Is Human Evolution Still Happening?

It is tempting to think of human evolution as something that happened to our ancestors: australopiths, Homo erectus, Neanderthals. We have clean water, antibiotics and Caesarean sections; surely natural selection has lost its grip?

It has not. Evolution doesn’t need harsh conditions or extinction. It needs only that people with certain genetic variants have, on average, more surviving children than people with others. That is true today, and researchers can measure it.

What counts as evolution?

There are four main forces:

• Mutation supplies new variation. Every baby carries roughly 60–70 brand-new mutations that neither parent had, and with about eight billion people, the total supply each generation is huge.
• Natural selection changes frequencies when variants affect survival or reproduction.
• Genetic drift changes frequencies by chance, especially in small groups.
• Gene flow mixes variants between populations. Modern travel means it is faster than at any time in history.

Even if selection were completely switched off, the other three would keep human gene pools changing.

The Bajau: sea nomads with bigger spleens

The Bajau of Indonesia, Malaysia and the Philippines have lived for centuries on boats and stilt houses, spearing fish while freediving to depths of 60 metres or more, sometimes for many hours a day. In 2018 Melissa Ilardo and colleagues scanned the spleens of Bajau divers in Indonesia and compared them with those of a neighbouring land-living group, the Saluan. Bajau spleens were about 50% larger, even among Bajau who did not dive.

The spleen releases oxygen-rich red blood cells when a diver holds their breath — something seals do too. The Bajau carry a variant near a gene called PDE10A, which affects thyroid hormone, and could plausibly control spleen size. Because size differences appear in non-divers, the difference is genetic, not simply trained.

Tibetans and a borrowed gene

Above 4,000 metres, most people respond to thin air by making more red blood cells, which thickens the blood and raises the risk of complications. Tibetans do the opposite: they keep haemoglobin levels low and breathe and circulate blood in different ways. A key gene is EPAS1 (you can view its protein in our 3D protein viewer). In 2014 Emilia Huerta-Sánchez and colleagues showed that the Tibetan version was inherited from Denisovans, who apparently passed it on through interbreeding. Selection has driven it to very high frequency, possibly within the last few thousand years — among the fastest changes seen in humans.

Malaria and other infections

Diseases are among the strongest selective pressures. Sickle-cell trait, carrying one copy of the sickle allele of the haemoglobin gene, protects against severe malaria but causes sickle-cell disease when two copies are inherited. In parts of West Africa, up to a quarter of people carry one copy. Other examples include the Duffy-null variant, near universal in much of West and Central Africa and protecting against one kind of malaria, and variants in immune genes that come from Neanderthal ancestors.

A claim that fell apart: height

In 2016 a study by Matthew Field and colleagues used patterns of rare variants in genomes from the UK to look for selection in the last 2,000 years. It reported several signals, including selection for greater height. For a while, this became a textbook case of recent human evolution.

In 2019 two independent teams (Sohail and colleagues, and Berg and colleagues) found the problem. The original height signal depended on a genetic-association dataset that contained subtle population-structure errors. When re-tested using the much larger, better-controlled UK Biobank data, most of the signal vanished. It is a useful lesson: traits influenced by hundreds of genes are extremely hard to test for selection, and dramatic results need to be checked.

Small changes in modern populations

Some studies look at present-day selection directly. A 2010 analysis of the multigenerational Framingham Heart Study in Massachusetts by Sean Byars and colleagues found that women with lower cholesterol and blood pressure, slightly shorter and stouter, and who had their first child a little earlier, tended to have more children. Projected over ten generations, the changes would be modest.

A few studies have also reported weak selection against genetic variants associated with more years of schooling, because in some populations more educated people have had fewer children. The effect is tiny — one Icelandic study estimated about one-hundredth of a standard deviation per decade — and it is dwarfed by environmental influences like nutrition and schooling, as shown by the way average test scores rose for much of the twentieth century.

Evolution you can see in the mirror?

A 2020 study by Teghan Lucas and colleagues reported that a small artery in the forearm, the median artery, which is usually present before birth and shrinks away, persists in adults more often than it used to: about 10% in 19th-century anatomical studies against about 30% in people born late in the twentieth century. The authors suggested it was a case of evolution in action, though other researchers point out that developmental or environmental factors could contribute and the picture is not settled.

What medicine changes

Medicine lets many more people survive to have children. That relaxes selection against some conditions, but doesn’t stop evolution: it shifts it. Differences in how many children people have are still selection. An extreme example is the tight fit of childbirth, where Caesarean sections may be allowing a modest increase in mismatches between baby and pelvis.

What to make of it

Human evolution has not stopped. It is just hard to see, because the fastest changes usually happen in small populations facing a specific stress — altitude, disease, diving — and because in a globally mixing species, gene flow spreads useful variants quickly. The biggest story of the last 10,000 years is not that we stopped evolving, but that we evolved fast, in many places, in different ways.