Recent human evolution

Genetic evidence suggests that a species dubbed Homo heidelbergensis is the last common ancestor of Neanderthals, Denisovans, and Homo sapiens. This common ancestor lived between 600,000 and 750,000 years ago, likely in either Europe or Africa. Members of this species migrated throughout Europe, the Middle East, and Africa and became the Neanderthals in Western Asia and Europe while another group moved further east and evolved into the Denisovans, named after the Denisova Cave in Russia where the first known fossils of them were discovered. In Africa, members of this group eventually became anatomically modern humans. Migrations and geographical isolation notwithstanding, the three descendant groups of Homo heidelbergensis later met and interbred. Archaeological research suggests that as prehistoric humans swept across Europe 45,000 years ago, Neanderthals went extinct. Even so, there is evidence of interbreeding between the two groups as humans expanded their presence in the continent. While prehistoric humans carried 3–6% Neanderthal DNA, modern humans have only about 2%. This seems to suggest selection against Neanderthal-derived traits. For example, the neighborhood of the gene FOXP2, affecting speech and language, shows no signs of Neanderthal inheritance whatsoever. Introgression of genetic variants acquired by Neanderthal admixture has different distributions in Europeans and East Asians, pointing to differences in selective pressures. Though East Asians inherit more Neanderthal DNA than Europeans; Their differences also suggest separate hybridization events for the ancestors of East Asians and other Eurasians. But gene flow did not go one way. The fact that some of the ancestors of modern humans in Europe migrated back into Africa means that modern Africans also carry some genetic materials from Neanderthals. In particular, Africans share 7.2% Neanderthal DNA with Europeans but only 2% with East Asians. A 2020 study of Africans unveiled Neanderthal haplotypes, or alleles that tend to be inherited together, linked to immunity and ultraviolet sensitivity. But later studies did not find this gene in the Neanderthal genome The promotion of beneficial traits acquired from admixture is known as adaptive introgression. ==Upper Paleolithic, or the Late Stone Age (50,000 to 12,000 years ago)==

Victorian naturalist Charles Darwin was the first to propose the out-of-Africa hypothesis for the peopling of the world, but the story of prehistoric human migration is now understood to be much more complex thanks to twenty-first-century advances in genomic sequencing. There were multiple waves of dispersal of anatomically modern humans out of Africa, with the most recent one dating back to 70,000 to 50,000 years ago. Earlier waves of human migrants might have gone extinct or returned to Africa. Moreover, a combination of gene flow from Eurasia back into Africa and higher rates of genetic drift among East Asians compared to Europeans led these human populations to diverge from one another at different times. Bird-bone flutes were invented 30,000 to 35,000 years ago, indicating the arrival of music. Experts believe this sudden "great leap forward"—as anthropologist Jared Diamond calls it—was due to climate change. Around 60,000 years ago, during the middle of an ice age, it was extremely cold in the far north, but ice sheets sucked up much of the moisture in Africa, making the continent even drier and droughts much more common. The result was a genetic bottleneck, pushing Homo sapiens to the brink of extinction, and a mass exodus from Africa. Nevertheless, it remains uncertain (as of 2003) whether or not this was due to some favorable genetic mutations, for example in the FOXP2 gene, linked to language and speech. A combination of archaeological and genetic evidence suggests that humans migrated along Southern Asia and down to Australia 50,000 years ago, to the Middle East and then to Europe 35,000 years ago, and finally to the Americas via the Siberian Arctic 15,000 years ago. People from warmer climates tend to have thicker lips, which have large surface areas, enabling them to keep cool. With regards to nose shapes, humans residing in hot and dry places tend to have narrow and protruding noses in order to reduce loss of moisture. Humans living in hot and humid places tend to have flat and broad noses that moisturize inhaled air and retain moisture from exhaled air. Humans dwelling in cold and dry places tend to have small, narrow, and long noses in order to warm and moisturize inhaled air. As for hair types, humans from regions with colder climates tend to have straight hair so that the head and neck are kept warm. Straight hair also allows cool moisture to quickly fall off the head. On the other hand, tight and curly hair increases the exposed areas of the scalp, easing the evaporation of sweat and allowing heat to be radiated away while keeping itself off the neck and