Cooking and the Hominin Revolution

Part Of: Anthropogeny sequence
See Also: Born to Run: a theory of human anatomy
Content Summary: 2100 words, 21 min read

The Universality of Cooking

Cooking is a human universal. It has been practiced in every known human society. Rumors to the contrary have never been substantiated. Not only is the existence of cooked foods universal, but most cuisines feature cooked foods as the dominant source of nutrition.

Cooking_ A Human Universal (1)

Raw foodists comprise a community dedicated to consuming uncooked food. Of course, compared to historical hunter-gatherers, modern raw foodists enjoy a wide variety of advantages. These include:

  1. Elaborate food preparation (pounding, purees, gently warming),
  2. Elimination of seasonal shortages (supermarkets)
  3. Genetically enhanced vegetables with more sugar content and fewer toxins.

Despite these advantages, raw foodists report significant weight loss (much more than vegetarians!). Further, raw foodists suffer from increasingly severe reproductive impairments, which have been linked to not getting enough energy.  

Cooking_ Consequences of Raw-Foodism (1)

Low BMI and impaired reproduction are perhaps manageable in modern times, but are simply unacceptable to hunter-gatherers living at subsistence levels.

The implication is clear: there is something odd about us. We are not like other animals. In most circumstances, we need cooked food.

The Energetics of Cooking

Life exists to find energy in order to make more copies of itself. Feeding and reproduction are the twin genetic imperatives.

Preferences are subject to natural selection. The fact that we enjoy cooked food suggests that cooking provides an energy boost to its recipients. The raw-foodist evidence hints towards this conclusion as well. But there is also direct evidence in rats that cooking increases energy gains.

In the following experiments, rat food was either processed/pounded, cooked, neither, or both. After giving this diet over the course of four days, rats in each condition were weighed.

Cooking_ Energy Benefits of Cooking (1).png

For starches (left) and meat (right), cooking is by far more effective at preventing weight loss and promoting weight gain. Tenderizing food can sometimes help, but that technique pales in comparison to cooking.  

The above results were taken from rats. But similar results have replicated in calves, lambs, piglets, cows, and even salmon. It seems to be universally true that cooking improves the energy derived from digestion, sometimes up to 30%.

How does cooking unlock more energy for digestion?

First, denaturation occurs when the internal bonds of a protein weaken, causing the molecule to open up. Heat predictably denatures (“unfolds”) proteins, and denatured proteins are more digestible because their open structure exposes them to the action of digestive enzymes.

Besides heat, three other techniques promote denaturation: acidity, sodium chloride, and drying. Cooking experts constantly harp on these exact techniques, because it aligns with eating preferences.

Second, tender foods is another boon to digestion, because they offer less resistance to the work of stomach acid.  If you take rat food, and inject air into the pellets, that does not augment denaturation. Nevertheless, softening food in this way improves the energy intake of the rat.

Cooking does have negative effects. It can cause a loss of vitamins, and give rise to long-term toxic molecules called Maillard compounds, which are linked to cancer. But from an evolutionary perspective, these downsides are overshadowed by the impact of more calories. In subsistence cultures, better fed mothers have more, happier, and healthier children. When our ancestors first obtained extra calories by cooking their food, they and their descendants past on more genes than others of their species who ate raw.

A Brief Review of Human Evolution

The most recent common ancestor of humans and chimpanzees lived 6 mya (million years ago). But the first three million years of our heritage are not particularly innovative, anatomically. The australopiths were essentially bipedal apes. They could walk comfortably, but retained their adaptations for tree living as well. There is some evidence that australopiths acquired food from a new source: tubers (the underground energy storage system of plants).

Climate change is responsible for the demise of the australopiths. Africa began getting dryer about 3 million years ago, making the woodlands a harsher and less productive place to live. Desertification would have reduced the wetlands where Australopiths found fruits, seeds, and underwater roots. The descendents of Australopithecus had to adapt their diet.

