Tech UPTechnologyHow natural selection has shaped modern humans

How natural selection has shaped modern humans

The publication of The Origin of Species baffled Victorian readers in 1859 , its ideas were excessively provocative, despite the fact that in its pages Charles Darwin barely explained how that revolutionary idea of evolution applied to humans. Twelve years later, with all the scientific and social uproar that his theory had aroused, he decided to address the issue. On February 24, 1871, his second book, The Origin of Man and Selection in Relation to Sex , was made public. The Spanish translation appeared a few years later, in 1880. The work was a new publishing success . His name on the cover, along with the primates, humans and sex within, made the book sell very well despite the provocative ideas expressed in its pages. In them he argued insistently in placing people together with other organisms. Neither more nor less, humans were subject to the same natural laws as animals : natural selection and sexual selection.

sexual selection

In his second book, Darwin further developed his concept of sexual selection , regarding it as a force as important as natural selection . His ideas about the role of sex in evolution were not only closely linked to the evolution of humans as a species, but he used them to develop his own theory about the origin of human races . Although today, the idea that there are different human races has been dismissed from a genetic and biological point of view by the scientific community, in the Victorian era it was a hot topic in academic circles. At the end of the 19th century, naturalists questioned the existence of the observed human variety. So much diversity was disconcerting. Why weren’t people the same everywhere? How had the different races originated?

There were those who considered that each race had a unique origin, as if they were different species. The different origin thus explained the differences between people from distant continents. This view, known as “polygenism” , was the main current of scientific thought in the United States, where academics and intellectuals helped to justify slavery from a scientific point of view . His ideas were also very popular among European academics. The Anthropological Society of London, which came to support the Confederacy during the American Civil War, held the same ideas: the races had different origins. Charles Darwin was against it. He was a supporter of the monogenic thesis , according to which there was a common origin for all humans. That was an essential point in his theory of evolution, but considering that all people had a single origin meant looking for an alternative explanation for current human diversity. If there had been a common origin, why did people move from one continent to another?

Darwin found his argument in sexual selection and considered that the people of a human group were naturally more attracted to the members of their group than to those of any other group. He imagined a remote time in which there were a few human tribes in the world, hardly differentiated morphologically from each other. But there would be small details between them that would make a person from a tribe feel more attracted to someone from their own community than to those from any other group. This attraction inevitably led to the gradual differentiation of the populations over time, giving rise to the current diversity. Darwin was nonetheless a man of his time who was influenced by his own biases around gender and race in the way he theorized and thought about science. It was inconceivable that a white person could be attracted to a black person, just as a black person could not be attracted to a white person.

Despite this, Darwin, unlike many of his contemporaries, openly opposed slavery; but at the same time, like most of his contemporaries, he did believe in a racial hierarchy, with Africans at the bottom and Europeans at the top.

Diversity product of evolution

Fortunately, these ideas have disappeared from the scientific realm. Genetic studies developed in recent decades have led to discard the idea that there are different human races , unlike what happens with other animals. The genetic diversity between populations of chimpanzees separated by a few kilometers can be up to seven times greater than what has been recorded between a person native to Asia and Africa or between someone native to Europe and America. It is obvious that the members of a population are genetically more similar to each other than the people of distant populations, but these population differences are progressive and relatively small, enough, however, to generate the amazing human diversity that is not without fascination. A diversity that is the product of evolution , in which human populations have been, and continue to be, subjected to the natural selection described by Charles Darwin, but not to the sexual selection proposed in 1871 in The Origin of Man and Selection in relation to sex . Like the rest of the organisms, human populations have adapted locally to their habitats.

Homo sapiens – a locally adapted global species

Although it is surprising to us, given the current ability to travel far and wide and visit the most remote places on the planet, local adaptation has been and continues to be important in human evolution. Since our species, Homo sapiens, emerged on the African continent now some 300,000 years ago, it has rapidly colonized and inhabited a wide variety of new environments, both in Africa and beyond. Not more than 75,000 years ago, some human groups ventured beyond the continent where the species was born to occupy any ecosystem they have encountered: temperate and tropical forests, islands, mountains, coasts, deserts and even arctic landscapes. . Humans have managed to settle in all kinds of territory.

The settlement of each of these habitats and the development of cultural practices have driven local adaptations. Today we know that local adaptations, the result of natural selection in each place, have contributed significantly to the genetic differentiation of populations . This has allowed humans to locally adapt to various diets, pathogens, average temperatures, UV exposure, and altitudes. Many of these adaptations have resulted in phenotypic differences between people from diverse backgrounds. Their knowledge not only helps us understand our evolution and history as a species, but also has an important practical component.

Many of the differences include health-related traits , the prevalence of particular diseases, and response to medications that can lead to population differences in genetic risk or disease prevalence, contributing to the potential for large health disparities between populations. This may be because genetic variants, once advantageous, are detrimental in modern societies. A clear example of this is the problems suffered by the populations of Samoa with a higher prevalence of type 2 diabetes, metabolic disorders and obesity. Its inhabitants present variants, such as the CREBRF gene, which allows rapid weight gain, something possibly advantageous when food was scarce and populations suffered frequent starvation, but a problem now that there is a greater availability of calories. In other populations, adaptations to low levels of amino acids, possibly from their historical agricultural diets, may lead to increased risk of celiac disease and intestinal inflammation problems. Evolution never stops. We humans are constantly subjected to it, and even speed it up. Our ability to alter ecosystems, obtain new ways of manipulating and altering resources has exposed us throughout history to new selective pressures. Not only the environment, but also culture has contributed to the differentiation of our populations, just as genetic changes can give rise to unique cultural practices.

very large spleens

This is the case of the Bajau, the inhabitants of the coasts of Indonesia, the Philippines and Malaysia , capable of submerging themselves under water to depths of up to 60 meters for almost a quarter of an hour. Most of us barely hold our breath for a minute or two, yet they can dive for long periods to fish.