shoulders. Epicanthic eye folds are believed to be an adaptation protecting the eye from overexposure to ultraviolet radiation, and is presumed to be a particular trait in archaic humans from eastern and southeast Asia. A cold-adaptive explanation for the epicanthic fold is today seen as outdated by some, as epicanthic folds appear in some African populations. Dr. Frank Poirier, a physical anthropologist at Ohio State University, concluded that the epicanthic fold in fact may be an adaptation for tropical regions, and was already part of the natural diversity found among early modern humans. and negritos. Some studies suggest that it could be related to adaptation to low ultraviolet light levels in tropical rainforests. Physiological or phenotypical changes have been traced to Upper Paleolithic mutations, such as the East Asian variant of the EDAR gene, dated to about 35,000 years ago in Southern or Central China. Traits affected by the mutation are sweat glands, teeth, hair thickness and breast tissue. While Africans and Europeans carry the ancestral version of the gene, most East Asians have the mutated version. By testing the gene on mice, Yana G. Kamberov and Pardis C. Sabeti and their colleagues at the Broad Institute found that the mutated version brings thicker hair shafts, more sweat glands, and less breast tissue. East Asian women are known for having comparatively small breasts and East Asians in general tend to have thick hair. The research team calculated that this gene originated in Southern China, which was warm and humid, meaning having more sweat glands would be advantageous to the hunter-gatherers who lived there. The frequency of 370A is most highly elevated in North Asian and East Asian populations. The most recent Ice Age peaked in intensity between 19,000 and 25,000 years ago and ended about 12,000 years ago. As the glaciers that once covered Scandinavia all the way down to Northern France retreated, humans began returning to Northern Europe from the Southwest, modern-day Spain. But about 14,000 years ago, humans from Southeastern Europe, especially Greece and Turkey, began migrating to the rest of the continent, displacing the first group of humans. Analysis of genomic data revealed that all Europeans since 37,000 years ago have descended from a single founding population that survived the Ice Age, with specimens found in various parts of the continent, such as Belgium. Although this human population was displaced 33,000 years ago, a genetically related group began spreading across Europe 19,000 years ago. Alleles predictive of light skin have been found in Neanderthals, but the alleles for light skin in Europeans and East Asians, KITLG and ASIP, are (as of 2012) thought to have not been acquired by archaic admixture but recent mutations since the LGM. Within the last 20,000 years or so, lighter skin has evolved in East Asia, Europe, North America and Southern Africa. In general, people living in higher latitudes tend to have lighter skin. Inuit adaptation to high-fat diet and cold climate has been traced to a mutation dated the Last Glacial Maximum (20,000 years ago). Humans living in Northern Asia and the Arctic have evolved the ability to develop thick layers of fat on their faces to keep warm. Moreover, the Inuit tend to have flat and broad faces, an adaptation that reduces the likelihood of frostbites. Australian Aboriginals living in the Central Desert, where the temperature can drop below freezing at night, have evolved the ability to reduce their core temperatures without shivering. ==Holocene (12,000 years ago to present)==

Neolithic or New Stone Age Impacts of agriculture The advent of agriculture has played a key role in the evolutionary history of humanity. Early farming communities benefited from new and comparatively stable sources of food, but were also exposed to new and initially devastating diseases such as tuberculosis, measles, and smallpox. Eventually, genetic resistance to such diseases evolved and humans living today are descendants of those who survived the agricultural revolution and reproduced. The pioneers of agriculture faced tooth cavities, protein deficiency and general malnutrition, resulting in shorter statures. Examples for adaptations related to agriculture and animal domestication include East Asian types of ADH1B associated with rice domestication, and lactase persistence. Migrations As Europeans and East Asians migrated out of Africa, those groups were maladapted and came under stronger selective pressures. Before this mutation, dairy farming was already widespread in Europe. A Finnish research team reported that the European mutation that allows for lactase persistence is not found among the milk-drinking and dairy-farming Africans, however. Sarah Tishkoff and her students confirmed this by analyzing DNA samples from Tanzania, Kenya, and Sudan, where lactase persistence evolved independently. The uniformity of the mutations surrounding