The paranthropes adapted by promoting tubers (underground storage organs of plants) from backup to primary food. In contrast, the habilines (e.g., Homo Habilis) took a different strategy: meat eating. These creatures inherited tool making from the late australopiths (Mode 1 tools, the Oldawan industry- was discovered in Ethiopia 2.6 mya), and used these tools to scrape meat off of bones). The habilines are more ecologically successful, and lead to:

  • 1.9 mya: The erects (e.g., Homo erectus/ergastor) with significantly larger brains and near-modern anatomies.
  • 0.7 mya: The archaics (e.g., Homo Heidelbergensis) appear, who eventually give rise to the Neanderthals, Denisovans, and us.
  • 0.3 mya The moderns (e.g., Homo Sapiens) emerge out of Africa, and completely conquer the globe.

Here is a sketch of how our body plans have changed across evolutionary time:

Cooking_ Hominin Anatomy Comparison

Explaining Hominization

The transition from habiline to erects deserves a closer look. We know erects evolved to be persistence hunters. But a number of paradoxes shroud their emergence:

  1. Digestive Apparati. The erect diet appears to be mainly meat and tubers. Both require substantial jaw strength and digestive apparati. Yet the Homo genus features a dramatically reduced digestive apparatus. How was smaller mouths, weaker jaws, smaller teeth, small stomachs, and shorter colons an adaptive response to eating meat and starches?
  2. Expensive Tissue. Australopiths brain size stayed relatively constant at 400 ccs (10% of resting metabolism). Erect brains began to grow. This transition ultimately yielded a 1400 cc brain (20% of resting metabolism) in archaic humans. How did the erects find the calories to finance this expansion?
  3. Time Budget. The above anatomical features of erects are geared towards endurance running, which suggests that their lifestyle involved persistence hunting. Chimps have about 20 minute intervals in between searching for & chewing food. Thus, chimps can only afford to spend 20 minutes hunting before giving up. How did erects perform the risky behavior of persistence hunting, which consumes 3-8 hours of time?
  4. Thermal Vulnerability. As part of their new hunting capabilities, erects became the naked ape (with a new eccrine sweat gland system to prevent overheating). But Homo Erectus also managed to migrate to non-African climates such as Europe. How did these creatures stay warm?
  5. Predator Safety. Erects lost their anatomical features for arboreal living, which suggests they slept on the ground. Terrestrial sleeping is quite dangerous on the African savannah. How did erects avoid predation & extinction?

All of these confusing phenomena can be explained if we posit H. erectus discovered the use of fire, and its application in cooking:

  1. Digestive Apparati. As we have seen, the primary role of cooking is to “externalize digestion”, and to increase the efficiency of our digestive tract. Cooked meat and starches are incredibly less demanding to process than their raw alternatives. This explains our reduced guts. By some estimates, the decrease in digestive tissue corresponds with a 10% energy savings by our erect ancestors.
  2. Expensive Tissue. Cooking increases the metabolic yield of most foodstuffs by ~30%. For reference, a 5% increase in ripe fruit for chimpanzees reduces interbreeding interval (time between children) by four months. 30% is an absurdly large energy gain, enough to “change the game” for the erects..
  3. Time Budget. Cooking freed up massive amounts of time otherwise spent chewing. Chimpanzees can take 4-7 hours per day chewing; humans only need one hour per day. This frees up massive amounts of time, which can be used for e.g., hunting.
  4. Thermal Vulnerability. It is very difficult to explain a hairless Homo Erectus thriving on the colder Asian continent without control of fire.
  5. Predator Safety. It is very difficult to explain how erects were not preyed upon to extinction without fire to identify & deter predators. Hadza hunter-gatherers comfortably sleep through the night, typically by taking turns “on watch” while the others rest.

Cooking_ Overall Argument (3)

The Archaeological Record

We are positing that erects learned to create and controlling fire 2 mya. Is that a feasible hypothesis?

Habilines had learned how to create stone tools 2.6 million years ago. By the time of the erects, techniques to create these tools had persisted for 600,000 years. So it is safe to say that our ancestors were able to retain useful cultural innovations.

Independent environmental reasons link fire-making with H Erectus. The Atlas mountain range is the most likely birthplace of this species, and this dry area fires triggered by lightning are an annual hazard. Hominins living in such environments would be more intimately familiar with fire than those with less combustible vegetation zones.

