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new synthesis
In contrast to progenesis, neoteny occurs when reproductive
maturity takes place at the normal time but physical
development slows down. The effect is the same: reproductively
mature individuals that resemble juveniles.
The principal example that is used to illustrate neoteny is
the human species. Even before the acceptance of evolutionary
theory, Georges Buffon (see Buffon, Georges) and Etienne
Geoffroy St. Hilaire recognized the remarkable similarity
between the skulls of humans and of juvenile apes, but not of
adult apes (see photo on page 291). Their work was developed
in an evolutionary context by the Dutch anatomist Louis Bolk,
whose major work was published in 1926. Bolk’s arguments
were later expanded by zoologist Sir Gavin de Beer. Human
characteristics contrast not just with those of adult apes but
with those of most adult vertebrates. Here are just a few of the
similarities between adult humans and juvenile nonhuman vertebrates:
• The head is relatively large. Everyone who has seen baby
guppies, puppies, kittens, calves, and birds knows that
juvenile vertebrates have relatively large heads. (This is true
even of some invertebrates such as grasshoppers.) Related
to this, the brain is relatively large in adult humans and in
juvenile nonhuman vertebrates. Brain development slows
very soon after birth in nonhuman apes but continues for
years in humans. The continuation of embryonic brain
growth into human adulthood gives humans the largest
encephalization quotient of any animal (see allometry).
Not surprisingly, the bones of the human skull retain the
juvenile ability to expand during this time of brain growth.
• The face is relatively vertical. During adult development
in nonhuman apes, the facial angle extends outward,
resulting in a forehead that slopes back from the nose and
mouth. Related to this, the foramen magnum (the hole in
the skull through which the spinal cord connects with the
brain) is underneath the skull in embryonic vertebrates
and in adult humans but is in the back of the skull in adult
nonhuman vertebrates. It is essential that the foramen
magnum be at the bottom of the skull for upright posture
(see bipedalism).
• The skull has no pronounced brow ridges. All adult nonhuman
apes, but no newborn apes, have ridges of bone above
the eyes (also called supraorbital processes). The lack of
brow ridges distinguishes adult humans from every other
adult modern ape and from all other known adult hominins
in the fossil record.
• The body has relatively little hair. Juvenile apes are born
with little hair aside from that on the head. Adult humans
have very little hair other than on the head, prompting
zoologist Desmond Morris to call the human species the
Naked Ape.
The human brain grows so rapidly during fetal development
that a human baby must be born at a stage of development
that is much earlier than that of other apes (see life
history, evolution of). Human babies are, effectively,
helpless embryos when born.
The juvenile characteristics of adult humans also include
behavior. Most young animals are playful, as they explore
their environments and practice at behaviors that they will
need when they are adults. Humans remain playful all of
their lives, as evidenced by activities as diverse as musical creativity,
scientific research, and sports. This neotenous playfulness
has resulted from but also enhanced the evolution of the
brain (see intelligence, evolution of) and has made the
evolution of human culture possible (see religion, evolution
of; language, evolution of).
The most important features that characterize modern
humans—large brains, upright posture, and creativity—have
resulted from neoteny. Neoteny serves as a unifying principle
for the suite of changes that have occurred in human evolution,
resulting perhaps from a few simple changes in genes
that control development (see developmental evolution).
Because all of these differences result from neoteny, it is not
necessary to find an adaptive explanation for every human
feature. One famous example is the chin. Humans are unique
among apes in having chins. Some anthropologists have
speculated that the chin evolved for some adaptive function,
perhaps as a threat display. Neoteny explains that the human
chin did not evolve in response to natural selection. In effect,
the human chin is not an entity; it is the product of differences
in growth rates of bones that occurred during neoteny. There
is no need to assign any adaptive function at all to the chin.
In modern humans, females display more neotenous features
(in head size, facial angle, and lack of body hair) than
males. Traditionally, males (including male scientists) have
considered themselves to be more advanced than females. But
if evolutionary advancement is tied to neoteny, then the case
could be made that women are more advanced than men.
What would happen if human physical development
continued past reproductive maturity, rather than stopping?
This question can only be addressed through fiction. Evolutionary
biologist Julian Huxley (see Huxley, Julian S.)
injected hormones into a neotenous amphibian. This caused
the amphibian to develop into an adult form that had never
been observed in nature. His brother, novelist Aldous Huxley,
had a grasp on evolution far beyond most other writers
of fiction and put his brother’s discovery to use. In his short
novel After Many a Summer, Aldous Huxley tells the story of
a rich couple who discovered the secret of immortality, but
they could not stop the continued development of their bodies.
They were discovered in a cave; at nearly 200 years of
age, they looked like chimpanzees.
Further Reading
Gould, Stephen Jay. “The Child as Man’s Real Father.” Chap. 7 in
Ever Since Darwin: Reflections in Natural History. New York:
Norton, 1977.
new synthesis See modern synthesis.
noncoding DNA All of the enzymes that work in the
human body, or in any other species, are encoded in genes
made of DNA (see DNA [raw material of evolution]).
Genes make up less than 10 percent of the DNA in human
chromosomes, and the same is true of most other eukaryotic
species. The remaining DNA, over 90 percent, is referred to as