Chapter 6 - Neoteny
Biologically, an organism becomes “sexually mature” or an “adult” when it is capable of reproducing. And it becomes “physically mature” when it acquires its adult form. The rate at which an organism matures physically and the rate at which it matures sexually are independently controlled by different genes. 1 A population can evolve so that individuals physically mature faster or slower, while keeping their rate of sexual maturation constant, or it can evolve so that individuals mature sexually faster or slower, while keeping their rate of physical maturation constant, or both can change.
A population can evolve so that individuals remain childlike in their adult form (“paedomorphism”) in two ways. It can evolve to speed up physical and sexual maturation so that individuals become both physically and sexually mature while they are still infants (“progenesis,” e.g., newts), or it can evolve to slow down or stop physical maturation so that the age of sexual maturation stays about the same, but individuals are childlike when they reach sexual maturity (“neoteny”). “Neoteny” (new-stretch) refers to a gene-controlled change in the way individuals mature, where they mature sexually at about a normal rate but, although the body grows in size as they become sexually mature, their juvenile features (and their ancestors’ juvenile features) are retained into adulthood and are not replaced by distinctively different adult features; in other words, a child becomes a larger, but sexually-mature, child.
The evolution of man was accomplished by a number of genetic changes, but one of the most important was neoteny. Humans are the most neotenic of all primates. Notice, in Figure 6-1, 2 the remarkable and important comparison of an adult and baby chimpanzee. The adult chimpanzee did not retain his babyish face, but instead replaced it with a very different face. The more human-looking face of the baby chimp is much flatter, while the adult has a very protruding jaw. 3 Because the adult did not retain the baby’s face, the chimpanzee is not neotenic. Now imagine that the baby chimpanzee grew up to become sexually mature, but his face did not change; then the chimpanzee would be neotenic and would look much more human.
Now that you know what neoteny is, it should not be difficult to see that man is neotenic. In fact, man is so neotenic that he has been described as a “sexually mature fetus.” 4 Many of our neotenic traits were vital to our evolution. As in most fetal mammals, including humans, the foramen magnum (the opening through which the spinal chord exits the skull) is more in the center of the base of the skull. As quadrupedal animals mature it moves to the rear (Table 9-2), but in humans, who are bipedal, it remains in its infant position (so the eyes are directed perpendicularly to the spine).In embryonic mammals, the vaginal opening is more to the front, and remains so in adult human females (for front-to-front intercourse) and does not move to the rear (for front-to-back intercourse), as in other mammals. Our big toe remains parallel to our other toes (for walking) and does not move to a 90° angle to them (for grasping) as in the great apes. Man’s neotenic traits also include a more gracile (i.e., less robust) skeleton, a skull that is larger (in proportion to body size), rounder, and more spherical with thinner bones, a flatter face with a less protruding jaw (“prognathism”) 5 and smaller teeth, little body hair, smaller arms, legs, fingers, and feet, and more fat under the skin, all traits found in primate babies. 6
Flesh-colored skin may also be neotenic in humans. Newborns of dark-skinned parents are lighter-skinned (Abner, 1998), then their skin darkens as they grow older. 7 It is interesting to note that young chimpanzees have flesh-colored skin which becomes blackish or black between ages 10 and 12 (Baker, 1974, p. 112); that suggests that our last common ancestor (LCA) with chimpanzees may also have had light skin when young. 8 There is some genetic evidence that “the common ancestors of all humans on earth had white skin under dark hair – similar to the skin and hair color pattern of today’s [young] chimpanzees.” 9
The hair of newborns is also straighter, even of African babies, and fetuses have an epicanthic fold (a fatty fold of skin that partially covers and protects the eyes, Figure 10-3), 10 so those traits are also neotenic. A white sclera (eyeball) may be neotenic as “most animals have sclera that darken with age, [but] humans retain white sclera all of their lives.” (Etcoff, 1999, p. 33).
What caused man’s neoteny? The obvious answer is that before man became neotenic, individuals differed slightly in how neotenic they were, just as they differ in nearly all traits; man would have stayed non-neotenic forever, but his environment changed. After that change, those individuals who were more neotenic had more reproductive success than those who lacked alleles for neoteny, and the entire population became more neotenic.