Erects would have seen sparks when they hit stones together to make tools. But the sparks produced by many kinds of rock are too cool to catch fire. However, when pyrites (a fairly common ore) are hit against flint, the results are used by hunter-gatherers to reliably produce fire. The Atlas mountain range is renowned for being exceptionally rich in minerals:

Why is Morocco one of the world’s great countries for minerals? No glaciers! Many of the world’s most colorful minerals are found in deposits at the surface, formed over time by the interaction of water, air and rock. Glaciers remove all of that good stuff (as happened in Canada recently, geologically speaking) –  and with no recent glaciation, Morocco hosts many fantastic occurrences of minerals unlike any in parts of the world stripped bare during the last Ice Age.

Since this mountain range contains pyrites, early erects could have found themselves inadvertently making fire rather often.

Once it is created, fire is relatively easy to keep going. And it does not take much creativity to stick food a fire. Moreover, modern-day chimps prefer cooked food over raw; it is hard to imagine H Erectus finding cooked food distasteful. All of these considerations suggest an early control of fire is at least plausible.

We can consult the archaeological record to see record of man-made fire (i.e., hearths). This is bad news for the cooking hypothesis! There is strong evidence for hearths dating back to 800 mya and the advent of archaic humans. Before then, there are six sites that seem to be hearths; but these are not universally acknowledged as such.

Cooking_ Archaeology Evidence (1)

But absence of evidence isn’t evidence of absence, right?

No! That idiom is wrong. Silence is evidence of absence. It’s just that the strength of the evidence depends on the nature of the hypothesized entity.

  • If you think an unidentified planet orbits the Sun, a lack of evidence would weigh heavily against the hypothesis.
  • If you think an unidentified pebble orbits the Sun, a lack of evidence doesn’t say much one way of the other.

Wrangham argues that evidence of hearths are more fragile than e.g. fossils, and points to facts like there are zero hearths recorded for modern humans during European “ice ages” – but we know these must have existed. It is possible that the contested hearth sites will ultimately be vindicated, and that we just can’t see much evidence.

Despite these claims about evidential likelihood, the silence of the archaeological record is undeniably a significant objection to the theory.

Weighing The Evidence

Is the cooking hypothesis true? Let us weigh the evidence, and contrast it with alternative hypotheses.

The most plausible alternative hypothesis is that archaic humans H. Heidelbergensis discovered cooking. But the emergence of that species involved an increase in brain size, and more sophisticated culture & hunting technology.  Neither adaptation seems strongly connected to cooking. In contrast, the H. Erectus adaptations would have all been strongly affected by cooking. 

Moreover, alternative hypotheses must still answer the five paradoxes of hominization:

  1. Digestive Apparati. Why did erects evolve smaller mouths, weaker jaws, smaller teeth, small stomachs, and shorter colons?
  2. Expensive Tissue. How did the erects find the calories to finance more brain tissue?
  3. Time Budget. How could erects afford spending 3-8 hours per day engaged in the risky strategy hunting?
  4. Thermal Vulnerability. Erects also managed to migrate to non-African climates such as Europe. How did these creatures stay warm?
  5. Predator Safety. Erects slept on the ground. How did they avoid predation?

The habilines ate meat. It is unclear how they did so (hunting or scavenging), but we have strong evidence that they did. Meat is a much higher quality food than tubers (cf. paranthropes) or fruit (cf. chimpanzees). The meat-eating hypothesis argues that meat eating was the primary driver of hominization.

Meat-eating resolves the Expensive Tissue paradox (meat allows for brain growth) and Digestive Apparati (carnivores are known to have smaller guts). But it doesn’t address why a meat-eater would develop smaller canines. And it struggles to explain how the reduction in gut size is compatible with the tuber component of the erect diet. And what about time budget, thermal vulnerability, and predator safety? The meat eating hypothesis fails to address these paradoxes entirely.

Which is more likely to occur in the next twenty years: undisputed evidence for early control of fire, or an alternate theory that resolves all five hominization paradoxes?

My money is on the former.


  • Wrangham (). Catching Fire: How Cooking Made Us Human
  • Aiello & Wheeler(1995). The expensive tissue hypothesis: the brain and the digestive system in primate and human evolution.

An Introduction to Domestication

Part Of: Anthropogeny sequence
Content Summary: 1300 words, 13 min read.

