The ability of modern medicine to keep us alive makes us think that human evolution has stopped. Improving health care is destroying a key driving force of evolution, as some people live longer than they could in their natural environment, making it more likely that their genes will be passed on. But if we look at the rate of evolution of our DNA, we will see that human evolution has not stopped - perhaps it is happening even faster than before.
Evolution is the gradual change in the DNA of a species over many generations. The process can occur through natural selection, where certain traits created by genetic mutations help the body to survive or reproduce. Thus, such mutations are likely to be passed on to the next generation, so they increase in the population. Gradually, these mutations and associated traits become more common among the entire group.
By looking at global studies of our DNA, we can see evidence that natural selection has recently made changes in us and continues to do so. Although modern health care protects us from many causes of death, in countries where there is no access to good health services, populations continue to "evolve." Survivors of infectious disease outbreaks contribute to natural selection by passing on their genetic resistance to their offspring. Our DNA carries evidence of resistance to deadly diseases such as Lassa fever and malaria. Natural selection in response to malaria still continues in regions where the disease remains prevalent.
People also adapt to their environment. Mutations that allow humans to live at high altitudes have become more common among the populations of Tibet, Ethiopia and the Andes. The spread of genetic mutations in Tibet is arguably the fastest evolutionary change in humans over the past 3000 years. This rapid increase in the frequency of a mutant gene that increases the oxygen content of the blood gives local residents an advantage in survival at high altitudes, resulting in more children surviving.
Diet is another source of adaptation. DNA evidence from the Inuit demonstrates their adaptability to the fat-rich diets of Arctic mammals. Research also suggests natural selection for a mutation that allows adults to produce lactase - an enzyme that breaks down milk sugars - which is why certain groups of people are able to digest milk. For more than 80% of Western Europeans, this is natural, but in parts of East Asia, where milk is drunk much less often, it is normal to be unable to digest lactose. As in the case of altitude adaptation, selection for milk digestion has evolved more than once in humans and can serve as an example of evolution.
We can easily adapt to unhealthy diets. A study of family genetic changes in the United States in the 20th century has shown an increase in survival for individuals who are able to maintain low blood pressure and cholesterol levels in modern diets.
Yet despite these changes, natural selection only affects about 8% of our genome. According to the theory of neutral evolution, mutations in the rest of the genome can freely change frequency in populations by chance. If natural selection is weakened, the mutations it would normally cleanse are not removed as efficiently, which can increase their frequency and therefore increase the rate of evolution.
But neutral evolution cannot explain why some genes evolve much faster than others. We measure the rate of evolution of genes by comparing human DNA to that of other species, which also allows us to determine which genes are evolving rapidly only in humans. One of the rapidly developing genes is human accelerated region 1 (HAR1), which is required during brain development. A random stretch of human DNA is, on average, more than 98% identical to a chimpanzee comparator, but HAR1 evolves so rapidly that it is only 85% similar to that of a monkey.
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Although scientists can detect these changes, we still do not fully understand, some genes evolve rapidly, while others are extremely slow. Originally thought to be the result of purely natural selection, we now know that this is not always the case.
Recently, attention has focused on the process of gene transformation that occurs when our DNA is passed through our sperm and egg. The creation of these germ cells involves breaking down DNA molecules, recombining them, and then repairing the gap. However, molecular repair is usually very unusual.
DNA molecules are made up of four different chemical bases known as C, G, A, and T. The repair process makes corrections using bases C and G rather than A or T. Although it is unclear why this offset exists, it makes G and C more common.
Increasing G and C at sites of regular DNA repair triggers ultra-fast evolution of parts of our genome, a process that can easily be mistaken for natural selection, since both cause rapid DNA changes in highly localized regions. This process affected about a fifth of our fastest growing genes, including HAR1. If GC changes are detrimental, natural selection usually opposes them. But with weakening selection, this process can go largely unnoticed and may even help accelerate the evolution of our DNA.
The very level of human mutations can also change. The main source of mutations in human DNA is the process of cell division, spermatogenesis. The older the males get, the more mutations occur in their sperm. Therefore, if their contribution to the gene pool changes - for example, if men delay having children - the mutation rate will also change. This sets the speed of neutral evolution.
Understanding that evolution does not happen only through natural selection makes it clear that the process is unlikely to ever stop. Freeing our genomes from the pressure of natural selection only opens them up to other evolutionary processes, and. as a result, it is even more difficult to predict what future people will be like. However, it is likely that with the advancement of modern medicine, future generations will have more genetic problems.
Original: The Conversation