In popular discourse, human evolution is often spoken of in the past tense. The assumption runs roughly as follows: natural selection shaped our ancestors through millions of years of environmental pressure, but modern civilisation — with its medicine, agriculture and technology — has removed the conditions under which evolution operates. We have, in this view, stepped outside the evolutionary process.
A significant and growing body of research is now suggesting that this assumption is wrong. Not only is human evolution continuing, but several independent lines of evidence point to the possibility that it is occurring at a faster rate than at any previous point in our species' history.
Why More People Means Faster Evolution
One of the most straightforward reasons for accelerating evolution is simple arithmetic. Evolution by natural selection operates by filtering genetic variants — mutations — through the sieve of differential reproductive success. The more mutations exist in a population, the more raw material there is for selection to work on. And mutations arise at a roughly constant rate per individual per generation.
A few thousand years ago, the global human population numbered in the tens of millions. Today, it exceeds eight billion. That eight-billion-fold increase in the total number of humans on the planet means that, at any given moment, an incomparably larger number of new genetic variants are being generated and tested by selection. Researchers studying the distribution of rare genetic variants in modern human populations have found patterns consistent with a dramatic expansion in evolutionary activity over the past ten thousand years — roughly coinciding with the agricultural revolution and the population growth it enabled.
What Traits Are Changing?
Several traits appear to be under active selection in contemporary human populations. Studies of large biobank datasets — including the UK Biobank, which contains genetic and health information on approximately half a million British volunteers — have identified genetic variants associated with disease resistance, reproductive timing and metabolic function that appear to be increasing in frequency more rapidly than would be expected under genetic drift alone.
One well-documented example concerns lactase persistence — the ability to digest milk sugar into adulthood. This trait, rare in most pre-agricultural human populations, has spread dramatically in populations with a long history of dairy farming, including much of northern Europe. Genomic analyses suggest it spread with remarkable speed: strong selection over just a few thousand years can account for its current prevalence in populations of British and Scandinavian descent.
"What the genomics data keep telling us is that the pace of evolution is not fixed. It varies with the size of the population, the nature of the selective pressures and the genetic architecture of the traits involved. In all three respects, modern conditions are exceptional."
Modern Selective Pressures
The nature of selective pressures has also changed, not disappeared. Infectious disease continues to exert evolutionary pressure, as evidenced by the persistence of protective genetic variants such as the sickle cell trait in populations with a history of malaria exposure. New infectious agents — including, most recently, novel respiratory viruses — may be generating selective pressure in real time, favouring genetic configurations that produce more robust immune responses.
Reproductive patterns also continue to impose selection. Even in high-income societies where most children survive to adulthood, differences in the age at which people have their first child, the total number of children they have, and the heritable traits that influence these behaviours mean that not all genetic configurations are reproductively equivalent. Studies of Icelandic and British populations with multi-generational genetic records have found evidence of ongoing directional selection for traits including age at first birth and educational attainment.
The Role of Culture
Perhaps the most conceptually interesting dimension of this accelerating evolution is its interaction with culture. Human beings are unusual among animals in the degree to which our environments are shaped by our own cultural products — our diets, our social structures, our technologies, our medicines. Each of these cultural innovations creates new selective environments, which in turn exert pressure on our genomes.
Researchers describe this as "gene-culture coevolution" — a feedback loop in which genetic changes enable cultural adaptations, and cultural innovations generate new genetic selection pressures. The lactase persistence story is a classic example: the cultural practice of dairy farming created selective pressure for genes enabling lactose digestion, and the resulting genetic change enabled further cultural elaboration of dairy practices.
This dynamic, operating over thousands of years and across billions of people, suggests that the human species is not standing still. It is, in the technical sense of the word, evolving — and perhaps faster than it ever has before.