The Domestication Syndrome

Since our emigration out of Africa 70,000 years ago, Homo Sapiens have domesticated many other species, including

  • dogs (18 kya, first domesticated in Germany)
  • goats, sheep (11 kya)
  • cattle, pigs, cats (10 kya)
  • llamas, horses, donkeys, camels, chickens, turkeys (5 kya)
  • foxes (50 years ago)

Consider the domestication of wolves into dogs. An important part of the environment of a species is other species- not merely its predators or pathogens but its symbionts. In this case, canines began to get food from human campsites. Dogs that were less aggressive were (by unconscious preference and conscious intent) more successful at extracting resources. This process is known as artificial selection.

Most ancient dogs kept by hunter-gatherers share a common body shape. More recently however, humans have conducted pedigree breeding: influencing the morphologies of different dog breeds. We have used this power to sculpt breeds as diverse as the Chihuahua and the Great Dane.

The defining feature of domestication is docility: a reduction in reactive aggression. All domesticated species exhibit this feature, in comparison to their wild counterparts. Not all species are capable of this sort of control. For example, humanity has tried for centuries to domesticate big fauna such as zebras, lions, and hippos. However, some breeds have reproductive and aggressive styles that prohibit domestication.

But domestication doesn’t just bring about a change in behavior. It also brings with it a bewildering number of anatomical changes, to essentially all domesticated species. The domestication syndrome include:

  • Docility (agreeableness, reduction in irritability)
  • Depigmentation (especially white patches, brown regions)
  • Floppy ears
  • Shorter ears
  • Shorter jaws
  • Smaller teeth
  • Smaller brains (10-15% reduction in volume)
  • More neotenous behavior (juvenile behavior that extends into adulthood).
  • Curly tails

Most domesticated species express some aspect of the domestication syndrome, as we can see in the following table:

Self-Domestication_ The Domestication Syndrome (1)

Three Theories of Domestication

The sheer complexity of the domestication syndrome requires an explanation. What is the link between floppy ears and docility?

Three hypotheses suggest themselves:

  1. Multiselection. Are the symptoms of domestication all expressions of human preferences? Do we simply like curly tails and floppy ears?
  2. Environment. Is there something about proximity to humans that incentivizes these changes?
  3. Byproduct. When the genes for aggression are altered, does that somehow incidentally cause these other changes?

Animal husbandry practices are lost to the sands of time. Nevertheless, there is a way to test multiselection directly: by creating a domesticated species in the laboratory.

In 1959, Dmitri Belyaev began trying to domesticate silver foxes. He used exactly one criterion for selection: he only bred pups that exhibited the least aggression. Skeptics thought it would take centuries to complete the domestication process. But changes in temperament were seen after only four generations. At twelve generations, “elite” foxes began to emerge with dog-like characteristics: wagging their tails, allowing themselves to be petted etc. At twenty generations, the entire population was considered fully domesticated.

Despite only selecting for docility, Belyaev’s foxes exhibited the full domestication syndrome. The foxes inexplicably developed floppy ears, curly tails, white patches, etc etc. The multiselection hypothesis is false.

Is there something about proximity to humans that selects for the domestication syndrome? The environment hypothesis seems false for two reasons.

  1. First, when they return to the wild, domesticated species take a long time reverting their characteristics. In fact, often domestication gives them a selective advantage over their wild cousins.
  2. Second, as we will see in the next section, self-domesticated species such as bonobos exhibit the syndrome despite their evolution not being influence by hominids.

The byproduct hypothesis is our only remaining explanation for the domestication syndrome. But what specific system produces these changes? 

The Biological Basis of Domestication

In order to fully explain aggression reduction, we must understand it at a biological level.

The primary basis of aggression reduction is a shrinking amygdala and periaqueductal gray (PAG). These modules comprise the negative valence system which learn which stimuli are negatively-valenced, and forward them to the mobilization system (e.g., snake → bad → run away). Serotonin inhibits the negative valence system, and domesticated animals have much high concentrations of serotonin receptors in these regions. Finally, it appears that these changes mostly act across development. The negative valence system comes online only slowly: there exists a socialization window in the first month of a wolf’s life, where it can learn “humans are okay”. Domestication primarily acts by increasing the socialization window from one to twelve months. If a dog isn’t exposed to a human in its first year, it’s now-active fear system will kick in: it will be wild for the rest of its life.

So what biological system is able to a) expand the socialization window, and b) induce the rest of domestication syndrome? The leading hypothesis involves a feature of development called the neural crest.

A blastocyst has no brain. To correct this unfortunate situation, every vertebrate genome contains instruction for constructing a neural tube. This structure emerges via folding.

