Human species has evolved over the past two thousand years and continues to evolve

Nine out of ten human characteristics have undergone evolution by natural selection over the past two thousand years. This type of evolution, focused on survival and reproduction, generated as accidental by-products skin cancer, irritable bowel syndrome, attention deficit disorder, and anorexia nervosa. These are the findings of a large study published in the journal Nature Human Behavior last week.

Weichen Song and the other authors of the study, the Shanghai Mental Health Center selected 800 complex human characteristics whose genetic causes were studied throughout. the genetic material (genome). In genetics, complex traits are those that have many genes involved, each with a small effect on development, but which are also malleable to the influence of the environment. Height is one of these characteristics, as is all behavior that has a genetic basis.

The characteristics were studied in different time scales: the present, recent history (two to three thousand years), the Neolithic period (the final phase of the Stone Age, encompassing the last ones 10 thousand years) and the time since the human species emerged (about 200 a thousand years).

Schizophrenia and attention deficit hyperactivity disorder (ADHD) were associated with a higher number of sexual partners. However, the genetic risk of having some of the complex characteristics that are disorders was not associated with a greater number of children.

The researchers also considered gender differences. Characteristics such as body mass index, height and the ability to tan the skin have a greater impact on the number of children for men than for women. Most notorious is that high intelligence reduces women’s fertility, but increases their number of sexual partners.

Adaptations in recent history

Natural selection is the survival of characteristics that in a given environment help the organism to continue alive and produce children, while the characteristics that hinder it become increasingly rare or disappear. The process necessarily leads to adaptation. Chinese scientists found that 48% of the characteristics studied they have signs of being adaptations reinforced by selection over the past two millennia. It is important to note that the genetics of most of them are based on the UK Biobank, a database that contains the genome of 88 thousand people from the UK, so the results mainly apply to that population.

The ease of tanning the skin is one of the adaptations with stronger marks of natural selection in recent years 200 generations (2.720 years old). Others are the behavior of consuming raw vegetables, alcoholism and the height when sitting. On the other hand, neurological characteristics such as brain structures showed little sign of natural selection in the period, which is plausible, as human beings changed little in their behavioral nature during this period.

With their methods, the researchers were also able to assess which characteristics natural selection acted against. High cholesterol had the biggest negative sign, which indicates that it greatly affected their carriers’ chances of surviving and having children. Surprisingly, some diseases have a positive selection signal, meaning that they accompany other characteristics that natural selection favors. This is the case with skin cancer, irritable bowel syndrome, ADHD and anorexia nervosa. What would they accompany, to be favored? Anorexia nervosa may accompany food-seeking behavior altered by the threat of hunger. The irritable bowel can accompany a stronger immune system defense activity against germs, which was an adaptation in times of unhygienic life, and does not hinder us enough today to be eliminated.

Ten thousand years before Christ2021 In the Neolithic period mankind developed agriculture, domesticated animals, and this is written in our genes. Culture, together with the physical environment, also acted as natural selection on our mutations and genetic variants. There are signs that we evolved to digest starch more easily, and in those societies with milk consumption, adults who retained the ability to digest lactose had an advantage: previously, lactose intolerance was the rule.

In the study by Song and colleagues, they used genetic information collected from bodies preserved in the last ten thousand years to compare with today’s human genome. Again, changes in the skin’s ability to tan and nutrition-related traits were the most marked by selection-driven change. The authors were able to show that the change in the ease of tanning was directly related to the latitude in which the ancestors lived, and that, in fact, the advent of the Neolithic shows that natural selection changed its agenda: characteristics before the period related to hunting and gathering were favored, and later characteristics related to agricultural life began to be favored.

Diseases that accompany other characteristics were also favored: Crohn’s disease (chronic intestinal inflammation), atopic dermatitis (dry itchy skin), periodontitis (inflammation in the gums, ligaments and bones around the teeth) and fractures.

As for the period since the birth of humanity in the last hundreds of thousands of years, the researchers indicate that natural selection paid attention to the body’s water mass, intelligence, lower chance of stroke, memory and schizophrenia. Certainly related to our larger brain and our unique ability to use sweat water to refresh the body during physical exertion.

We continue to evolve

The study confirmed that the intensity of natural selection acting on us has changed dramatically over the past few millennia. But that doesn’t mean we stopped evolving. Evolution, in biology, means changing the characteristics of populations over time. It is possible to evolve in a neutral way regarding survival and reproduction, or by natural selection, which is in the direction of maintaining or improving them. Our post-industrial era presents environments that are different from those in which our hunter-gatherer, herder and farmer ancestors lived. Today, most of us no longer work in the field. In the next few millennia, our genomes will tell another story.