Homosexuality – a Survival Advantage for Early Man

John Hayman is a retired pathologist with experience in the diagnosis of diseases associated with HIV infection. He has looked for a reason as to why same sex sexual orientation, with greatly diminished genetic survival prospects for the one individual, should be present with such high frequency in all human populations. ‘Kin selection’ offers an explanation;  survival prospects as a whole are enhanced in those family groups having one or more members with same sex orientation.



History of Homosexuality


              Homosexuality is not a recent phenomenon; it is recorded in the earliest human writings and is depicted in petroglyphs. It is well documented in the Greek and Roman civilizations, in the cuneiform writings of the earlier societies along the Tigris and Euphrates rivers, and in the later history of the middle ages.(1) It is present and is common in all races and cultures, including present day hunter-gather societies.(2) However those with same sex orientation are not spread uniformly through Western society communities; many tend to segregate in enclaves within all major cities and many cities such as San Francisco have large congregations. The annual Gay Festival in Rio de Janeiro attracts some 1,500,000 participants.


              Any apparent variation in the prevalence of homosexuality in different cultures and races will relate to the level of acceptance in that society. As the late Vern Bullough wrote: ‘… at various times in history they (homosexuals) have been put in asylums, imprisoned, medicated, psychoanalysed, and ostracized …’.(1) To this list may be added incinerated (burnt at the stake) and subjected to aversion therapy.(3) A society that vilifies homosexuality will seem to have fewer homosexuals although the actual number in that society is unchanged – overt behavior becomes hidden.


              Supposed causes of homosexuality are as numerous as attempted treatments. Homosexuals are ‘born that way’, have been enticed into homosexuality by an adult, been turned into homosexuals by the lack of a strong parent, been made homosexual by a dominating parent, been trapped in the gang stage of sexual development, been unable to attract a person of the opposite sex, been oversexed or sexually deficient, been at a lower level of human evolution, been rebels against a bourgeois materialistic society, or been victims of various kinds of traumatic experiences.(1) Communist countries have viewed homosexuality with disfavour and as ‘an aberration produced by capitalism’. A favourite, still in vogue among those with fundamental beliefs of one sort or another, is that the homosexual is ‘possessed by the Devil’.(4)


Early Man, Human Evolution and Human Migration


              Early man (Homo sapiens) and the hominids that preceded him (Homo erectus) lived in family groups within larger tribal areas.(5) Within these groups there was division of labor with the adult males hunting as a team and the females caring for children and foraging for food. The males were generally more successful in their acquisition of food, acquiring more calories/hour than the females and passing excess provisions on to the females and their progeny.(6)


Our success as a species was in part by the development of reasoning and by the acquisition of coordination skills necessary for the use of weapons and tools.(7) We also developed forms of communication, allowing individuals to work together in such teams. The continuing development of these skills required progressive increase in the size of the human and pre-human hominid brain.(5)


              The larger brain required a larger cranium. The increasing infant head size was accommodated by modifications to the size and shape to the female pelvis, relaxation of the pelvic ligaments at term, and molding of the neonatal head during birth.(8) These adaptions are limited due to the necessity for maintaining the stability of the female pelvis and the possibility of neonatal brain damage if there is excessive molding. The further increases in human brain size occurred through brain growth after birth. An immature brain at birth, however, resulted in infant almost totally dependent on the mother and two and three year old children who still required considerable parental support. Man is unique among the mammalia in having offspring that are totally dependent on the mother for at least a year and who are partially dependent for some years later.


The anatomical changes in the female pelvis and the need to support children restricted the mobility of the female in comparison to that of the human and hominid male; this in turn led to the division of labor and provisioning of females and children by the adult males. Sexual proclivity altered in both sexes to maintain pair bonding and provisioning.


              Despite this restriction on mobility, modern man migrated out of Africa and moved into Asia, into India, Europe and south-east Asia. Man reached Australia some 65,000 years ago, probably less than 2000 years after the successful African egression. Later movement occurred across the Bering Strait into North and South America. This ability to spread across the entire globe and to settle in six continents is another major factor in our success as a species.


Value of Homosexuality


              Homosexual practice in early hunting parties increased the necessary bonding within the team, making it a more effective unit in hunting, particularly in the killing of larger animals.(9) Like the sacred band of Thebes,(10) the men would hold their ground in the face of the charging beast, mortally wounding it even though sustaining injury themselves. They would also acquit themselves well in the inevitable fighting with neighboring tribes.


              The increased sexual proclivity would tend to restrict absence from the unit base area for long periods but a self-sufficient group of males would be freed from this restriction. A hunting, exploring party far from base would find new, more suitable areas to occupy. When such an area was located part or all of the family unit would relocate, moving more slowly as a group; the men carrying their weapons and the women supporting the children, carrying their babies and everything else. The tribe that moved would have advantages over those remaining with new food resources, uncontaminated ground and water and less conflict with neighbors.


Kin Selection and Inclusive Fitness


              These concepts provide an explanation for altruistic behavior, how an individual will lessen his own survival or reproductive opportunities for the benefit of the herd or group. One cockatoo (Cacatua galarita) in the Australian bush will keep watch from a tree while the remainder of the flock eat the farmer’s wheat on the ground. He lessens his survival prospect by screeching loudly when the farmer with shotgun approaches. Such behavior may be entirely instinctive, the result of an inherited pattern of reaction. The concept was formalized by the late W. D. Hamilton in the form of a mathematical formula, known as ‘Hamilton’s Rule’, which may be loosely stated that intrinsic behavior will increase in frequency where the benefit to the total number of related recipients exceeds that of the cost (survival or reproductive cost) to the altruistic individual.(11) This action is seen and is explained in the extreme example of the eusocial insects such as bees and ants where reproduction is entirely forgone by one cast in favor of the reproduction by selected members of the colony.(12)


              Homosexual behavior in early human societies may be shown to conform to Hamilton’s Rule. If the reproductive fitness of one individual is sacrificed (say from four offspring to nil) and the surviving offspring of 20 related others is increased from four to five from improved hunting return and nutrition, then there is clearly increased inclusive fitness in that individual’s group.