The neural crest resides between the epidermis and the neural tube. These neural crest cells (NCCs) then proceed to migrate to a certain number of other anatomical structures to assist development. When the NCC migration malfunctions, the resultant disease is called a neurocristopathy. Many neurocristopathies result in outcomes similar to the domestication symdrome! For example, here is the effect of piebaldism:

Self-Domestication_ Piebalism

The mild neurocristopathy hypothesis (Wilkins et al, 2014) holds that domestication syndrome is a byproduct of changes to the NCC migration pattern.

Self-Domestication_ Mild Neurocristopathy Hypothesis

The hypothesis, however, is not very detailed (how exactly is NCC migration changed? What are the genomic and epigenomic contributions?). It is more of a promissory note than a mechanistic account. And there are other holistic hypotheses on offer, including genetic regulatory networks (Trut et al 2004) and action of the thyroid gland (Crockford 2000). It seems clear that, in the coming decades, a detailed mechanistic theory of domestication will emerge to vindicate the byproduct hypothesis.

Two Kinds of Domestication

Natural selection explains why the “design requires a designer” trope is obsolete. For the same reason, domestication can occur in the absence of a domesticator. More precisely, change in a species ecological niche can itself select against aggression.  Because aggression is very relevant to survival, we see plenty of species that have increased, and plenty that have decreased their rates of aggression. We call those less aggressive species self-domesticated: they became more peaceful in the absence of humans. What’s more, these species also exhibit the domestication syndrome.  

Another example is embedded in Foster’s Rule. Islands tend to be geologically more recent than continents, so their populations derive from the continent rather than vice versa. Islands tend to have fewer predators, but also fewer resources. Reduced predation increases the size of small animals (e.g., dodos evolved from pigeons), but limited resources decreases the size of big animals (e.g. the 3ft tall dwarf elephant).  

Self-Domestication_ Foster's Rule

Because islands have fewer predators, they also tend to have higher population densities; as such, reactive aggression is a less useful strategy. Selection favors the less aggressive. And we can see the domestication syndrome in island species. For example, the Zanzibar red colobus monkey has diverged from the continental red colobus along the same trajectory as dogs diverged from wolves.

Other examples of self-domestication can be found with group size reduction (ungulates, seals) and low-energy habitats (extremophile fish).

Finally, bonobos provide a particularly relevant example of self-domestication. Because food is more plentiful (don’t have to compete with gorillas for vegetation), females can spend time close to one another. Proximity produces bonding, and female coalitions exert pressure on bonobo behavior.

  • In chimps, bullying women increases reproductive success. Chimps will systematically beat up all females in their group as a coming-of-age ritual.
  • In bonobos, female coalitions retaliate against male aggression, making it unprofitable. Sexual selection then acts against reactive aggression.

So we can see that domestication (i.e., reduction in aggression) can come in two flavors: traditional vs self-domestication.

Self-Domestication_ Categories of Aggression Reduction (1)

As we will see next time, Homo Sapiens is yet another example of a self-domesticated species. See you then!

Related Resources

  • Wilkins et al (2014). The “domestication syndrome” in mammals: a unified explanation based on neural crest cell behavior and genetics

[Excerpt] Replicators and their Vehicles

Original Author: Richard Dawkins, The Selfish Gene
See Also: [Excerpt] The Robot’s Rebellion
Content Summary: 800 words, 4 min read

The First Replicator

Geochemical processes gave rise to the “primeval soup” which biologists and chemists believe constituted the seas some three to four thousand million years ago. The organic substances became locally concentrated, perhaps in drying scum round the shores, or in tiny suspended droplets. Under the further influence of energy such as ultraviolet light from the sun, they combined into larger molecules. Nowadays large organic molecules would not last long enough to be noticed: they would be quickly absorbed and broken down by bacteria or other living creatures. But bacteria and the rest of us are late-comers, and in those days large organic molecules could drift unmolested through the thickening broth.

At some point a particularly remarkable molecule was formed. We will call it the Replicator. It may not necessarily have been the biggest or the most complex molecule around, but it had the extraordinary property of being able to create copies of itself.