the lactase gene suggests that lactase persistence spread rapidly throughout this part of Africa. According to Tishkoff's data, this mutation first appeared between 3,000 and 7,000 years ago. This mutation provides some protection against drought and enables people to drink milk without diarrhea, which causes dehydration. Lactase persistence is a rare ability among mammals. However, Dan Ju and Ian Mathieson caution in a study addressing 40,000 years of modern human history, "we can assess the extent to which they carried the same light pigmentation alleles that are present today," but explain that Early Upper Paleolithic hunter-gatherers "may have carried different alleles that we cannot now detect", and as a result "we cannot confidently make statements about the skin pigmentation of ancient populations." Eumelanin, which is responsible for pigmentation in human skin, protects against ultraviolet radiation while also limiting vitamin D synthesis. Variations in skin color, due to the levels of melanin, are caused by at least 25 different genes, and variations evolved independently of each other to meet different environmental needs. Eye color Blue eyes are an adaptation for living in regions where the amounts of light are limited because they allow more light to come in than brown eyes. They also seem to have undergone both sexual and frequency-dependent selection. In 2018, an international team of researchers from Israel and the United States announced their genetic analysis of 6,500-year-old excavated human remains in Israel's Upper Galilee region revealed a number of traits not found in the humans who had previously inhabited the area, including blue eyes. They concluded that the region experienced a significant demographic shift 6,000 years ago due to migration from Anatolia and the Zagros mountains (in modern-day Turkey and Iran) and that this change contributed to the development of the Chalcolithic culture in the region. Bronze Age to Medieval Era Resistance to malaria is a well-known example of recent human evolution. This disease attacks humans early in life. Thus humans who are resistant enjoy a higher chance of surviving and reproducing. While humans have evolved multiple defenses against malaria, sickle cell anemia—a condition in which red blood cells are deformed into sickle shapes, thereby restricting blood flow—is perhaps the best known. Sickle cell anemia makes it more difficult for the malarial parasite to infect red blood cells. This mechanism of defense against malaria emerged independently in Africa and in Pakistan and India. Within 4,000 years it has spread to 10–15% of the populations of these places. Another mutation that enabled humans to resist malaria that is strongly favored by natural selection and has spread rapidly in Africa is the inability to synthesize the enzyme glucose-6-phosphate dehydrogenase, or G6PD. The allele in question is named SLC11A1 1729+55del4. Scientists found that among the residents of places that have been settled for thousands of years, such as Susa in Iran, this allele is ubiquitous whereas in places with just a few centuries of urbanization, such as Yakutsk in Siberia, only 70–80% of the population have it. Evolution to resist infection of pathogens also increased inflammatory disease risk in post-Neolithic Europeans over the last 10,000 years. A study of ancient DNA estimated nature, strength, and time of onset of selections due to pathogens and also found that "the bulk of genetic adaptation occurred after the start of the Bronze Age, <4,500 years ago". Adaptations have also been found in modern populations living in extreme climatic conditions such as the Arctic, as well as immunological adaptations such as resistance against prion caused brain disease in populations practicing mortuary cannibalism, or the consumption of human corpses. Inuit have the ability to thrive on the lipid-rich diets consisting of Arctic mammals. Human populations living in regions of high altitudes, such as the Tibetan Plateau, Ethiopia, and the Andes benefit from a mutation that enhances the concentration of oxygen in their blood. This mutation is believed to be around 3,000 years old. As maritime hunter-gatherers, the ability to dive for long periods of times plays a crucial role in their survival. Due to the mammalian dive reflex, the spleen contracts when the mammal dives and releases oxygen-carrying red blood cells. Over time, individuals with larger spleens were more likely to survive the lengthy free-dives, and thus reproduce. By contrast, communities centered around farming show no signs of evolving to have larger spleens. Because the Sama-Bajau show no interest in abandoning this lifestyle, there is no reason to believe further adaptation will not occur. Advances in the biology of genomes have enabled geneticists to investigate the course of human evolution within centuries. Jonathan Pritchard and a postdoctoral fellow, Yair Field, counted the