Saturday, May 29, 2010

The Neotony Hypothesis - How did human intelligence evolve?

The origin of human intelligence has been debated for centuries. The path to intelligence suggested here leads not only to a plausible explanation of how we came to be so clever, but also to some startling conclusions about human psychology in general.
It was Dr. Desmond Morris, in his popularization of anthropology: "The Naked Ape", who first drew my attention to the role of neoteny. This is the term used to describe the extended childhood period in humans that sets us apart from all other species.
A calf or a foal is on its feet within half an hour after birth, humans take about a year to reach the same stage. The very thought of human children becoming sexually mature within 2 months, like the rat, or 3-4 years, like the horse, simply boggles the mind!
Chimpanzees, our closest living relative, are sexually mature at about 6-8 years old (roughly ¼ of their total life-span in the wild) whereas humans reach puberty much later, between the ages of 14-17 (½ of the estimated lifespan of humans in the hunter-gatherer era).
Morris described neoteny as a slowing down or extension of the maturation process that allows for an extended learning period in human development, during which our large and adaptable brains can absorb a relatively huge amount of knowledge, through observing our elders, absorbing cultural and traditional information, practicing skills and making individual discoveries. Indirect support for the neotony theory is the fact that humans are more or less hairless, a juvenile trait of primate species.
'Stephen Jay Gould was an advocate of the view that humans are a neotenous species of chimpanzee; the argument being that juvenile chimpanzees have an almost-identical bone structure to humans, and that the chimpanzee’s ability to learn seems to be cut off upon reaching maturity.' (Wikipedia).
If we accept that neoteny may have had a role in human development, the question remains: how might such an extended childhood and delayed sexual maturity have arisen?
Perhaps the only other example of such extended or delayed development is in the 17-year Cicada, an insect whose underground larval stages last exactly 17 years, before the adults all emerge together from the soil. But that strategy is based on the life cycle of their food plant, and is not related to education or intellectual development.
David F. Bjorklund, a developmental psychologist, wrote:
"One possible explanation for our slow rate of maturation is that it is an adaptation—that is, natural selection may have favored a long childhood because it had benefits that outweighed its costs. However, most scientists who have examined this issue have assumed that immaturity has no inherent advantages and that our extended period of development must therefore be a by-product of selection for some other characteristic.
The most popular candidate has been intelligence. A big and complex brain takes a lot of time to develop, and in humans much of that development must occur after birth, because bipedalism limits birth-canal width, which has in turn constrained the head size of newborns. More specifically, social intelligence has often been postulated as the driving factor. In this view, as humans achieved ecological dominance, they became one another's principal competition for resources. Consequently, the ability to manage social relations and alliances was selected for, in what evolutionary biologist Richard Alexander has characterized as a cognitive "arms race" within the species. The result is that we are much smarter than we would need to be simply to succeed at hunting and gathering, and we are thus capable of creating complex cultures."

Another explanation is that our large and adaptable brain requires a long time to upload data and integrate learning skills and information, and so neoteny developed as an adaptation, extending brain plasticity and learning time to "take advantage" of the increasing brain capabilities.
The trouble with this explanation is that it requires the process of natural selection (NS) to plan for future goals. In effect it wants NS to tell the genes: "Slow down development because this gives more time for a big brain to develop and learn, which will pay survival and adaptability dividends in the future" But NS can hardly be expected to reduce a species' fitness temporarily for the sake of future advantages, that would be equivalent to a river flowing uphill. Neoteny, after all, comes as a heavy price.