              Man is no insect but there may be other examples of inclusive fitness ingrained in our genome. Color vision is encoded by a recessive gene on the X chromosome;(13) defective color vision of the common variety is seen mostly in males. The color defective early hunter may not have seen the approaching saber-toothed tiger (decreased survival fitness) but he would have spotted the camouflaged prey animal in the bushes and led the party back to base in the twilight of their hunting day (increased inclusive fitness).(14) Left-handedness may also have had inclusive survival fitness, with the left-handed hunter on the left flank of the animal cordon, club in his left hand.


Inheritance of Homosexuality


              A behavioral variation that may reduce reproductive fitness to zero is unlikely to be directly inherited. What is inherited, and inherited by all of us, is the random prospect that we will have same sex orientation.


Same sex orientation is more common in brothers of male homosexuals than in the general population, indicating that selection is not truly random but that genetic or epigenetic factors are involved in a switching process. Studies have shown that when one twin is homosexual, same sex orientation is then more common in monozygous twins (52%) than in dizygotic twins (22%) and in non-twin siblings (11%),(15) compared to a self-reported incidence of 2.8% in the general population.(16)


Division of a single zygote to form monozygotic twins may occur any time from the first day of fertilization up to ten days, rarely even later. Cleavage of the zygote after 8 days results in monoamniotic twins, later cleavage may result in conjoined twins.(17) If the incidence of same sex orientation in monozygotic is compared to that in dizygotic twins the determination of sexual orientation may be estimated to happen five to six days after fertilization. At this time some 50% of the zygotes destined to become twin pregnancies have undergone cleavage.(18) The zygote is in the early blastocyst formation stage and the embryonic cells are commencing specification.(19) Cleavage before five days would result in embryos free to undergo their individual sexual orientation subject to differing epigenetic influences; division after six days results in embryos with concordant orientation. (Other characteristics, such as left or right-handedness, would also be determined at this same stage of development.)




              Homosexuality has been present in humans long before there was ever a deliberate record of our existence. Same sex orientation deceases the survival fitness of the individual but at an earlier time in our evolutionary history increased the inclusive fitness of the family group or tribe of which that individual was a member. Homosexuality is still of inclusive benefit to civilized man but in ways that are very different from when it evolved in tribal societies at least two hundred millennia ago.


1.           Bullough VL. Homosexuality: A History. New York: NAL; 1979.

2.           Schneebaum T. Keep the River on Your Right. New York: Grove Press; 1969.

3.           Bancroft J. Aversion therapy of homosexuality. A pilot study of 10 cases. Br J Psychiatry. 1969 Dec;115(529):1417-31.

4.           Dimond P. FAQ- Does God Create Homosexuals? http://wwwmostholyfamilymonasterycom/does_God_create_homosexualshtml (accessed 7 September 2010) [serial on the Internet]. 2010.

5.           Leakey R. The Origin of Humankind. London: Wedienfeld & Nicolson; 1994.

6.           Hill K. Hunting and Human Evolution. Journal of Human Evolution. 1982;11:521-4.

7.           Darwin C. The Descent of Man and Selection in Relation to Sex. London: John Murray; 1870.

8.           Russell JG. Moulding of the pelvic outlet. J Obstet Gynaecol Br Commonw. 1969 Sep;76(9):817-20.

9.           Mackey WC. A cross-cultural analysis of recruitment into all male groups: An ethological perspective. Journal of Human Evolution. 1981;10(3):281-92.

10.        DeVoto JG. The Thebian Sacred Band. The Ancient World. 1992;23(2).

11.        Hamilton WD. The Evolution of Altruistic Behaviour. The American Naturalist. 1963;97(896):354-6.

12.        Attenborough D. Life on Earth: A Natural History. London: Little, Brown & Co; 1981.

13.        Jackson CE, Symon WE, Mann JD. X Chromosome Mapping of Genes for Red-Green Colorblindness and Xg. Am J Hum Genet. 1964 Dec;16:403-9.

14.        Morgan MJ, Adam A, Mollon JD. Dichromats detect colour-camouflaged objects that are not detected by trichromats. Proc Biol Sci. 1992 Jun 22;248(1323):291-5.

15.        Bailey JM, Pillard RC. A genetic study of male sexual orientation. Arch Gen Psychiatry. 1991 Dec;48(12):1089-96.

16.        Kendler KS, Thornton LM, Gilman SE, Kessler RC. Sexual orientation in a U.S. national sample of twin and nontwin sibling pairs. Am J Psychiatry. 2000 Nov;157(11):1843-6.

17.        Dickinson JE. Monoamniotic twin pregnancy: a review of contemporary practice. Aust N Z J Obstet Gynaecol. 2005 Dec;45(6):474-8.

18.        Hall JG. Twinning. Lancet. 2003 Aug 30;362(9385):735-43.

19.        Suwinska A, Czolowska R, Ozdzenski W, Tarkowski AK. Blastomeres of the mouse embryo lose totipotency after the fifth cleavage division: expression of Cdx2 and Oct4 and developmental potential of inner and outer blastomeres of 16- and 32-cell embryos. Dev Biol. 2008 Oct 1;322(1):133-44.





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