singletons, or changes of single DNA bases, which are likely to be recent because they are rare and have not spread throughout the population. Since alleles bring neighboring DNA regions with them as they move around the genome, the number of singletons can be used to roughly estimate how quickly the allele has changed its frequency. This approach can unveil evolution within the last 2,000 years or a hundred human generations. Armed with this technique and data from the UK10K project, Pritchard and his team found that alleles for lactase persistence, blond hair, and blue eyes have spread rapidly among Britons within the last two millennia or so. Britain's cloudy skies may have played a role in that the genes for light hair could also cause light skin, reducing the chances of vitamin D deficiency. Sexual selection could also favor blond hair. The technique also enabled them to track the selection of polygenic traits—those affected by a multitude of genes, rather than just one—such as height, infant head circumferences, and female hip sizes (crucial for giving birth). They found that natural selection has been favoring increased height and larger head and female hip sizes among Britons. Moreover, lactase persistence showed signs of active selection during the same period. However, evidence for the selection of polygenic traits is weaker than those affected only by one gene. A 2012 paper studied the DNA sequence of around 6,500 Americans of European and African descent and confirmed earlier work indicating that the majority of changes to a single letter in the sequence (single nucleotide variants) were accumulated within the last 5,000-10,000 years. Almost three quarters arose in the last 5,000 years or so. About 14% of the variants are potentially harmful, and among those, 86% were 5,000 years old or younger. The researchers also found that European Americans had accumulated a much larger number of mutations than African Americans. This is likely a consequence of their ancestors' migration out of Africa, which resulted in a genetic bottleneck; there were few mates available. Despite the subsequent exponential growth in population, natural selection has not had enough time to eradicate the harmful mutations. While humans today carry far more mutations than their ancestors did 5,000 years ago, they are not necessarily more vulnerable to illnesses because these might be caused by multiple mutations. It does, however, confirm earlier research suggesting that common diseases are not caused by common gene variants. In any case, the fact that the human gene pool has accumulated so many mutations over such a short period of time—in evolutionary terms—and that the human population has exploded in that time mean that humanity is more evolvable than ever before. Natural selection might eventually catch up with the variations in the gene pool, as theoretical models suggest that evolutionary pressures increase as a function of population size. Early Modern Period to present A study published in 2021 states that the populations of the Cape Verde islands off the coast of West Africa have speedily evolved resistance to malaria within roughly the last 20 generations, since the start of human habitation there. As expected, the residents of the Island of Santiago, where malaria is most prevalent, show the highest prevalence of resistance. This is one of the most rapid cases of change to the human genome measured. Geneticist Steve Jones told the BBC that during the sixteenth century, only a third of English babies survived until the age of 21, compared to 99% in the twenty-first century. Medical advances, especially those made in the twentieth century, made this change possible. Yet while people from the developed world today are living longer and healthier lives, many are choosing to have just a few or no children at all, meaning evolutionary forces continue to act on the human gene pool, just in a different way. For a long time, medicine has reduced the fatality of genetic defects and contagious diseases, allowing more and more humans to survive and reproduce, but it has also enabled maladaptive traits that would otherwise be culled to accumulate in the gene pool. This is not a problem as long as access to modern healthcare is maintained. But natural selective pressures will mount considerably if that is taken away. Population geneticist Emmanuel Milot and his team studied recent human evolution in an isolated Canadian island using 140 years of church records. They found that selection favored younger age at first birth among women. Human evolution continues during the modern era, including among industrialized nations. Things like access to contraception and the freedom from predators do not stop natural selection. Among developed countries, where life expectancy is high and infant mortality rates are low, selective pressures are the strongest on traits that influence the number of children a human has. It is