They have been living for centuries from the resources they obtain from the sea, a way of life that they could not possibly lead if it were not for the fact that they have a genetic mutation that makes them have proportionally larger spleens than the rest of the people. Disproportionately large spleens have also been observed in some marine mammals such as seals and walruses, which carry out much of their activity underwater, relative to those of terrestrial species.

On the other hand, there are numerous adaptive responses as a result of agricultural practices throughout the planet. Cultural habits have promoted the selection of some variants, the best known case being that of lactose tolerance . Some populations have a genetic mutation that allows lactase enzymes to digest the lactose sugars present in milk. The daily consumption of milk was not possible until the Neolithic and the domestication of sheep, goats and cows. Genetic studies have shown that 4,000 years after the introduction of agriculture and animal husbandry in central Europe, only 12% of people were in possession of the variant that confers tolerance to lactose, while a few centuries later Later, in the Late Middle Ages, the percentage of tolerant people was already 60% . Today, in these countries, the population with this variant ranges between 70 and 90%. Historical changes in its frequency constitute evidence of positive selection among human populations, although what evolutionary advantage it entails for its carriers remains poorly understood, although it has been suggested that the high energy value of milk may have provided greater chances of survival during periods of food scarcity to those able to digest it.

Shaping contemporary Homo sapiens

Hybridization with archaic forms and natural selection have shaped contemporary Homo sapiens . Much has been learned in recent decades about the origin of differences between human populations. Racist prejudices have been banished in order to understand the evolutionary processes and mechanisms behind so much diversity. Sometimes, some traits can be attributed to the product of migrations, as well as random genetic drift; but in most of them the consequences of the natural selection described by Darwin can be deduced. Today it is known that selection can act on new mutations that appear randomly in the genome , and are advantageous when they appear.

Cross between Homo sapiens and Neanderthals

When this happens, the frequency of the advantageous mutation tends to increase rapidly among the population under selection, especially when it is strong or if the new allele is dominant or beneficial in heterozygosity. Sometimes, selection takes place on previously existing variations in populations, which cease to be neutral and become advantageous when there is an environmental change that alters the selective pressure. In these cases, the selected variation is older than the selective pressure. It is quite possible that many local human adaptations have been from previously existing variations , since the recent rapid dispersal and selection of populations out of Africa has provided little time for the appearance of new mutations.

Another source of diversity on which natural selection ends up acting is the mixture between individuals from genetically different populations . This allows new beneficial variants to be introduced. The spread of genes from one population to another has been constant throughout human history, for example, variants that confer lactose tolerance have been observed to spread from East African pastoralists to Khoisan populations from southern Africa, just as in earlier times the Neolithic settlers of Anatolia transmitted them to European populations.

One of the most fascinating findings in human evolution in recent years has been the discovery that gene exchange has not only taken place between populations, but also as a product of interbreeding between modern humans ( Homo sapiens ) in the past. and other archaic humans like Neanderthals and Denisovans. This exchange of genes is called “introgression” and its knowledge in humans was not possible until 2010 with the improvement of molecular techniques that have allowed researchers to recover ancient DNA from human remains thousands of years old and obtain reference genomes of Neanderthals and Denisovans. The first human groups to leave Africa interbred repeatedly with groups of Neanderthals in Eurasia and with Denisovans in Asia, these two archaic forms and modern humans had diverged about 800,000 years ago. The exchange of genes between them allowed modern humans to appropriate genes that had been inhabiting what for them were new conditions for thousands of years. Neanderthals and Denisovans had adapted to the temperate regions of Eurasia as well as the tropics of Asia. They had been living with their climates and their pathogens for hundreds of thousands of years.

It has been found that most of the exchanged alleles were harmful to the people who carried them, so that most of the genes acquired from Neanderthals were quickly deleted . Within a few generations, most of the Neanderthal DNA was purged down to the 1-4% currently seen in Eurasian and American populations. A percentage that has persisted until today, either because its effects are neutral or because they have been selected as an advantage.

Because only some human populations experienced introgression from Neanderthals and Denisovans, their adaptive contributions have also contributed to population differences. The importance of adaptive introgression is one of the great novelties in the study of human diversity. The benefits of these hybridizations of the past have been demonstrated among the inhabitants of the most extreme environments: in the adaptations to the cold of the Inuit of Greenland and the adaptations to hypoxia of the Tibetans. However, also in the immune functions of many populations, where Neanderthal and Denisovan introgressions were able to act as vaccines and help fight pathogens from new environments by bringing new strains to the immune system.

Neanderthal introgressions have been suggested to be particularly important among European populations in resistance to RNA viruses . More than 300 genes of Neanderthal and Denisovan origin have already been identified with some effect, although they are not always positive. Some introgressions have been linked to an increased risk of depression, diabetes, Alzheimer’s and other pathologies. During the COVID-19 pandemic, it has been seen that a Neanderthal variant could increase the risk of having complications from a SARS-CoV-2 infection, while other variants seem to reduce the risk of complications.

Natural selection is dynamic

The truth is that, despite advances in recent decades, much remains to be learned about the causes and mechanisms of local adaptation that have generated the current human diversity, as well as its current consequences. Life is a continuous change. The adaptations of the past can be a burden in the present. What was selected in its day because it turned out to be useful, today can become harmful. The higher incidence of cardiovascular diseases that have been associated with Neanderthal introgressions has to do with our lifestyle, just as it has happened to the people of Samoa when they changed their diets. Natural selection is dynamic, changing, we ourselves have changed it repeatedly. Evolution cannot stop. We have been, and always will be, subject to natural selection in one way or another.

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