Archaic Adaptive Introgression
Certainty styling is being phased out topic by topic.
Hover over keys for definitions: An interesting emerging feature of human evolution is the presence and persistence of DNA with non-modern human origins. Prior to the dispersal of the first modern humans from Africa, the precursors to other archaic human groups, namely Neanderthals and Denisovans, migrated and settled across Eurasia. Preceding modern humans by at least a few hundred thousand years, these groups diverged genetically and phenotypically from out last common ancestor, in part due to natural selection imposed by their novel environments. As modern humans emerged from Africa, they overlapped spatially and temporally with these divergent archaic groups. Modern DNA sequencing has revealed a number of regions of the modern human genome that appear to have been introgressed from limited number of human-archaic (i.e., Neanderthal) matings. Although much of the introgressed DNA appears to have undergone negative selection, a few notable regions appear to have been positively selected. These regions include gene involved in immunity, hair and skin pigmentation, freckling, metabolism and high altitude resistance. The positive selection of some introgressed regions suggest that as modern humans migrated from Africa and eventually replaced the archaic groups, sporadic interspecific breeding events may have allowed modern humans to take advantage of the evolutionary history of these groups and more rapidly adapt to their new environments. There is considerable debate about how formative these events were for the emergence of our species, but for at least some human populations they may have been essential for survival.
At current it is unclear how prevalent instances of archaic adaptive introgression are across great apes as the genetic and demographic studies have not been performed. It may be reasonable to presuppose that, among great apes, this evolutionary process is a uniquely human feature stemming from the unusual successive global migrations to diverse environments made by modern humans and their archaic sister species. The construction of the hybrid zones and successive backcrossing required to support archaic adaptive introgression appears to be generally rare in ecology, although it is also an understudied phenomenon.
Adaptive Introgressed Regions
Gene/Region containing Function Likely Source Presence in Evidence Citations
Introgressed haplotype Modern Populations
HLA-A, HLA-B, HLA-C Immune Response Neanderthal Europeans, East Likely 1
and Denisovan Asians, Melanesians
HLA-DPB1 Immune Response Neanderthal Europeans Mixed 2,3
STAT2 (Haplotype N) Pathogen Defense Neanderthal Non-Africans Likely 4
STAT2 (Haplotype D) Pathogen Defense Denisovan Melanesians Speculative 4
OAS1 Viral Immune Response Denisovan (or Melanesians Mixed 5,6,7
unknown group)
OAS gene cluster Viral Immune Response Neanderthal Non-Africans Speculative 6,7
HYAL2 Cellular Response to Neanderthal East Asians Strong 8,9,10,11
UV Radiation
BNC2 Skin Pigmentation and Neanderthal Europeans Strong 5,12,13,14
Freckling
POU2F3 Keratinocyte Neanderthal East Asians Strong 5,12
Differentiation
TMEM136 Unknown Neanderthal East Asians Strong 5,12
MC1R Melanocyte Function; Neanderthal Non-Africans Mixed 8,15
Red-hair, freckles and
fair skin
SLC16A11 Lipid Metabolism Neanderthal Native Americans Likely 16
SLC16A13 Lipid Metabolism Neanderthal Native Americans Likely 16
DMD Skeletal Muscle Neanderthal Non-Africans Speculative 17,18
EPAS1 Response to High Denisovan East Asians Strong 19,20
Altitude Hypoxia (perhaps Tibetans only)
Multiple regions Multiple genes Neanderthal Europeans Likely 21
identified by a involved in
genome-wide scan integumentary system
Multiple regions Multiple genes Neanderthal Europeans and Likely 12
identified by a involved in lipid East Asians
genome-wide scan catabolism
Multiple regions Multiple genes Neanderthal Europeans and Likely 5
identified by a involvedin keratin East Asians
genome-wide scan filament, sugar metabolism,
musclecontraction and
oocytemeiosis
Multiple regions Not characterized Neanderthal Europeans, East Speculative 22, 23
identified by a and Denisovan Asians, Melanesians
genome-wide scan
Table adapted from Racimo et al 2015.
Strong Evidence = Multiple published reports supporting AAI
Likely Evidence = A single published report supporting AAI
Speculative Evidence = Proposed AAI but no published statistical test
Mixed Evidence = Inconsistent published support for AAI
Because of the unique way in which humans and their archaic sister species successively spread across the globe at time intervals that allowed local adaptation, they may be the only great ape to have receive adaptive introgressed DNA. It is unclear if the dispersion and migration of other great apes could have allowed for such a process.
Adaptive Introgression is likely present in most to all human populations, although any one introgressive event may be present in only a subset of populations.
The introgressed DNA was selected via natural selection. Only a few beneficial segments were retained while as a majority of introgressed DNA was selected against.
This type of DNA transfer among sister species appears generally rare in ecology, although it is understudied. There are numerous hybrid zones and interbredding between sister species, but the type of long separation and replacement with introgression has only been observed in a few distant species, namely, salamanders and mice. Archaic adaptive introgression has also been observed in some plant species.
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