The next question is, “What environmental changes would make neotenic traits more advantageous?” A smaller, non-protruding jaw and less robustness (smaller bones and muscles) would be a major disadvantage in a fight. But, if man had advanced enough to develop tools and weapons, those traits would be unnecessary, a waste of the body’s resources and energy, and would reduce speed and agility. What other traits do babies have that, if an adult had them, would make that adult more likely to survive?
Another possibility is a larger brain. In proportion to body size, babies have larger brains than adults, 11 and more neotenic adults usually have larger brains than less neotenic adults. It is also true that there is a moderate 12 correlation (r = 0.44, Lynn, 2006a, p. 214) between intelligence and brain size. 13 It is not a perfect correlation – people with large brains can still be stupid – but it is still a significant correlation. So it is possible that if the change in the environment required more intelligence to survive, then individuals who were more neotenic and therefore had larger brains and greater intelligence, would be selected. 14 If a population migrated from the tropics, where there is little seasonal change, to the north where there are four distinct seasons, including a long, cold, winter, more intelligence would be an asset in planning for the winter and provisioning food. Thus, seasonal differences in climate would select for more intelligence and therefore for more neotenic individuals.
How severe the selection for intelligence would be is hard to estimate. Small brains are, after all, capable of provisioning for the winter – squirrels do it all the time and, in proportion to body size, their brains are far smaller than man’s were. Moreover, the brain is the body’s most costly organ, as it requires more energy (per unit weight) than any other organ. 15 An adult brain is about 2% (Leakey, 1994, p. 54) or 3% (Foley, 1995, p 170) of body weight but uses 20% of the body’s energy 16 and the average newborn’s brain consumes an amazing 75% of an infant’s daily energy needs. 17 A bigger brain may help solve more problems, but it is extra weight to carry around and requires extra food to keep it functioning. To see which way the assets and liabilities shifted, it is necessary to see how much intelligence in the north actually increased, which we will examine later in this book.
Babies almost anywhere, except possibly in the tropics, must be kept warm to prevent death by hypothermia. Because of their small size (high surface area to mass ratio) they need to conserve heat and minimize the burning of calories. They have many traits that help them do this, which would be useful to adults who migrated north, one of which is baby fat. Babies have extra fat under their skin evenly distributed over their bodies which stores energy for their rapidly-growing brains, provides some protection against bumps, and keeps them warm. Other neotenic traits useful in colder climates include an epicanthic fold and traits that reduce surface area, 18 e.g., a flatter face, small hands and feet (Baker, 1974, p. 307), and a thick trunk, all of which are characteristic of northern Asian populations. This suggests that neoteny could be strongly selected for in a population that migrated into a colder climate.
The most neotenic people on the planet are the East Asians and the most neotenic East Asians are the Koreans, who have the most subcutaneous fat, 19 followed closely by the Han Chinese and other Mongoloids. 20 Just like babies, East Asians have a round head with a flat chubby face, a small nose, short arms and legs, very little body hair, and extra fat evenly distributed over their entire body. Their “third eyelid” (epicanthic fold) and smaller eye sockets help to protect their eyes from the cold. Clearly, these people evolved to live in a cold climate and, since they became so neotenic, that suggests that neoteny was advantageous in that climate. (Chap. 4, Rule 11).
The European lineage became neotenic as well, but much less so than the Asians. Europeans have longer heads, more hair, longer limbs, and the fat under their skin is less uniformly distributed; instead, it accumulates in unsightly bunches around the abdomen, hips, and thighs, providing a good source of income for the weight-loss industry. Most Africans are still less neotenic, but their lineage is more complicated, giving different African populations some very different traits. (Chap. 26).
