Noted public figures like David Attenborough have previously claimed that human evolution is over, but many researchers studying human evolution firmly disagree. We know that humans have altered our environment in innumerable ways–changing the very air, water, and soil that we rely on as the most successful “ecosystem engineers” on Earth. It can be easy, amid all that change, to assume that we’ve conquered biology and eliminated the effects of evolution and natural selection on our species. But that’s not what the science says.
“Of course humans are still evolving,” says Jason Hodgson, an anthropologist and evolutionary geneticist at Anglia Ruskin University in England. “All living organisms that are in a population are evolving all the time.”
Evolution is a process, not an outcome, and it doesn’t always happen linearly.
“Humans are definitely still evolving,” agrees John Hawks, a paleoanthropologist at the University of Wisconsin-Madison.
“Humans are still evolving, as are virtually all other populations of organisms,” says Stephen Stearns, an emeritus professor of ecology and evolutionary biology at Yale University.
Yet the way that evolution acts on our species may not be what you imagine. In biological terms, evolution is a change in gene variant (a.k.a. “allele”) frequency in a population over time. It is not a force directing a species’ trajectory towards a certain goal. However, biologists can still observe human evolution in action and identify recent examples of it.
How does evolution happen?
Before we get into the research, it’s important to understand how evolution works. There are a handful of ways organisms can evolve. Every person is born with about 70 new genetic mutations on average, which aren’t derived from their parents’. Once emerged, mutations can be passed onto subsequent generations, thereby changing population-level allele frequency. There’s drift, where randomness influences what gene variants are passed on in what proportions–this unfolds the quickest in small populations. There’s gene flow, where individuals and populations migrate, bringing their genetic material to new places. Sexual selection happens when people mate non-randomly (which accounts for most of human mating).
Then there’s natural selection–often conflated with evolution itself–where environmental conditions that impact survival and reproduction dictate what alleles are most likely to persist through future generations. Counter to drift, natural selection happens faster in larger populations, says Hodgson, as beneficial alleles are more likely to emerge when there are more people around. “This is the largest the human population size has ever been, so this is probably to some extent the greatest change for natural selection to act in humans,” he adds.
“Humans are still evolving, as are virtually all other populations of organisms.”
Through all of these above mechanisms, humans evolved from our last common ancestor with our two closest living relatives–chimpanzees and bonobos. We did not evolve from these great apes, but rather from a lineage of now extinct primates which diverged from the predecessor of chimpanzees and bonobos from a shared point. Long before Homo sapiens were a branch on the tree of life, previous divergences led to primates and mammals and vertebrates. Slowly, over billions of years, evolution has diversified all organisms from the last universal common ancestor. Our bodies still carry the evidence of our species’ evolutionary past. For instance, in our now useless (or “vestigial”) tail bones and appendixes.
And all of the above mechanisms are still at play in humans today, though our societal organization and sheer numbers may have altered which ones are acting the quickest, in what populations, and in what ways. Big evolutionary changes impacting conspicuous traits, like the loss of tails, happen very slowly, but there is still less apparent change being wrought all the time.
The oft-stated belief that modern medicine has eliminated natural selection “is not a serious view,” says Hodgson, because it relies on many untrue assumptions. For instance that we’ve resolved all health issues with science or that access to healthcare and contraception is equal. In reality, “it’s not even close to being equal,” he emphasizes. Therefore, there is also unequal reproduction and unequal survival and people everywhere remain exposed to all manner of selective mechanisms.
Concurrently, there’s also other drivers of human evolution that are likely unique to our species, at least in their intensity. In many instances, our cultures influence with whom, how, and if people reproduce, note both Hodgson and Hawks. Those things also go on to affect the frequency of gene variants across time.
“We are maybe somewhat tweaking the course of evolution, but it doesn’t at all mean we’re stopping it from happening,” says Hakhamanesh Mostafavi, an associate professor of genetics and genomics at New York University
How do we know evolution is still happening?
Many studies of humans’ genetic past provide illustrative examples of evolution in action. There’s the rise of malarial resistance in Madagascar, linked to the proliferation of a specific gene variant in the population, as described in 2014 research from Hodgson. That example of evolution, he says, occurred within the past 2,000 years–very recent on the order of evolutionary time.
There’s the emergence and spread of alleles that enable adult lactose digestion in some Middle Eastern, European, and African populations, following the spread of herding. “Even within the past 1,000 years, lactase persistence as an allele is increasing,” Hawks says.
And even more recently than that–in the last century–Stearns and his research colleagues attributed height decreases and other population-level changes to natural selection in a classic 2010 study of people in Framingham, Massachusetts, based on a data collection endeavor begun in 1948.
Studies of large genomic datasets also reveal changes that wouldn’t otherwise be observable on a trait-level. A 2022 paper identified two small changes in the human genome, responsible for creating functional proteins, which emerged since our species split from other primate lineages.
“We are maybe somewhat tweaking the course of evolution, but it doesn’t at all mean we’re stopping it from happening.”
In a 2017 study led by Mostafavi, he and his colleagues examined gene differences between age groups in the UK Biobank, a repository of about half a million British peoples’ genetic and health data. They were looking for widespread alleles that were less common among older people, and therefore likely linked with longevity and survival. Across the whole dataset, they only found two such gene variants–one related to heavy smoking in men and one related to Alzheimer’s risk. Despite the fact that many differences in health are genetic, there were relatively few identifiable, harmful single gene variants circulating in the study population. Which means evolution likely already filtered out those genes.
“The absence [or more deleterious alleles] is a strong suggestion for selection,” says Mostafavi. The two genes that were identified in the study are liable to be filtered out with additional time as well, he says. The fact that they’re currently common might mean that they’ve only recently become harmful.
The type of evolution demonstrated in Mostafavi’s research is purifying selection: The removal of harmful mutations from a population over time. It’s less flashy than selection in favor of beneficial traits, but it’s much more common.
Another common form of not-so-flashy selection is stabilizing selection, where extremes remain rare, in favor of a tendency towards the middle ground. Birth weight and human height are standard examples of where stabilizing selection acts, he says.
“Maybe we don’t always see stabilizing [and] purifying selection, maybe they’re not obvious… but we know for a fact that many human [traits] are somehow kept at some optimum,” indicating that both forms of natural selection remain ongoing.
So what are we evolving into?
“Most people want to know, ‘are we directionally changing as a species into something else?’ That is: Is there a destination at the end of this?” says Hawks. The answer, at least on the timescale scientists can study it, is unclear.
Evolution is a process, not an outcome, and it doesn’t always happen linearly. “Today’s environments are changing really fast in various ways. We don’t know for sure which changes will be sustained over time, so we don’t know what changes might add up to anything. [Many] changes might reverse and go the opposite direction just as quickly as they evolved in the last generation or two,” Hawk adds.
“I personally think that our genetics are going to continue to change, probably at an accelerated rate,” he says, “but I do not have a good basis for predicting how.”
