Recent human evolution refers to evolutionary adaptation, sexual and natural selection, and genetic drift within Homo sapiens populations, since their separation and dispersal in the Middle Paleolithic about 50,000 years ago. Contrary to popular belief, not only are humans still evolving, their evolution since the dawn of agriculture is faster than ever before. It has been proposed that human culture acts as a selective force in human evolution and has accelerated it; however, this is disputed. With a sufficiently large data set and modern research methods, scientists can study the changes in the frequency of an allele occurring in a tiny subset of the population over a single lifetime, the shortest meaningful time scale in evolution. Comparing a given gene with that of other species enables geneticists to determine whether it is rapidly evolving in humans alone. For example, while human DNA is on average 98% identical to chimpanzee DNA, the so-called Human Accelerated Region 1 (HAR1), involved in the development of the brain, is only 85% similar.
Following the peopling of Africa some 130,000 years ago, and the recent Out-of-Africa expansion some 70,000 to 50,000 years ago, some sub-populations of Homo sapiens have been geographically isolated for tens of thousands of years prior to the early modern Age of Discovery. Combined with archaic admixture, this has resulted in relatively significant genetic variation. Selection pressures were especially severe for populations affected by the Last Glacial Maximum (LGM) in Eurasia, and for sedentary farming populations since the Neolithic, or New Stone Age.
Single nucleotide polymorphisms (SNP, pronounced 'snip'), or mutations of a single genetic code "letter" in an allele that spread across a population, in functional parts of the genome can potentially modify virtually any conceivable trait, from height and eye color to susceptibility to diabetes and schizophrenia. While approximately 2% of the human genome codes for proteins and a slightly larger fraction is involved in gene regulation, the remainder has no known function. If the environment remains stable, the beneficial mutations will spread throughout the local population over many generations until it becomes a dominant trait. An extremely beneficial allele could become ubiquitous in a population in as little as a few centuries whereas those that are less advantageous typically take millennia.
Human traits that emerged recently include the ability to free-dive for long periods of time, adaptations for living in high altitudes where oxygen concentrations are low, resistance to contagious diseases (such as malaria), light skin, blue eyes, lactase persistence (or the ability to digest milk after weaning), lower blood pressure and cholesterol levels, retention of the median artery, reduced prevalence of Alzheimer's disease, lower susceptibility to diabetes, genetic longevity, shrinking brain sizes, and changes in the timing of menarche and menopause.
Archaic admixture
Palaeogenetic evidence suggests that the last common ancestor of modern humans, Neanderthals and Denisovans lived approximately 765,000–550,000 years ago, although its morphology, geographical distribution and taxonomic identity remain uncertain. Homo heidelbergensis was historically proposed as this common ancestor, but fossils assigned to the species may not represent a single coherent lineage, and many Eurasian specimens traditionally classified as H. heidelbergensis are now considered more closely related to the Neanderthal–Denisovan lineage than to Homo sapiens.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; East Asians, South Asians, Australo-Melanesians, Native Americans, and Europeans all share Neanderthal DNA, so hybridization likely occurred between Neanderthals and their common ancestors coming out of Africa. Their differences also suggest separate hybridization events for the ancestors of East Asians and other Eurasians.
Following the genome sequencing of three Vindija Neanderthals, a draft sequence of the Neanderthal genome was published and revealed that Neanderthals shared more alleles with Eurasian populations—such as French, Han Chinese, and Papua New Guinean—than with sub-Saharan African populations, such as Yoruba and San. According to the authors of the study, the observed excess of genetic similarity is best explained by recent gene flow from Neanderthals to modern humans after the migration out of Africa. 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.
Some climatic adaptations, such as high-altitude adaptation in humans, are thought to have been acquired by archaic admixture. An ethnic group known as the Sherpas from Nepal is believed to have inherited an allele called EPAS1, which allows them to breathe easily at high altitudes, from the Denisovans. A 2014 study reported that Neanderthal-derived variants found in East Asian populations showed clustering in functional groups related to immune and haematopoietic pathways, while European populations showed clustering in functional groups related to the lipid catabolic process. A 2017 study found correlation of Neanderthal admixture in modern European populations with traits such as skin tone, hair color, height, sleeping patterns, mood and smoking addiction. A 2020 study of Africans unveiled Neanderthal haplotypes, or alleles that tend to be inherited together, linked to immunity and ultraviolet sensitivity.
The gene microcephalin (MCPH1), involved in the development of the brain, likely originated from a Homo lineage separate from that of anatomically modern humans, but was introduced to them around 37,000 years ago, and has become much more common ever since, reaching around 70% of the human population at present. Neanderthals were suggested as one possible origin of this gene. But later studies did not find this gene in the Neanderthal genome nor has it been found to be associated with cognitive ability in modern people.
The promotion of beneficial traits acquired from admixture is known as adaptive introgression.
A study concluded only 1.5–7% of "regions" of the modern human genome to be specific to modern humans. These regions have neither been altered by archaic hominin DNA due to admixture (only a small portion of archaic DNA is inherited per individual but a large portion is inherited across populations overall) nor are shared with Neanderthals or Denisovans in any of the genomes of the used datasets. They also found two bursts of changes specific to modern human genomes which involve genes related to brain development and function.
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.
Around 65,000 to 50,000 years ago, a variety of new technologies, such as projectile weapons, fish hooks, porcelain, and sewing needles, made their appearance. Bird-bone flutes were invented 30,000 to 35,000 years ago, indicating the arrival of music. Artistic creativity also flowered, as can be seen with Venus figurines and cave paintings. Cave paintings of not just actual animals but also imaginary creatures that could reliably be attributed to Homo sapiens have been found in different parts of the world. Radioactive dating suggests that the oldest of the ones that have been found, as of 2019, are 44,000 years old. For researchers, these artworks and inventions represent a milestone in the evolution of human intelligence, the roots of story-telling, paving the way for spirituality and religion. 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.