When I give public lectures, I am invariably asked where evolution will lead us, and what humans will look like in the future, and equally invariably I try to avoid answering such tricky questions. I have, though, taken a different view in public from my geneticist friend Steve Jones over whether human evolution is over. Jones suggests that modern culture and its benefits like medical care have removed the power of natural selection to affect humans, since virtually everyone now reaches reproductive age. I disagree because, first, changes in our genome are occurring all the time, whether we can detect them or not; some calculations suggest that each of us could have about 50 new mutations compared with our parents’ DNA. Second, life in the developed world has its own differential costs in terms of reproduction and health, with the general availability of contraception, but also of junk food, alcohol and drugs. Third, and even more significant, at least a quarter of the world’s population is still denied the benefits of decent health care and the necessities of healthy living conditions and diets. Thus selection is operating strongly on those billions of people, and I cannot see that stopping any time soon. From my perspective, evolution is certainly still working away on Homo sapiens, and there is even evidence that its effects have accelerated rather than diminished over the last 10,000 years.
Science fiction images of humans of the future often show us with a huge brain but big brains are not necessarily the best brains — witness the extinct Neanderthals — and if anything our brains have actually shrunk in size over the last 20,000 years. In practical terms, unless the process of birth is bypassed, our brain size is already at the limit at which the female pelvis can cope with delivery. Then there is the sheer cost in energy of running a big brain, and the evidence that larger brains are not necessarily as efficient at some tasks. And anyway, so much of our memorising and thinking is done externally now — in the brains of other people or in the processors of our computers. All of these factors could be responsible for our shrinking brains, as well as more mundane factors like an overall reduction of body size compared with our Stone Age ancestors.
More realistically for our future evolution, there is the prospect of genetic engineering, which is already happening on a small scale. Genetic counselling is available to advise potential parents about harmful DNA mutations that could be passed on to their children, and to give them the choice about whether to proceed. As this becomes more common and wider in its reach, future gene pools will be affected. Even more ambitiously, gene therapy could be applied to a faulty organ in the body, and germ-line therapy could plant a permanent change in the genome of an unborn baby. There are formidable ethical questions to be addressed here, not to mention the scientific ones. For example, we know that the actions of genes are often interrelated, and that a single gene may perform more than one function. So great care would be needed to ensure that the targeted change in DNA achieved only what was intended. And the social consequences of even giving people the simple choice of a male or a female child are enormous, let alone providing opportunities to enhance that child’s beauty, talents, or intelligence.
And we should bear in mind that selective changes may not benefit everyone; there can be winners and losers, as there has been with the rise of sickle-shaped cells in the blood of African-derived populations. Sickling has benefited those who are heterozygous for the sickle-cell gene (that is, they have only one copy of it) by conferring some immunity against the malarial parasite. But without medical intervention, those born with two copies of the gene will be highly anaemic and will die prematurely. The frequency of a mutation in the leptin receptor gene has increased dramatically in East Asia, linked with changes in the body mass index and a tendency to store fat. This may have been beneficial for adaptation to colder climates but now is a cause of high blood pressure and obesity. Some researchers have also argued that long and stressful sea voyages, whether forced in the case of the slave trade or voluntary in the case of the colonisation of Polynesian islands, would have selected physiques and physiologies that were best able to survive the rigours of those journeys. The survivors then went on to found much larger populations who now live under very different conditions, perhaps explaining the prevalence of salt-sensitive hypertension in American blacks, and of diabetes and obesity in parts of Oceania.
Similarly, as the anthropologist Peter Ellison pointed out, it is possible that the apparent increasing frequency of conditions like autism, schizophrenia, allergies, asthma, autoimmune diseases and reproductive cancers are the modern downside of genetic changes that were beneficial under more ancient human environments and lifestyles. These comparisons between the past and the present are the basis of a whole new field of science called evolutionary medicine.
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Those figures of human species (usually males) marching boldly across the page have illustrated our evolution in many popular articles, but they have wrongly enshrined the view that evolution was simply a progression leading to us, its pinnacle and final achievement. Nothing could be further from the truth. There were plenty of other paths that could have been taken; many would have led to no humans at all, others to extinction, and yet others to a different version of “modernity”. We can only inhabit one version of being human — the only version that survives today — but what is fascinating is that palaeoanthropology shows us those other paths to becoming human, their successes and their eventual demise, whether through failure or just sheer bad luck. Sometimes the difference between failure and success in evolution is a narrow one, and we are certainly on a knife edge now as we confront an overpopulated planet and the prospect of global climate change on a scale that humans have never faced before. Let’s hope our species is up to the challenge.
Extracted from The Origin of Our Species, by Chris Stringer, available now in paperback (Penguin, £9.99). The author is Research Leader in Human Origins, Natural History Museum