Have modern humans stopped evolving?
The popular notion dates back at least to 2013, when broadcaster and naturalist David Attenborough suggested that natural selection had been halted.
As the argument goes, medical ingenuity now keeps many people alive who would have otherwise died from disease or genetic abnormality, thus allowing their genes to be passed on to future generations and stymying natural selection.
So, don’t expect your progeny to have eyes in the back of their heads, or an extra arm, as beneficial as both of those attributes would be for wrangling small children or bringing in the washing.
The humans of the future will look and function very much as they do today.
But make no mistake, say evolutionary geneticists, Homo sapiens are still evolving.
So too are other animals and plants.
And opinion is divided about what that might mean for our ability to respond to the stresses and extremes of future climate change.
Evidence for ongoing human evolution
For scientist Michio Kaku, a distinction needs to be drawn between different forms of ongoing adaptation.
“Evolution is still taking place. It takes place every time two people mate, it takes place inside our bodies, in our immune systems, in our body chemistry,” he told RN’s Future Tense.
“But gross evolution has pretty much gone.”
In other words, just because it isn’t obvious doesn’t mean it isn’t happening.
Laurence Hurst, the director of the Milner Centre for Evolution at the University of Bath, says there are numerous recent examples of human evolutionary change, often relating to regional isolation and geographic variance.
He notes that human DNA shows adaptation to life in low oxygen environments like the Tibetan Plateau, the South American Andes and the Ethiopian high plateau — though recent research has suggested that, at least in the case of the Tibetans, this adaptation could be tens of thousands of years old.
“And in sub-Saharan Africa, we have very recent evidence for resistors to things like malaria and ongoing resistance to malaria as well,” he says.
A community of human ‘sea otters’
For evolutionary genomicist Melissa Ilardo, the notion that the evolutionary process has been halted is a “first-world” perspective.
Her recent research for the University of California, Berkley, has focused on the Bajau people of Indonesia.
The Bajau live in a small coastal community and practice a traditional marine hunter-gatherer lifestyle, living on houseboats and spending much of their time in the ocean.
Thousands of years of experience has given the Bajau exceptional diving skills, but they also have a unique ability to hold their breath for long periods of time.
An examination of their physiology, Dr Ilardo says, shows community members have unusually large spleens.
“The spleen is kind of a weird organ,” she says.
“When you dive, when you hold your breath, one of the things that happens as a part of what’s called the mammalian dive reflex is that your spleen contracts. This is important because your spleen is holding red blood cells that are carrying oxygen.
“When it contracts, it pushes those red blood cells into your system and gives you an oxygen boost.
“So, through natural selection, over thousands of years what happens is that the people who had larger spleens were either less likely to die during dives, because breath-hold diving can actually be quite dangerous.”
By contrast, she says, analysis of people in neighbouring communities with a long tradition of farming, rather than diving, found no evidence of spleen enlargement.
And given the Bajau show no sign of giving up their sea-going ways anytime soon, there’s nothing to suggest that further adaptations won’t continue to occur, Dr Ilardo says.
How fast is fast, when it comes to evolution?
From a biological perspective, the speed of evolution is determined by the pace of generational change.
Evolutionary adaptation in bacteria, for instance, happens at a rapid rate, because the lifespan of bacterial organisms is comparatively short.
But how fast can genetic adaptation occur in humans?
DNA changes evident in the Bajau happened over hundreds, if not thousands of years, but Professor Hurst says evidence suggests it can occur over much shorter timeframes.
He points to a significant long-term heart study that began in the US in the late 1940s.
“The Framingham study came out with the result that there is genetic evolutionary change with respect to cholesterol levels and with respect to blood pressure,” he says.
“These are two things that have been profoundly affected by the modern Western diet, which is very high in salt, which puts up the blood pressure.
“So, what we are seeing is selection against high blood pressure and selection against cholesterol levels, all as if we were adapting to the diet.”
And that adaptation, if correct, has occurred over the course of just three generations.
Can evolution keep pace with climate change?
Rapid evolution has also been recorded in some plants and fish.
Researchers at the University of New South Wales have documented significant morphological modifications in the DNA of South African beach daisies, introduced into Australia in the 1930s.
And Australian scientists studying marine species in both the Antarctic waters of the Southern Ocean and the much warmer coastal environments off South Africa have found increased speciation — the divergence of separate species — caused the isolating effects of variant water temperatures.
Researcher Claire Brandenburger says such revelations help us better understand the evolutionary process.
“What we are learning is that such rapid evolution is much more common than we previously thought,” she says.
“More and more evidence is showing that rapid evolution can occur in as little as 10 to 100 years.
“Hopefully this is a little ray of good hope for plants in the face of climate change.
“For plant species that don’t move or are only dispersed by their seeds, people are worried that climate change is really going to hit plants hard. And this study gives us hope that perhaps there are some plants that can be able to adapt very quickly.”
Luciano Beheregaray, a professor of biodiversity genetics at Flinders University, is less optimistic.
“Climate change is happening very fast in the world’s oceans,” he says.
“In fact, coastal marine species are shifting their distribution because along many regions the temperature of water is increasing much more rapidly than the average temperature on land.
“Some species might be able to [adapt], but the evidence at the moment suggests that the change in temperature driven by human influences is happening too fast.”
His concern is shared by Tim O’Hara, the senior curator of marine invertebrates at Museums Victoria.
“Climate change won’t extinguish life, by any means, there will be things that can adapt,” he says.
“But to recover that enormous biodiversity that exists on the planet now will take tens of millions of years.
“Antarctica is still responding to an extinction event tens of millions of years later. So, there’s really a serious conservation message in this.”
Setting the clock on a potential genetic timebomb
And there are specific concerns about how humans will fare in the future.