Human evolution is speeding up. Around 40,000 years ago our genes began to evolve much faster. By 5000 years ago they were evolving 30 to 40 times faster than ever before and it seems highly likely that we continue to evolve at this super speed today.
Our population explosion and rapidly changing lifestyles seem to be the drivers of this acceleration, the discovery of which contradicts the widely held notion that our technological and medical advances have removed most of the selection pressures acting upon us.
This stunning insight into humanity’s development comes from a wide-ranging study of human gene variants gathered by the international HapMap project. Investigators led by John Hawks of the University of Wisconsin, Madison, studied 3.9 million simple differences in DNA called single nucleotide polymorphisms (SNPs, pronounced “snips”) from 270 individuals, including people of Han Chinese, Japanese, Yoruban and northern European extraction. This revealed several pieces of evidence that provide clear support for the idea that human evolution is accelerating.
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The first is that the genomes of the people within the study group contain a relatively high number of new genetic traits, marked in the genome by the presence of a relatively new SNP. Such SNPs are known to be linked to particular genes and affect their activity, and these mutations have been linked to significant changes in our lifestyle. For example, we evolved greater resistance to the cold as people migrated north out of Africa, and to infectious diseases such as smallpox, yellow fever and typhus, which became important killers when we began settled living. The advent of agriculture and changes to our diets also influenced our genome.
This high rate of mutation was caused by the explosive increase in the population – as more people are born, more mutations can be introduced into the gene pool. Darwin himself predicted that larger populations would evolve more quickly than smaller ones, something that has since been shown in insects and bacteria.
However, if humans had always evolved at the same fast pace, you would then expect to see relatively few SNPs surviving today, as selection would have weeded out most of the unfavourable genes they are linked to. “But when we look at the genome, we see that the variation is relatively high,” says Hawks. In fact, the researchers managed to confirm that around 1800 genes, or roughly 7 per cent of the total in the human genome, have changed under the influence of natural selection within the past 50,000 years, a figure they first revealed in 2005 after conducting two similar but smaller genetic analyses. That is roughly the same proportion of genes that were altered in maize when humans domesticated it from its wild ancestors.
That high level of variation means our rate of evolution must have speeded up considerably, as there has not been time for many SNPs to be selected out. The researchers say this started around 40,000 years ago. Evolution then continued to accelerate until a peak, which they found occurred in Europeans and Yoruban Africans 5250 and 8000 years ago respectively (, ). However, these dates are almost certainly artificial. The researchers believe it is likely that evolution has continued apace, but too little time has elapsed for more recent adaptive mutations to emerge in the study’s sample.
The research also explains why there are just 40,000 or so differences in the number of adaptive SNPs seen between humans and chimps. If humans had always been evolving at a constant rate, instead of undergoing a recent acceleration in evolution, then this number would be in the millions.
“If humans always evolved at a constant rate, then the genetic differences between us and chimps would be far greater”
These findings flout the conventional wisdom that humans had reached their fully modern form in the mid-Palaeolithic, as stated in textbooks, says anthropologist Clark Larsen, of the Ohio State University in Columbus, who was not involved in the study.
“People have always thought that the force of selection had decreased, because it became easier to survive,” says Hawks. In fact, he says, disease, population growth, sedentary lifestyles, and changes in diet, technology and social group size have greatly increased the forces of selection.
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Walking tall to carry a baby
One of the most important evolutionary adaptations for humans occurred far earlier in our history, when our ancestors took to walking upright on two legs.
Explanations of why bipedalism evolved have focused on how it freed the hands, saved energy or helped with balancing on branches. But there is a more beneficial reason for walking tall, say some researchers: it enabled our ancestors to better support their children.
Apes rely on infants clinging to their fur at least some of the time so they can keep both hands free, but a mechanical analysis of ape hair suggests our ancestors’ pelt may not have been up to the job. This could have forced mothers to manually carry their babies to the extent that walking upright became preferable.
Lia Amaral, a physicist at the University of São Paulo in Brazil, measured the strength of gorilla and orang-utan fur and how humidity affects fur grippiness. “The results suggest the safe weight limit for a baby clinging to fur alone is about 5 kilograms for African apes,” says Amaral. Bigger babies are carried on the back, but while this works fine in knuckle-walking chimps and gorillas, Amaral’s calculations show that in an upright position, the fur cannot support their weight. Orang-utans, which spend a lot of time upright in trees, carry older babies on the hip instead and have thicker skin and longer, stronger hair.
Amaral speculates that an increasingly dry environment made our ancestor’s fur more slippery and also caused hair reduction, to allow for heat loss without using upa lot of water in sweating. This would have made fur less safe for carrying infants and forced mothers to use their hands to support them, thus favouring bipedal walking (Naturwissenschaften, ).
“It’s an interesting idea,” says primatologist Bill Sellers at the University of Manchester, UK, who has shown that carrying infants on the hips is energetically expensive (Journal of Human Evolution, ). “There clearly must have been a strong environmental selection pressure for active, upright infant carrying to evolve,” he says.
Robin Crompton of the University of Liverpool, UK, is less convinced: “Orang-utans often use their hands to support their infants. Our ancestors probably had a similar lifestyle, and we don’t know if terrestrial bipedalism began in a dry rather than a wet forest.”
But while Amaral admits that determining the timing of hair reduction is crucial, she insists that baby carrying deserves more consideration in evolutionary models. “It correlates with hair and skin characteristics, infant development and social bonds – factors we must seriously focus on.”
Walking upright affected pregnancy, too, and the burden of carrying a baby has left its mark on the female spine. Women have evolved differently shaped vertebrae to deal with a shifting centre of gravity during pregnancy.
One hallmark of our upright posture is curvature of the lower spine to align the upper body above the hips. A pregnant woman’s bump disturbs this delicate balance, but she won’t topple over because the spine has evolved to allow increased curvature during pregnancy, says Katherine Whitcome of Harvard University. Her team measured the vertebrae of 59 male and 54 female lower spines from a bone collection and discovered that women have three wedge-shaped vertebrae while men only have two. The extra wedge permits a steeper curve during pregnancy. In addition, the dorsal protrusions of the vertebrae are larger and oriented differently to further improve stability when pregnant (Nature, vol 450, p 1075).
“Until recently, women were pregnant or carrying children pretty much all the time, so it makes sense that natural selection quickly shaped their spines to cope with this,” says Whitcome. Similar differences in the vertebrae of male and female Australopithecus fossils suggest that our ancestors 2 to 3 million years ago already used the same adaptations to balance their bumps while walking upright.
Nora Schultz
Walking small to have a baby
We also now have an answer to another enigma about human evolution – how to explain the short stature of pygmies. Size, it seems, isn’t important in itself. Instead, being small may be the side effect of a strategy of having children early because of low life expectancy.
Andrea Migliano of the University of Cambridge studied the growth rates and life histories of pygmies and discovered that they grow fast but stop young, at around age 13. “There is competition between putting energy into growth and reproduction,” she says.
She proposes that finishing growth early allows women to start having babies younger, an essential strategy when mortality is high (Proceedings of the National Academy of Sciences, ). Most pygmies die before their early twenties, one or two decades earlier than other hunter-gatherers.
Reasons for high mortality likely include infection and challenging environments. But if these conditions cause early death rather than directly causing small body size, pygmies may not be that well adapted to their habitats, says Migliano. “Mortality isn’t an adaptation. We shouldn’t just accept that their life expectancy has to be so low.”
Nora Schultz