A molecule which makes copies of itself is not as difficult to imagine as it seems at first, and it only had to arise once. Think of the replicator as a mold or template. Imagine it as a large molecule consisting of a complex chain of various sorts of building block molecules. The small building blocks were abundantly available in the soup surrounding the replicator. Now suppose that each building block has an affinity for its own kind. Then whenever a building block from out in the soup lands up next to a part of the replicator for which it has an affinity, it will tend to stick there. The building blocks which attach themselves in this way will automatically be arranged in a sequence which mimics that of the replicator itself. It is easy then to think of them joining up to form a stable chain just as in the formation of the original replicator. Should the two chains split apart, we would then have two replicators, each of which can go on to make further copies.

Replicator Competition

The primeval soup was not capable of supporting an infinite number of replicator molecules. For one thing, the earth’s size is finite, but other limiting factors must also have been important.

But now we must mention an important property of the copying process: it is not perfect. mistakes will happen. I hope there will be no misprints in this book, but if you look carefully you may find one or two. We do not know how accurately the first replicator molecules made their copies. Their modern descendants, the DNA molecules, are astonishingly faithful compared with the most high-fidelity human copying process, but even they occasionally make mistakes, and it is ultimately these mistakes which make evolution possible. Mistakes were made, and these mistakes were cumulative.

Replicators with a comparatively worse design must actually have become less numerous because of competition, and ultimately many of their lines must have one extinct. There was a struggle for existence among replicator varieties. They did not know they were struggling, or worry about it; the struggle was conducted without any hard feelings, indeed without feeling of any kind. But they were struggling, in the sense that any mis-copying which resulted in a new improved level of stability, or a new way of reducing the stability of rivals, was automatically preserved and multiplied.

This process of replicator improvement was cumulative. Ways of increasing stability and of decreasing rivals’ stability became more elaborate and more efficient. Some of them may even have ‘discovered’ how to break up molecules of rival varieties chemically, and to use the building blocks so released for making their own copies. These proto-carnivores simultaneously obtained food and removed competing rivals. Other replicators perhaps discovered how to protect themselves, either chemically, or by building a physical wall of protein around themselves. This may have been how the first living cells appeared.

Replicator Self-Improvement

Replicators began not merely to exist, but to construct for themselves containers, vehicles for their continued existence. The replicators that survived were the ones that built survival machines for themselves to live in. The first survival machines probably consisted of nothing more than a protective coat. But making a living got steadily harder as new rivals arose with better and more effective survival machines. Survival machines got bigger and more elaborate, and the process was cumulative and progressive.

Was there to be any end to the gradual improvement in the replicators’]techniques? What weird engines of self-preservation would the millennia bring forth?  Four thousand million years on, what was to be the fate of the ancient replicators?

They did not die out, for they are past masters of the survival arts. But do not look for them floating loose in the sea; they gave up that cavalier freedom long ago. Now they swarm in huge colonies, safe inside gigantic lumbering robots, sealed off from the outside world, communicating with it by tortuous indirect routes, manipulating it by remote control..

They are in you and in me; they created us, body and mind; and their preservation is the ultimate rationale for our existence. They have come a long way, those replicators. Now they go by the name of genes, and we are their survival machines.

[Excerpt] Self-domestication and human homosexuality

Excerpts are not my writing! This comes from Richard Wrangham’s book:

The Goodness Paradox: The Strange Relationship Between Virtue and Violence in Human Evolution

It was a fun read. Recommended!

Human homosexuality is not adaptive

The hypothesis that human homosexuality is adaptive (genetically advantageous) has not been rejected lightly. Homosexual behavior can be frequently found among wild animals, and traits that are widespread are likely to be adaptive.

So when evolutionary biologists began to study human homosexual behavior, they tended to search for ways to explain how a same-sex preference might have been favored in natural selection. Homosexual behavior among other animals offered some ideas.

Close study reveals how homosexual behavior can be adaptive.

  1. Scarcity of opposite-sex partners. Among Laysan albatrosses in Hawaii, two parents are needed for chicks to be reared successfully. When there are not enough males, females pair together. Their sexual behavior includes courtship and pseudo-copulation. Females in same-sex pairs are fertilized by an already mated male, who then ignored the resulting eggs and chicks. The female pair brings them up without male help.
  2. As a prosocial device. In animals whose choice of sexual partner is not a response to a shortage of opposite-sex partners, homosexual behavior sometimes appears to be adaptive by promoting useful social relationships. In troops of Japanese monkeys, females form temporary homosexual mating partnerships even when other males are available. Among savanna baboons, males form alliances that they use in fights against others. Allies reciprocally fondle one another’s genitals, apparently to demonstrate their commitment to the bond.

