While reading Still Life with Woodpecker by Tom Robbins in the early 1980s, for the first time I came across the word “neoteny.” Robbins may have been familiar with Stephen J. Gould’s work, Ontogeny and Phylogeny, published in 1977. Ontogeny and Phylogeny is considered the most important modern exploration of neoteny. Gould believed neoteny explained how human beings evolved.
Gould described the dynamics of heterochronic theory. Neoteny is one of the six processes of heterochrony. Here we’ll mostly talk about neoteny and neoteny’s opposite, acceleration. Both describe how changing the rate of maturation can result in lightning-fast evolution. Changing the timing of maturation also affects evolution.
When describing neoteny, Gould and others have found it convenient to consider the chimpanzee as representative of a human-like forebear perhaps five million years old. If you note a chimpanzee infant’s relatively small chin, large head relative to body size, large eyes relative to face and upright positioning of the head on the neck, you get the idea that a chimpanzee infant exhibits features much like a contemporary human adult. There are a number of features retained by a chimpanzee infant, and the infant of what would be our ancient ancestor, which over a period of thousands of generations grew slowly to appear in the features of descendant adults, moving upwards through toddlers to preteens to adolescents. That is the basic principle of neoteny. Ancestor infant features prolong or extend themselves over the course of evolution into later and later maturational phases until infant traits appear in the adults of descendants. Not just physical features are affected. Ancient chimpanzee infant or human forebear child personality characteristics such as curiosity, playfulness, displays of affection and social behavior all prolong themselves, through neoteny, to appear in the features of adult descendants, modern humans.
Acceleration is the opposite of neoteny. Imagine an animal, over time, condensing or withdrawing adult features backward, or earlier, over generations. Ancestor adult features appear earlier and earlier with successive generations until the traits of adult ancestors appear in the children of their descendants. For example, if a relatively hairless elephant forebear had evolved into the mammoth when an ice age came, the hairy aspect of an adult may have resulted, over time, in hairy babies. Evolution would in this case encourage the acceleration of maturation so that adult features appear in the very young, with the adults becoming even hairier over time.
Prolonging maturation results in neoteny. Neoteny is when ancestor infant features appear in descendant adults. Withdrawing or condensing maturation creates acceleration, or the moving backward through ontogeny, or development, so that adult characteristics emerge in the young of descendants.
By adjusting only the rates and timing of maturation over generations, evolution can be dramatically sped up. Mutation is not required. Evidence suggests that a host of environmental influences in addition to sexual selection can directly influence the rate and timing of maturation, resulting in neoteny and its opposite.
Foxes have been observed to evidence dramatic changes in look and behavior in as few as 20 years of targeted breeding. Individuals were selected to breed that exhibited “tame” behavior, or the neotenous features of cooperation, playfulness and displays of affection. Coats changed, ears flopped down, barking emerged and even estrus varied, prolonging reproduction periods. This happened in less than a dozen generations. When humans selected cooperative individuals to mate, cooperation became a feature of descendants. Physical changes accompanied selection for behavior.
In perhaps the most unique exhibition of neoteny and acceleration, in Mexico there is a salamander-like creature called an axolotl. It has external gills and spends its whole life in the water. Change the axolotl environment and remove the water, and the axolotl, over a generation, will adjust to become a creature virtually identical to the North American salamander. The North American salamander lives on land and uses lungs.
The axolotl is the larval or embryonic stage of the salamander. This creature can evolve or adjust maturation to offer descendants a choice of a larval version (living in the water) or an adult version (living on the land). Both forms reproduce. The axolotl features neotenous characteristics of the salamander. Or, you might say that the salamander exhibits acceleration regarding axolotl features.
The absence or presence of water determines which form this axolotl/salamander takes, an environmental effect. With wolves or foxes, generations change according to which features humans select when assigning mates. Humans also change, exhibiting neoteny or acceleration, based upon environmental influences or features selected when mates are chosen. With humans, I hypothesize that those environmental features that influence levels of testosterone and estrogen affect the rate and timing of maturation. In addition, how humans select each other as mates, their sexual selection behavior from within social structures, also determines the speed and intensity of maturation-rate-and-timing-determined evolution.
Testosterone controls the rate of maturation. Estrogen controls the timing. Human evolution is controlled by these two hormones.
Specific environmental variables influence the levels of testosterone and estrogen in males and females. Light, diet, stress, exercise, drugs, alcohol, tobacco use and touch all influence testosterone and often estrogen. Sometimes they affect males and females in opposite directions.
I hypothesize that if a mother with an embryo behaves in ways or is exposed to factors that increases or decreases these hormone levels in her womb, the maturation rates and timing of her children will be affected. I posit that a result can be the prolongation or acceleration of maturation, accompanied by changes in the timing of maturational events. For example, increasing a mother’s testosterone and estrogen levels will decrease her son’s testosterone and estrogen levels. A lower rate of testosterone slows maturation. A lower level of estrogen delays the timing of specific maturational events. A net result may not only be a slower maturing son, but a son with less synapse pruning of the right cerebral hemisphere when very young. A hallmark of facile language use is a smaller right cerebral hemisphere. If this pruning is diminished or delayed, the son may be challenged in the use of language. Adjusting the timing of maturation can result in differences in adult predilections or behaviors. This is how I connect neoteny with autism.
If an aboriginal society is introduced to high-fat diets, the age of puberty drops. If high-fat diets continue, the age of puberty drops further with successive generations. The timing of maturation has been affected with elevated estrogen levels causing an earlier surge in pubertal synapse-pruning testosterone. Puberty comes earlier. In the West, with several generations of increased fat, the age of puberty has dropped three to four years.
If an infant has testosterone and fat or estrogen levels that are too low, language-encouraging testosterone surges may be delayed. Children emerge from their parents with hormonal constellations based upon their parents’ testosterone/estrogen balance and changes that may have occurred while in the womb. Testosterone and estrogen naturally ally themselves to specific social structures, patrifocal or matrifocal. The mating of a high-testosterone woman with a low-testosterone man constitutes a central core of matrifocal social structure. The mating of a high-testosterone man with a low-testosterone woman is central to patrifocal social structure. Varying levels of estrogen impact social structure, compelling additional variations. The net result is that how people select mates leads to the promotion of specific characteristics that can change over time as the environment adjusts levels of testosterone and estrogen.
For example, let’s say a patrifocal aboriginal society experiences climatic change. Increases in rainfall result in the blossoming of high-fat foods. Putting on bulk, females experience increases in both testosterone and estrogen. Male testosterone diminishes. The age of puberty drops. Females with a more authoritative hormone distribution begin to pick males that behave cooperatively. Domineering men grow less common. Over generations, birth rates increase, accompanied by a shift in social structure and sexual selection. Commanding woman and cooperative man become the new convention.
The females exhibit an acceleration of characteristics that exhibit authority at younger and younger ages. The males feature more and more neoteny over generations as they prolong the trait of childlike cooperation into the behavior of adults. Evolution in humans features a balance of both neoteny and acceleration as sexual selection within social structure in combination with environmental influences compel a balanced transformation.
Bands of proto humans over time drift in neotenous and accelerated directions, transforming quickly in just the way that we breed dogs. Still, there is an overall trend in a neotenous direction. Playful, curious, creative, social and affectionate adults become more and more highly valued over time.
Lightning-fast human evolution is a direct result of sexual selection and environmental effects influencing testosterone and estrogen to adjust maturation rate and timing to speed up physical and behavioral transformation. Wolves, foxes and salamanders evolve in just this way. So do humans.