cambridge, MA, told the BBC.
"That's [...] the take-home message of the Framingham study, that we are continuing to evolve, that biology is going to change with the culture, and it's just a matter of not being able to see it because we're stuck right in the middle of the process right now."
Dr. Pardis Sabeti
Why are the Dutch so tall?
A 2015 study published in Proceedings of The Royal Society B asked the question, "Does natural selection favor taller stature among the tallest people on Earth?" The researchers behind the study tested this by looking at the tallest people on Earth: the Dutch.
But the Dutch were not always the tallest people on Earth. The researchers observe that in the mid-18th century, the average height of Dutch soldiers was 165 centimeters, which was well below the average of soldiers from other European countries and tiny compared with American soldiers, who were 5–8 centimeters taller than the average Dutch soldier.
But Dutch men have experienced a relatively sudden growth spurt, adding an extra 20 centimeters to their average height over the past 150 years.
During the same period, American men have only added 6 centimeters to their average height, and men from other European countries have struggled to keep pace with their neighbors from the Netherlands.
But why? The authors took into account disparities between the Netherlands and the United States in diet, social inequality, and the availability and quality of healthcare, but they concluded that it was natural selection that was driving up the height of the Dutch.
Put simply, Dutch women were more likely to find tall men attractive and were therefore more likely to have children with them. Tall Dutch men, the study confirmed, have more children than shorter Dutch men.
And, although the study found that tall Dutch women were less likely to have children than middling-height Dutch women, the tall women who did have children had more children than their shorter countryfolk.
In combination, these preferences exert a powerful natural selection effect on the average height of people in the Netherlands.
While this might not exactly be Marvel Cinematic Universe levels of genetic mutation — we are sad to report that we did not find any studies suggesting that the human race is about to acquire a telepathy gene — these examples illustrate how evolution works in terms of modern humans.
Evolution is persistent, everywhere, pushing our species forward in tiny increments. It might even be occurring with an accelerated regularity.