The future of human evolution

It is a continuous and lengthy process happening right under our noses; but what does the future hold for human evolution? As a species, we are still evolving; we look different to humans 200,000 years ago, and our appearance will probably have changed in another 200,000 years. However, our evolution is no longer shaped by natural selection alone; medical advances are sculpting the future of human evolution with modern medicine having had a profound effect. Compared to the 19^th century, more children now reach their adult years, and thanks to our use of vaccines, antibiotics and pharmaceuticals, we have an increasingly ageing population. “Modern medicine, extending the major contributions of clean water with vaccines and antibiotics, has significantly changed the selection pressures operating on humans, primarily through a dramatic reduction in mortality rates and cultural management of reproduction with contraceptives,” says Professor Stephen Stearns, an evolutionary biologist from Yale University. We are becoming increasingly reliant on medicines to survive – and the very adversaries we hope to destroy with medication are increasingly becoming resistant to their effects. Selective pressure through disease has lessened: the weak survive to pass on their genes and survival of the fittest has a much lesser impact. “The fact that everybody stays alive, at least until they’re sexually mature, means ‘survival of the fittest’ has got nothing to work with,” says Steve Jones, genetics professor at University College London. “That part of the Darwinian fuel has gone.” But medical advances can offer much more than prolonging life. It already has a helping hand in creating it, and could go further by opening up the possibility of parents being able to choose the traits of their offspring before they are even conceived. “Parents could basically choose which sperm and egg get to meet up to produce a baby based on genetic information about which genes contribute to which physical and mental traits,” says Geoffrey Miller, an evolutionary psychologist from the University of New Mexico. “I see no major moral issues with genetic engineering to avoid birth defects and genetic diseases expressed later in life, but then I am not influenced in that judgement by religion, and many are. I respect their opinions,” Professor Stearns says. “As for genetic engineering for intelligence, beauty, athletic performance ... that will be difficult and very expensive, therefore accessible only to the rich. In all these cases the technical problems are immense and the fundamental science is not yet well understood, so you don't have to hold your breath.” While it is possible to eliminate undesirable mutations, parents might also unwittingly eliminate those which are the strongest, or most beneficial to the species as a whole. We could be inadvertently selecting weak traits, or traits that would have died out naturally.

Attempts to predict are useless, for culture is changing much faster than biology can respond

This isn’t evolution in the traditional sense, but it could change the future of our species. Medical advances mean that we are creating a false evolution. Designer babies and disease treatment and prevention mean that we are artificially selecting the genes to pass on to future generations, whether through a conscious choice or simply allowing them to survive. Through this engineering we could be stunting our own evolution; by artificially selecting genes in this way, we could be preventing the emergence of significant mutations required for evolution to occur. Combine this with our tendency to travel all over the globe ­–­ opening up the possibility of inter-population breeding – evolution could be weakened. “Everything we know about evolutionary change suggests that genetic innovations are only likely to become fixed in small isolated populations,” says Ian Tattersall, an anthropologist from the American Museum of Natural History. “If populations aren’t isolated, crossbreeding makes it much less likely for potentially significant mutations to become established in the gene pool – and that’s exactly where we are now.” It is likely that humans will converge genetically as populations mix and we will probably all begin to look the same – it could even mean natural blondes and red-heads become an endangered phenotype. “I kind of view us all as eventually having chocolate-coloured hair and medium stature, getting rid of all extremes,” says Peter Ward, a palaeontologist from the University of Washington at Seattle. “I want us to have big heads and be smarter. I suspect we will all be tan, shorter, and unhappy for much of the future.” Professor Stearns worries about the accuracy of such long-term predictions: “We are able to predict some changes in the very short term. In the 20th century there was selection for shorter, chubbier women in Massachusetts and taller, thinner women after 1975 in The Gambia, but those are both short-term trends, and they probably will not amount to much,” he says. Advances in medicine aren’t the only influence we are having on our evolution. Historically, key human adaptations have been the result of environmental instability. A 2013 study showed that human evolution was driven by short pulses of rapid environmental change in East Africa’s Rift Valley. “It seems that modern humans were born from climate change, as they had to deal with rapid switching from famine to feast – and back again – which drove the appearance of new species with bigger brains and also pushed them out of East Africa into Eurasia and South Africa,” says Professor Mark Maslin from University College London. “We have long recognised that many key events in human evolution, including the appearance of modern humans, occurred in East Africa,” says co-author Dr Susanne Shultz from the University of Manchester. “Our study highlights how important the Rift Valley climate was in driving the evolution of our species.” A 2010 study found that the Yoruba people of West Africa – who are exposed to the dry conditions of the Sahara desert – had a variation in a gene known to be involved in water retention. Andreas Moreno of Stanford University compared DNA samples of 20 Yoruba, 20 Europeans and 20 East Asians; this interspecies comparison showed the gene – FOXI1 – was under human-specific selection and was positively selected for in the Yoruba people. The study suggests the change occurred in the last 10,000 to 20,000 years. “Climate change might be related to this recent adaptive event in humans,” Moreno writes in Evolutionary Biology. “Of multiple functions of FOXI1, its role in kidney-mediated water-electrolyte homeostasis is the most obvious candidate for explaining a climate-related adaption.” Shultz argues that despite substantial evidence to suggest that climate extremes were important in our evolution – speciation in times of plenty, extinction in times of scarcity – we are not as sensitive to the climate now because of the complex technology developed to help us overcome climate variation. Instead, human evolution is likely to be guided by more immediate selection pressures such as competition for somewhere to live as well as for food and water. Unseasonable weather is already affecting crop yields, and plants are beginning to adapt their flowering time to reflect the changing climate which will only help to compound the problem of available resources. “There are potentially bigger issues with food security and biodiversity loss. Starvation can affect people differently. Individuals that are better able to store fat reserves and have lower metabolisms should cope with famine conditions better. Moreover, food production changes in the future to cope with changing climate could impose new selection pressures on people. There could be novel pressures in the future,” Shultz says. And the survival of our species is dependent on the survival of others. The role of pollinators like bees has been widely discussed; our use of insecticides and destruction of their habitat means fewer bees to pollinate crops, so fewer crops and more competition for food. As humans we will continue to evolve, change and adapt; exactly what changes will be selected for are impossible to predict. Our way of life is changing the selection pressures acting upon us and we can’t predict whether we will evolve to our varying conditions quick enough. “The recognition that smoking, cholesterol, and obesity are major health risks is leading to lifestyle changes that are also altering selection pressures. In brief, human culture has become a major player in human evolution,” Professor Sterns says. “In the longer term, attempts to predict are useless, for culture is changing much faster than biology can respond, and we have no idea what the selection pressures on humans will be just a few decades from now, much less for the next several millennia, the time period needed to produce significant responses.” Author Kerry Taylor-Smith Kerry is web editor on Laboratory News and has a degree in Natural Sciences from the University of Bath