In his 1895 novel, The Time Machine, H.G. Wells describes a world in which subterranean and forest-dwelling subspecies of humans coexist. To a distant relative of Homo sapiens, the concept of modern humans was likely as foreign as Wells’ world is to us. Still, there is no doubt in the mind of scientists that humans are still evolving.
Image Credit: Pixabay
The main driving forces of evolution are known as selection pressures. How well an individual can overcome these pressures, like predators and diseases, will determine how long it survives and whether it reproduces. By evolving, and passing on genes considered ‘successful’, a species can develop features to help overcome these selection pressures and live longer, reproduce more and ultimately become better suited to the environment in which it lives.
The major advantage we humans possess is the ability and the intelligence to alter our environment, while other species must adapt to better survive within theirs. Farming, healthcare, and lack of predators remove most selection pressures towards humans, and can lead to changes that may well be mistaken for evolution. Is the increasing human lifespan due to evolution or healthcare? We can’t say for sure.
Saying that, there are certain physiological changes that are definitely evolutionary. If wisdom teeth have been as painful a problem for you as for me, you might be glad to know that humans are predicted to lose them altogether. Our jaws are becoming smaller and more bullet-shaped, as cooking and utensils have reduced the need for large, strong jaws with extra teeth for eating tough food.
In addition, our brains are getting smaller, but are being ‘rewired’ to become faster and more efficient. The number of blue-eyed individuals worldwide is increasing. This is thought to be because blue eyes, fair hair and pale skin are linked, and allow greater vitamin D production is low-light environments such as northern Europe. Dark eyes, hair and skin provide more protection from harmful UV rays in hotter regions such as Africa and the Caribbean.
The environment in which human populations live is a major factor when considering evolution, predominantly due to food availability and disease. Seeing as different diseases are more or less common in different areas in the world, humans have evolved different mechanisms to survive.
A huge selective pressure on humans is malaria, a disease transmitted by mosquitos between the tropics of Capricorn and Cancer. In these regions, multiple types of resistance have evolved (and are evolving) separately, such as the sickle cell trait and a type of anaemia called thalassemia. Both of which alter the red blood cells – where the malaria parasite grows in humans. Some mechanisms that have evolved, such as Pyruvate Kinase deficiency, would not be beneficial in malaria-free regions. PKD means your cells cannot make enough energy, leading to many serious health problems. But these health problems are less severe than malaria, so these mutations are beneficial (a type of bet-hedging).
In remote Papua New Guinea, the Fore tribe are the only population of humans that are commonly infected with Kuru, a rare neurodegenerative disease caused by misfolded prion proteins in the brain. Within this tribe, a change in this protein grants resistance to Kuru, and is found nowhere else in nature. Studying human resistance to disease shows how selection pressure has directed mutations within an isolated population.
A map of where different heritable genetic resistances to malaria can be found, the majority of which coincide with the malaria belt. Image Credit: Wikicommons
In terms of how far evolution can go, if you’re hoping that humans will suddenly sprout wings and take to the skies, then I’m sorry to disappoint. It is far more likely that subtle changes will take place to help us better suit the environment in which we live. In rural, developing countries, this may include resistance to diseases to compensate for less effective medical options. However, in more technology-driven communities, eye size is likely to increase and become more efficient in low-light. Facial features are likely to become more objectively appealing, with perfect left:right symmetry.
On a worldwide scale, gene flow as a result of genetic mixing between ethnicities may lead to a future in which humans look extremely similar, with many of the differences between races being lost – a human ‘standard’ of darker skin, large heads and large eyes. The global nature of the modern world, ease of transportation and even social media make this convergence increasingly likely.
Another more alarming possibility is the increase in less favourable traits that become widespread as a result of the modern lifestyle. For example, celiac disease (gluten intolerance), is becoming increasingly common. 50 years ago, it is estimated that 0.2% of people had this disease in the U.S., compared to one in 100 people today. With global trade, humans are becoming less reliant on staples such as grains and wheat, meaning the selection pressure of having celiac disease is less of an issue and individuals can eat enough to survive – increasing the number of people with this disease. Within developing countries where staples are still essential, celiac disease is much rarer.
This is also occurring in type one diabetes, with insulin therapy so effective that patients can live near normal lives, with normal lifespans, and potentially pass the condition onto their children. Furthermore, the battering of our perfectly adapted gut bacteria with prescription antibiotics reduces much innate resistance to disease, and may also affect individual traits such as weight gain and digestive health.
Of course, the possibility of editing human embryos is still a possibility. But it remains to be seen whether this technology is ever routinely used in practice. All in all, the impact of selection has been reduced in humans because of our own intelligence. Evolution will never cease, but just continue adapting humanity to the ever-changing conditions in which we live, with each major change or event providing more ground for evolution to work with.