speculated that alleles influencing sexual behavior would be subject to strong selection, though the details of how genes can affect said behavior remain unclear. All this changed in the twentieth century as Caesarean sections (also known as C-sections) became safer and more common in some parts of the world. Larger head sizes continue to be favored while selective pressures against smaller hip sizes have diminished. Projecting forward, this means that human heads would continue to grow while hip sizes would not. As a result of increasing fetopelvic disproportion, C-sections would become more and more common in a positive feedback loop, though not necessarily to the extent that natural childbirth would become obsolete. ), which raises blood pressure.|left Researchers participating in the Framingham Heart Study, which began in 1948 and was intended to investigate the cause of heart disease among women and their descendants in Framingham, Massachusetts, found evidence for selective pressures against high blood pressure due to the modern Western diet, which contains high amounts of salt, known for raising blood pressure. They also found evidence for selection against hypercholesterolemia, or high levels of cholesterol in the blood. The data indicates that the women were not eating more; rather, the ones who were heavier tended to have more children. Stearns and his team also discovered that the subjects of the study tended to reach menopause later; they estimated that if the environment remains the same, the average age at menopause will increase by about a year in 200 years, or about ten generations. All these traits have medium to high heritability. Pickrell and Przeworski also investigated 42 traits determined by multiple alleles rather than just one, such as the timing of puberty. They found that later puberty and older age of first birth were correlated with higher life expectancy. In South Africa, where large numbers of people are infected with HIV, some have genes that help them combat this virus, making it more likely that they would survive and pass this trait onto their children. If the virus persists, humans living in this part of the world could become resistant to it in as little as hundreds of years. However, because HIV evolves more quickly than humans, it will more likely be dealt with technologically rather than genetically. In 2020, Teghan Lucas, Maciej Henneberg, Jaliya Kumaratilake gave evidence that a growing share of the human population retained the median artery in their forearms. This structure forms during fetal development but dissolves once two other arteries, the radial and ulnar arteries, develop. The median artery allows for more blood flow and could be used as a replacement in certain surgeries. Their statistical analysis suggested that the retention of the median artery was under extremely strong selection within the last 250 years or so. People have been studying this structure and its prevalence since the eighteenth century. Multidisciplinary research suggests that ongoing evolution could help explain the rise of certain medical conditions such as autism and autoimmune disorders. Autism and schizophrenia may be due to genes inherited from the mother and the father which are over-expressed and which fight a tug-of-war in the child's body. Allergies, asthma, and autoimmune disorders appear linked to higher standards of sanitation, which prevent the immune systems of modern humans from being exposed to various parasites and pathogens the way their ancestors' were, making them hypersensitive and more likely to overreact. The human body is not built from a professionally engineered blueprint but a system shaped over long periods of time by evolution with all kinds of trade-offs and imperfections. Understanding the evolution of the human body can help medical doctors better understand and treat various disorders. Research in evolutionary medicine suggests that diseases are prevalent because natural selection favors reproduction over health and longevity. In addition, biological evolution is slower than cultural evolution and humans evolve more slowly than pathogens. Whereas in the ancestral past, humans lived in geographically isolated communities where inbreeding was rather common, modern transportation technologies have made it much easier for people to travel great distances and facilitated further genetic mixing, giving rise to additional variations in the human gene pool. It also enables the spread of diseases worldwide, which can have an effect on human evolution. Furthermore, climate change may trigger the mass migration of not just humans but also diseases affecting humans. Besides the selection and flow of genes and alleles, another mechanism of biological evolution is epigenetics, or changes not to the DNA sequence itself, but rather the way it is expressed. Scientists already know that chronic illnesses and stress are epigenetic mechanisms. ==See also==