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Scarcity

Human fertilization is the story of scarcity: the man who succeeds in impregnating a woman reserves him 9 months to 1 year of his life, without even counting the time that will devote to his child elsewhere. This question of availability (which is not unique to the human species) requires men to strong sexual competition to reserve this rarity 1

More generally, all things being equal, distinguishing donor and recipient:

1. If the receiver has little availability, then:

   • The donor:

     • Must search for the maximum of receivers
     • Must prevent other donors from reaching the recipient

   • The receiver :

     • Either he must be careful and selective
     • Either there is another internal selection process (eg sperm war)

2. If the receiver has a lot of availability, then:

   • The donor:

     • Must always search for the maximum of receivers
     • A lot less need to block other donors (but still a little)

   • The receiver :

     • Must search for the maximum number of donors

Is sex used to make children?

Human fertilization is in case 1. above: it is enough to replace "donor" by "man" and "recipient" by "woman" to have a synthesis of sexual relations (excluding Paternal investment [^ ipm].

There are, however, several points that contradict the application of this model to human relations:

• Contraception: The low probability of conception is most often not perceived as a problem, but as a risk that remains, and history shows that humans have always looked for ways to reduce this probability to zero.

• Nonfecundant sexual acts: humans (like many other species) practice a great deal of sexual acts that do not allow fertilization (oral sex, anal sex, etc.)¹ ;

• Risky sexual behavior: Despite all warning campaigns, unprotected sex with unknown partners is important.

• The low selectivity of women: women are not as "cautious" as the demand for scarcity of their offer and they have much more sex with strangers than the search for a sire 3 .

The usual explanations

Two types of explanations are advanced to resolve these contradictions:

• Redundancy: when the realization of an action is important enough for our fitness, our body does not encourage us several programs, including that of pleasure. Nonfecundant sex can be partly explained by the fact that we respond to this redundant program.

• Strengthening the bonds of the couple: orgasm causes the production of oxytocin ("the hormone of attachment") that will strengthen the links between sexual partners. It has been argued that this effect would have been selected for investment in children 4 .

Super-organism

It may be noted that these contradictions are entirely due to scarcity (case 1. of the model above): it suffices to remove it (case 2.) so that they are perfectly explained.

However, as already reported on Evopsy (15 Dec 2013), DNA is not the only genetic code carried by humans: it must be added that of all microbiome bacteria and the transmission of these is not limited by scarcity (case 2.).

Another essential difference with DNA transmission is that in the case of the latter, the donor can be easily distinguished from the recipient whereas in the case of the microbiome, all partners are both donors and recipients.

Hypothesis: the influence of the microbiome

The hypothesis is that the partial objective of sex is to transmit the genes of the many bacteria that we carry and that these bacteria influence our behavior for this purpose. 5

This hypothesis is based on two conditions:

1. Bacteria are transmitted during sex

2. These bacteria have an influence on the brain encouraging sexual intercourse

The first is confirmed (example: Sexually Transmitted Diseases).

The second is not proven, but it is not an extreme hypothesis: the opposite is the assumption that bacteria have no influence that would be difficult to support.

The hypothesis that bacteria have an influence on sexual behavior is not new, it is the parasitic approach of Ewald and Cochran (already presented on Evopsy : 01 Nov 2004: Psychiatrie : l'hypothèse parasitaire):

"Phenomena that strongly reduce the evolutionary fitness of their bearers cannot be maintained by strictly genetic causation at frequencies far above the rates at which they could be generated by mutation. The fitness costs of male homosexuality place it in this category [124]. Perhaps more importantly, each of the hypotheses that have been put forward to explain male homosexuality have critical flaws that, if not sufficient to cause their outright rejection, are sufficient to severely weaken them.
(...)
In contrast with difficulties of noninfectious explanations of homosexuality, the hypothesis of infectious causation does not incorporate critical logical flaws or contradictions of fundamental biological principles. Indeed, anecdotal reports indicate that changes in human sexual orientation have occurred following changes in the limbic area due to trauma or infection [131, 132]. One possible route would be sexual, whereby homosexual behavior could facilitate spread because of the larger numbers of partners homosexual males may have on average, relative to heterosexual males. Alternatively, transmission could be partly or entirely by one or more nonsexual routes, and homosexual orientation be a side effect of the infection that is unrelated to transmission."
Cochran, Ewald, & Cochran (2000, pp 437-438)

The hypothesis defended here, however, differs by two novelties:

1. It is not limited to external parasites, but includes the entire microbiome 6 . Rather than the infectious origin of sexuality, we should rather speak of co-evolution (Johnston & Foster, 2018).