Table of Contents
1. Sexual and physical maturation rates are controlled by only a few Hox (homeobox) genes, genes that turn on or off a host of other genes, in this case genes that regulate physical and sexual maturation, so genetically changing the physical or sexual maturation rate does not necessarily require a large number of mutations in order to occur. Neoteny may “work” by halting the additive process (Chap. 4, Rule 1) that occurs in the fetus. Back
2. From (Naturwissenschaften, Vol. 14, 1926, pp. 447-448). Figure 6-1 shows common chimpanzees. The differences are less striking for the more-neotenic bonobo chimpanzee. When the smaller baby chimp grew into the larger adult chimp, its skull cap did not enlarge; unlike humans, the chimp brain stops growing at a much earlier age. The difference between the young and adult orangutan is so great that an early naturalist (Saint-Hilaire, in 1836) thought they were not even in the same genus. Back
3. The protruding jaw appears by the age of sexual maturity, when males fight for access to females. The absence of this menacing jaw in the baby makes it appear harmless and arouses caring emotions. Back
4. “If I wished to express the basic principle of my ideas in a somewhat strongly worded sentence, I would say that man, in his bodily development, is a primate fetus that has become sexually mature.” Bolk,L.; Bolk, 1926). Back
5. “Young monkeys and young negroes, however, are not prognathous like their parents, but become so as they grow older.” (Cartwright, 1857, p. 45). Back
6. Baker (1974, p. 312) implies that wide-apart eyes are neotenic, though bonobos are neotenic and have eyes close together. (id, p. 113). Back
7. "Negro children and white children are alike at birth in one remarkable particular – they are both born white, … “ (Cartwright, 1857, p. 45). "Apes when new born have very much lighter skins than adults; additional pigment becomes deposited during later development, and the same is true of the Negro. In this respect the white races are neotenous, for they retain the embryonic conditions of other forms. (de Beer, 1951, pp. 58-59). Back
8. "It is likely, then, that the common ancestor of humans and chimpanzees had light skin covered with dark hair, ..." (Jablonski, 2006, p. 26). "Skin color of the infant langur, baboon, and macaque is pink, in contrast to the almost black skin of the older infant or adult." (Frost, P. "Parental Selection, Human Hairlessness, and Skin Color," Evo and Proud, Apr. 1, 2007). Back
9. (Rogers, 2004). “[Chimpanzees] are extraordinarily variable in skin color, running from a grayish pink that is almost white to black, with several yellowish shades between. Their color range is essentially the same as in the races of man …” (Coon, 1962, p. 145). Back
10. Epicanthic folds develop in fetuses of all races during the third to sixth month but disappear in Caucasians. Children with Down syndrome also have them. (Wikipedia, “Down Syndrome”). Back
11. At birth, a baby’s brain is 24% of its adult weight, while its body is only 5% of its adult weight (Coon, 1962, p. 78). Back
12. A “weak” correlation is less than 0.4, a “moderate” correlation is between 0.4 and 0.6, and a “strong” correlation is greater than 0.6. The correlation squared times 100 gives the percentage explained, e.g., a correlation of 0.6 explains 36% of the effect. Back
13. (Witelson, 2006; McDaniel, 2005). Back
14. Genius today is often associated with youthfulness. (Charlton, 2006). Back
15. “Grey matter is the gas-hog of our bodies.” (Sloan, C.P., National Geographic, Nov., 2006, p. 159). Back
16. Compared to 9% for a chimpanzee. (Arsuaga, 2001, p. 38). Back
17. The acquisition and loss of traits, e.g., brain size, tails, ability to run, behavior (agriculture, seasonal migrations), and reproductive strategy (number, size, and frequency of offspring), can often be best explained in terms of energy expended and energy acquired. (Foley, 1995, p. 171, 176). Back
18. A sphere has the least amount of surface area (for the volume contained) of any three-dimensional shape, hence a rounder head retains more heat. Minimizing projections, such as the arms, legs, fingers, and toes, makes a body more spherical and therefore helps to retain heat. (Allen’s Rule). Back
19. From 1910 to 1945, the Japanese used completely naked Korean women, well-insulated by subcutaneous fat, as pearl divers. (Rennie, 1962). Back
20. “…the yellow races are nearest to the infantile condition.” Havelock Ellis. Han Chinese males lack hair on their arms, legs, and chest and also lack beards, having only head hair and some auxiliary and pubic hair. They don’t even have "peach fuzz" on the arms and legs. Most pure Han women have only sparse hair on the mons pubis. Koreans are nearly as hairless. Back