The price of a long childhood
The price of neoteny is two fold: Firstly, it extends the time for which the individual is a highly vulnerable and totally dependent baby and then a still vulnerable and moderately dependant child. All studies of predator species show that infant and immature members of prey species are most frequently preyed upon. Lions, hyenas, leopards and wild dogs select the smaller, more vulnerable individuals for special attention. Killing a mature adult bull may yield more meat, but there's a greater risk that the bull will either be fast enough to get away or gore the predator with its horns in the struggle.
Secondly, delayed puberty means that the species and the genes get no reproductive dividend for the considerable food and other resources consumed by the growing child for a full 15 years or more! That’s a very long term investment plan in biological terms! Modern wild apes spend most of their time foraging for food; the demands of human-like offspring that took 15 years to grow up would impose an impossibly heavy burden on them (so much for the Tarzan story!). This sort of reality must have faced our pre-human ancestors, living in small groups in the forest and on the veldt of Africa, several million years ago.
My conclusions from this are 3-fold:
Before neoteny could become selectable as an advantageous trait, our pre-human ancestors must …
a) Have had access to a plentiful food supply
b) Have been able to defend their children from predators over a much longer childhood
c) There must have been some powerful selective pressure that made the regular short animal childhood a disadvantage.
My hypothesis is that all three conditions came about through the invention of new tools.

The effects of tool use on behavior and evolution
Modern chimps have been observed intentionally throwing stones at people and making crude spears and using them to kill small prey animals We can safely assume that pre-humans (an admittedly vague term used here to define our ancestors before the emergence of Homo Sapiens without specifying species or timeline) discovered the power achieved from throwing rocks and stones. Groups of pre-humans learned to by throw stones in order to stun and disable prey animals and to effectively scare predator species. This learned, cooperative ability dramatically increased the supply of available food and provided significant protection to the young.
It is interesting to note here that bipedal walking is observed back in Australopithecus, when brain size was still relatively small. Bipedalism is associated with human tool use because it frees the forelimbs to carry, manipulate and throw. Earliest human tool use may go back as far as 2.5 MY. That puts tool use as starting well before advanced brain development.

Increasing weapon range from zero
Thrown stones and other weapons became the first short/medium range weapons in evolutionary history. Up until then, (with a few specialized exceptions, such as spiders' webs and the venom of the spiting cobra) animals had always used weapons that were integral parts of their bodies: claws, teeth, horns etc.. In military parlance, these are all zero range weapons. If animal A claws or bites animal B, if B is not much smaller in size or strength, chances are that B will claw A in self defense. Even if A wins the fight, he will probably get deeply scratched. These scratches may become infected and lower A's chances of surviving and his chances of passing on his own genes.
The consequences of this "victor vulnerability" are to be seen everywhere:
Predator species are mostly larger and more powerful than their prey species, with strong jaws, heavily muscled body and heavy bone structure (lions, tigers.. birds of prey, Tyrannosaurus Rex etc). Those that are smaller than their prey species (dogs, Hyenas, etc) hunt in packs and overcome their prey by co-operative hunting.
The invention of stone weapons was a real qualitative revolution in predation, because a thrown or dropped stone has a much higher kinetic energy than any tooth or horn or talon, and because the thrower (with a bit of luck!) can remain at some distance from the prey animal. These factors enabled the relatively small pre-humans to hunt and kill large prey animals, so becoming top predators. Simple quantitative analysis shows small body size and large preying capacity is a winning combination, in terms of population growth and evolutionary fitness.
It is probable that the earliest pre-human hunters hunted in groups, because an individual's accuracy in stone throwing would have been low, and the best way of increasing the likelihood of a kill would have been to increase numbers. Large groups of modern baboons and chimps have sometimes been observed hunting and killing small deer or other monkeys. Young chimps have been observed to scatter stones in play and aggressive behavior. So it's reasonable to suppose that stones and sticks were used as throwing tools to stun prey and/or ward off predators very early on, before the development of large brains.