2. It is not limited to homosexual relationships but includes all sexual intercourse, so also all that can be fertile.

Consequences

To consider that sex has as complementary objective to transmit the genes of the microbiome and that this one manipulates us for this purpose makes it possible to explain numerous behaviors:

• Non-fertile sexual behaviors do have a goal of genetic transmission: they are just other species.

• Risky and hypersexual behaviors (same reason).

• The scraper shape of the penis is not intended to take sperm out (Gallup, 2003, Gallup & Burch, 2006) or to reduce the acidity of the vagina (see Evopsy : 8 Aug 2003) but also to extract the bacteria present.

• The relationship between gender and women's morale (Gallup et al., 2002, Burch & Gallup, 2006) is explained by the impact of the microbiome on depression.

Synthesis

The goal of sex is to transmit our genes, on the condition that we consider ourselves a super-organism comprising the genes of our microbiome.

The hypothesis that certain bacteria in the microbiome influence human behavior in order to encourage increased sexual intercourse and even risky behavior, presents no theoretical difficulty and makes it possible to better explain non-fertile sexual behaviors and the obsession with contraception. .

This hypothesis does not call into question the other approaches based on the transmission of human DNA (and in particular the sperm war theory) but supplements them.

Sources

Links

• " Semen is' good for women 's health and helps fight depression' ." By Eddie Wrenn. The Daily Mail. 21 August 2012

• Have a Nice (Orgasm) Day . Robert J King Ph.D. Hive Mind - Psychology Today . July 31, 2013

• Evopsy:

  • 8 Aug 2003 : "A quoi sert un gros Pénis ?"

  • 1 Nov 2004 : "Psychiatrie : l’hypothèse parasitaire"

  • 15 Dec 2013 : "L’alimentation peut changer le microbiome en 5 jours…"

  • 30 Apr 2018 : "Pourquoi les microbiotes nous manipulent"

References

Burch, RL & Gallup, GG (2006). The Psychobiology of Human Semen. in Female Infidelity and Paternal Uncertainty: Evolutionary Perspectives on Male Anti-Cuckoldry Tactics . (SM Platek & TK Shackelford, Eds.) (258). Cambridge University Press. ISBN: 0521845386. Chap. 8, pp 141-172

Cochran, GM, Ewald, PW, & Cochran, KD (2000). Infectious causation of disease: an evolutionary perspective. Perspectives in Biology and Medicine , 3 (43), 406-448.

Gallup, G. (2003). The human penis as a semen displacement device. Evolution and Human Behavior , 24 (4), 277-289. doi: 10.1016 / S1090-5138 (03) 00016-3

Gallup, GG, Burch, RL, & Platek, SM (2002). Does semen have antidepressant properties? Archives of sexual behavior , 31 (3), 289-93. doi: 10.1023 / A: 1015257004839

Gallup, GG & Burch, RL (2006). The semen-displacement hypothesis: semen hydraulics and the intra-peer copulation proclivity model of female infidelity. in Female Infidelity and Paternal Uncertainty: Evolutionary Perspectives on Male Anti-Cuckoldry Tactics . (SM Platek & TK Shackelford, Eds.) (258). Cambridge University Press. ISBN: 0521845386. Chap. 7, pp 129-140.

Goetz, AT & Shackelford, TK (2006). Mate retention, semen displacement, and sperm competition in humans. in Female Infidelity and Paternal Uncertainty: Evolutionary Perspectives on Male Anti-Cuckoldry Tactics . (SM Platek & TK Shackelford, Eds.) (258). Cambridge University Press. ISBN: 0521845386. Chap. 9, pp 173-190

Johnson, KV-A., & Foster, KR (2018). Why does the microbiome affect behavior? Nature Reviews Microbiology . doi: 10.1038 / s41579-018-0014-3

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Notes