HYPOTHESES ON LONGEVITY HORMESIS 11variables vis-a-vis the allometric regression line has been termed vertical
allometry.51
These characteristics, as well as such traits as large brain size, erect pos
ture, small teeth, thin nails, prolonged growth period, late development of
sexual maturity, ventral positioning of the vagina (permitting us to express
our sexuality facing one another), relative hairlessness, etc., all result from
a phenomenon termed neoteny.ss As defined by Gould,52 neoteny is the
“retention of formerly juvenile characters by adult descendants... pro
duced by retardation of somatic development.” As it specifically relates to
humans, it is “the preservation in adults of shapes and growth rates that
characterize juvenile stages of ancestral primates”56 (see Figure 1.1 for an
illustration57). As a result of neoteny, we are born at an earlier stage of
physical development than other primates and do not mature as far along
their developmental path.55 58 The retardation feature of neoteny is believed
to have provided the only “escape” from our ancestral allometry, permitting
our species to embark evolutionarily toward a favored adaptation.52 With
respect to our aging, it is precisely our neotenous nature that enhances our
longevity relative to other mammalian species of similar size.
Figure 1.1. Baby and adult chimpanzees. Note the differences in facial morphology as a
function of age, but also the strong resemblance between the baby chimp and
adult humans. Naef57 commented: “ Of all the animal pictures known to me,
this [baby chimpanzee] is the most man-like.” As a result of neoteny, we
humans develop more slowly than other mammalian species and, as adults,
retain many juvenile features of our phylogenetic ancestors. While we did not
descend from chimpanzees, the juveniles of our phylogenetic ancestors
probably looked similar to the baby chimp illustrated here. This figure, and
one similar to it, were previously used by Gould52 and Montagu55 to illustrate
the same point. See Montagu55 for an excellent monograph on neoteny.
Reprinted from Naef57 by permission of Springer-Verlag Publishers.