The Invention of Murder
However, the new technology of throwing /dropping stones, sticks etc would also have begun to destabilize the hierarchical structure common to all primate societies, as soon as some individuals discovered that such tools could also be used against their fellow pre-humans. In many animal societies, competitive sexual aggression is ritualized, with snarling, bearing of teeth and other displays often taking the place of actual fighting. In the pre-tool era, when all weapons (teeth, claws etc) were zero range, both sides in any fight (including the winner) risked injury and consequent lowering of competitive fitness.
Suddenly, with the realization that thrown stones could stun or kill, young, barely mature males of 3-4 years old found themselves able to maim or kill large, powerful alpha males, who'd previously only had to growl at the youngsters to send them running off scared, without getting themselves wounded in the process. Killing the alpha males brought the rebellious young individuals some clear advantages such as greater and earlier access to females. This means that the discovery of stone throwing would have raised the selective advantage of more aggressive individuals with fewer inhibitions or fears about killing other members of the social group. Such shifts in behavior patterns could conceivably come about by increasing the frequency of genes that increased testosterone production, or reducing the frequency of genes that produce inhibitors of testosterone
So Murder had been invented, as aggression towards other males became more profitable than ever before. The trouble with this strategy, of course, was that it was available just about anyone in the group – who could simply bash even the most aggressive individual on the head as he slept. No one was safe any more, and the consequent disruption lowered the ability of the group of pre-humans to survive.
One might suppose that this state of affairs would cause NS to favor less aggressive individuals, but no: simple game-theory would suggest that highly aggressive individuals would have a strong breeding advantage over less aggressive members of the same group.
Neotony kicks in
My hypothesis sets this as the juncture at which neotony began to have selective advantages, for the following reasons:

  • · Prolonged sexually immaturity would keep young males "out of trouble" for much longer, by making as they were less likely to be targeted by aggressive, mature males
    · Slow-maturing individuals would benefit from parental care, protection and food provision for a longer period (provided that an adequate food supply was assured by tool use in hunting)
    · Slow-maturing individuals would have more time to engage in play activities, and cooperation with their peers and elders and acquire learned skills. Simply knowing their peers and other group members for longer periods would probably inhibit extreme aggressive behaviors.
    · Slow-maturing individuals would, when they finally did reach sexual maturity, enjoy major selective advantages over rapidly- maturing individuals, such as more learned skills and strategies, and stronger ties with peers and other group members.

Note that slow-maturing individuals would not necessarily be genetically type set to be less aggressive than their rapidly- maturing peers. The point is that their aggressive behavior would be inhibited. This distinction is very important, since an inhibited behavior still exists in potential, as a "dormant" network of cellular connections, and can reappear under the right conditions. Inhibition is a well known type of nerve cell activity that occurs down at the level of the synapse.

Eternal debates
So what we have here is not a theory about a set of genes that produce human behavior, but a theory about the dynamic clash between two sets of often contradictory behaviors. On the one hand you have individuals or selfish genes (as Dawkins would say) that code for aggression so as to maximize sexual encounters and reproduce themselves, while on the other hand you have genes that code for the repression of aggression against children (immature individuals) and cause instinctual feeding of children, thereby protecting the genes that those children (who are usually closely related) carry. The way this dynamic clash works out will vary very widely, and will be determined far more than circumstances and events than by the opposing gene sets.
This is a crude yet telling model for the me/us/country/class versus you/they/foreigner/alien disputes that dominate human morality, history and politics. Replace "aggressive behavior to secure mates, at all costs" with "maximize my profits and power at any cost" and replace "protect children who share my gene pool" with "serve society / Allah at any cost".
Even the most rarified academic debates about economic policy may contain assumptions disputed by these opposing mechanisms within our brains.

Neoteny and Language
To get back to our slowly-maturing pre-human ancestors in their hunter-gatherer society, how exactly did slow maturity increase brain size and intelligence?
The answer may lie in the in the relative weakness and dependency of children. About the only way they had to assure themselves some provision of food and care would have been to cry out. Human children are notorious for making a lot of noise and persistent demands. Mothers in all primate species communicate with their children with cries and other sounds, but a limited selection of whimpers and cries would not have been enough to keep a pre-human child, maturing over a large number of years, alive and fed in a not very luxurious hunter-gatherer clan, where mothers could die. Developing a much wider range of vocalizations for communication with adults and cooperation and play peers, seems to be the only available strategy. I believe that this was the main trigger for the development of language. Its first words and phrases would have included: "help me!, feed me! It's me I'm your child, I want more, teach me how to do that…."
This could well have been the factor that led NS to favor larger brain size and intelligence. Children are far better than adults at picking up a new language, because language skills would have been crucial to their survival in prehistoric times. As others have pointed out, meat-eating would have provided the protein fuel that NS needed to produce larger brains, but it seems that the language skills of children are the best candidate for a NS driver to bring about such a change.

Shimon Ein-Gal May 2010


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