Researchers have sought evidence that the kinds of reproductive or social benefits that animals gain from same-sex sexual interaction might be found in human. In theory, humans could form same-sex partnerships in response to a short supply of members of the opposite sex. Certainly, partner availability influences us. Women and men in single-sex institutions such as prisons, schools, monasteries, and ships often temporarily shift their sexual activity toward their own sex. Nevertheless, of course, many individuals feel an exclusive attraction to members of their own sex, regardless of the availability of the opposite sex.

Further, homosexual couples tend to have smaller families than same-sex couples, and there is a lack of evidence that their sexual orientation leads them to give exceptional help to their genetic kin. These data suggest that homosexual behavior in humans is not biologically adaptive.

Unfortunately, the conclusion that same-sex behavior is not adaptive has sometimes been associated with a negative view of homosexuality. But normative value and biological purpose are independent considerations. Many tendencies that we regard as morally reprehensible clearly evolved, including numerous kinds of sexual coercion, lethal violence, and social domination. Equally, many morally delightful tendencies did not evolve, such as charity to strangers and kindness to animals. Our decisions about which behavior we like or dislike should never be attributed to adaptive value.

The biological basis of homosexuality

Same-sex sexual attraction is often stable over a lifetime, and there is good evidence that is is partly heritable. These features make human homosexuality different from most animal homosexuality.

One particular area of the brain responds to androgens (sex hormones) in the fetal stage: the third interstitial nucleus of the anterior hypothalamus (INAH3). The INAH3 is larger in heterosexual men than in women, and has been found to be intermediate-sized in homosexual men. In an adult rams, experimentally reducing the comparable nucleus (oSDN) causes them to change his sexual-partner preference from female to male.

Homosexual preference is more likely in males who receive low testosterone exposure before birth. A standard method for assaying prenatal testosterone exposure is to measure the length of the ring finger (the fourth finger) compared to the length of the second finger: increased prenatal exposure to testosterone tends to be associated with relatively long ring fingers. The largest surveys of homosexual men in the United States, China, and Japan have found a tendency for homosexual women to have relatively long ring fingers, whereas homosexual men have relatively short ring fingers. Homosexual men also tend to have somewhat feminized face shapes and shorter, lighter bodies than heterosexual men, most likely from relatively low exposure to testosterone in the womb. In general, females who have been exposed to higher-than-usual levels of androgens, and males who have been exposed to lower-than-usual levels, appear to have a higher likelihood of being homosexual.

Homosexuality as a by-product of self-domestication

The evidence that exclusive homosexual preference is common but not adaptive makes it a prime candidate for being an evolutionary by-product.

Elsewhere, I have presented the self-domestication hypothesis: the theory that H. sapiens domesticated itself; that is, it selected against reactive aggression. Testosterone is involved both in male violence, but also sexual preference.

But since reduced testosterone is a common effect of domestication, homosexual orientation in this species appears to be explicable ultimately as an incidental consequence of selection against reactive aggression.

Some additional evidence are suggestive:

  • The only nonhuman animal in which exclusive homosexual preference is known is a domesticated species – namely, sheep.
  • At least 19 species of domesticated animals show homosexual behavior, though it occurs in their wild relatives as well.
  • Our two closest primate relatives are chimpanzees and bonobos. Chimpanzees are non-domesticated (highly aggressive) and have long ring fingers suggesting high prenatal exposure to testosterone. Bonobos are self-domesticated (placid), and have short index fingers.
  • Homo neanderthalensis morphology indicates they were quite an aggressive species (non-domesticated), and they shows a large finger-length ratio. The 100,000-year-old H. sapiens at Qafzeh is in-between the ratios for living humans and the five Neanderthals.

Thus, it may be that self-domestication (the source of our species’ remarkable ability for cooperation) yielded homosexual behaviors as a by-product.



[Sequence] History

I have blogged some on the history of ancient Israel, here:

I have also done some research on the Middle Ages, and the Bronze Age collapse of civilization. I’m hoping to someday present these data, in context of the theory of cliodynamics.

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[Sequence] Anthropogeny


Primary Sequence

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