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