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What biological characteristics set us apart from other animals? The
answer seems easy enough. To start with, we have a large brain, or more
precisely, a large cerebral cortex. Then come bipedality, a complex material
culture and our decidedly peculiar reproductive biology (where else in nature
do you find permanent breasts, concealed ovulation, a conspicuous penis
and continuous sexual receptivity in both sexes?). Those in the know might
add to this list our unique combination of large molar and small incisor
and canine teeth. Yet the chances are that few biologists and anthropologists
would mention the adolescent growth spurt.

This is an odd omission. For no other mammals, primates included, experience
the equivalent period of growth, and at no other time in our lives do our
physical and social attributes change quite so dramatically. Indeed, the
uniqueness of our adolescent dash for maturity makes for some challenging
puzzles, not least concerning its evolutionary origins. Why did adolescence
evolve only in humans and not other primates? What, if any, advantage did
it offer our ancestors?

We must first be clear about what adolescence is and where it falls
within the human life cycle. Infancy starts at birth and ends when the child
is weaned, which in preindustrial societies occurs most often at 18 to 24
months. Childhood is defined as the period following weaning, when the youngster
still depends on older people for feeding and protection. It usually spans
the ages of two to eight years, whereupon the child becomes a juvenile.
In girls, the juvenile period ends at about the age of 10, two years before
it usually ends in boys; the difference reflects the earlier onset of puberty
in girls. The adolescent stage begins with puberty, marked by some visible
sign of sexual maturation such as pubic hair. Adolescence ends with the
attainment of adult stature, which occurs at about age 17 in girls and 21
in boys.

The clearest evidence for these developmental stages comes from studies
of human growth rates. During infancy growth rate plummets, to be followed
by a period of slower decline during childhood and the juvenile stage. The
onset of adolescence is marked by a sudden and rapid increase in growth
rate, which peaks at a level unequalled since early infancy.

Most other mammals progress from infancy to adulthood seamlessly, experiencing
no childhood and no adolescent growth spurt. Indeed, animals such as mice,
guinea pigs, rabbits and cattle all reach sexual maturity with their growth
rates in decline: puberty follows hard on the heels of weaning. This trend
is broken by only the most social mammals – primates, wolves, elephants
and so on – which follow infancy with a period of juvenile growth and behaviour,
when they no longer need parental care but are not yet sexually mature.
But in these animals, too, puberty occurs while the rate of growth is still
decelerating and there is no detectable growth spurt.

That this is true even in our closest living relatives, the apes , makes
the evolutionary origins of adolescence all the more puzzling. Why did our
ancestors evolve the growth spurt? The conventional theory rests on the
observation that humans alone require prolonged stages of infant, childhood
and juvenile growth to learn the complex technical and social skills that
make up human culture. The growth spurt, so the theory goes, evolved because
at the end of this period our ancestors were left with proportionately less
time for procreation than most mammals, and therefore needed to attain sexual
maturity quickly. In an age when life was ‘brutish and short’, youngsters
who matured and reached adult size quickly would have produced more offspring
than their more sluggish cousins.

So genetic traits encouraging an adolescent growth spurt emerged in
humans not because of any intrinsic value but to compensate for time ‘lost’
to learning in early life. In a sense, childhood begot adolescence: that,
at least, is the idea.

Growing up ain’t easy

But surely this cannot be the whole story. For one thing, the argument
that adolescence evolved to compensate for a prolonged childhood does not
explain its timing. Girls experience the growth spurt before becoming fertile,
but for boys the reverse is true. Why the difference? More fundamentally,
the conventional theory assumes a simple, cosy relationship between adult
stature and fertility, and between fertility and reproductive success.
The reality is more complex: there is much more to raising a child than
fertilising an egg, and body size is not linked in a simple way to sexual
development.

In case you are in any doubt, historical sources describe the castrati,
male opera singers of the 17th and 18th centuries who were castrated as
boys to preserve their soprano voices, as being unusually tall for men.
And contemporary research by Andrea Prader at the Zurich Children’s Hospital
in Switzerland shows that children who are born without gonads or who have
them removed surgically before puberty (due to diseases such as cancer)
eventually grow to be normal-sized adults, even though they experience no
adolescent growth spurt. Height and growth spurt are clearly governed by
different biological factors. Whereas the steroid sex hormones produced
by gonads – principally testosterone in boys and oestradiol in girls – are
needed for the spurt, height is ‘fixed’ by substances from the pituitary
gland, in particular human growth hormone.

The complexity of the links between adolescence, fertility and body
size suggests that the human growth spurt has its own intrinsic value, and
is not just a by-product of slow prepubertal development. In short, it evolved
because it somehow made our ancestors better equipped to reproduce.

Human beings are a reproductive success story. Even people lacking the
benefits of modern medicine raise half their infants to adulthood (chimpanzees
manage to rear less than 36 per cent of their offspring to adulthood). The
physical roots of this reproductive prowess undoubtedly lie in our capacity
for learning complex behaviours and survival skills – language, cooperation,
hunting, tool making and so on. And these, in turn, depend in part on the
dramatic growth of the brain early in life. Yet the question remains: when
and how do young people learn all those exclusively adult behaviours related
to sex and child rearing?

As children and juveniles, perhaps. The problem here is that prepubescent
boys and girls look very similar in terms of size and the amount of muscle
and fat that they carry. Not looking like reproductive beings, they are
unlikely to be treated as such by adults. Moreover, prepubescents have
very low levels of testosterone and oestrogens – hormones thought to play
an essential part in priming a young person’s interest in adult sexual
and social behaviour. Prepubescents are ill-placed to learn from adults
the social behaviours that underpin reproductive success.

Even with the onset of puberty, when girls and boys become hormonally
and physically attuned to sexual behaviour, the road to reproductive maturity
is long and winding. In girls, the first outward sign of puberty is the
development of the breast bud and wisps of pubic hair. This is followed
by the laying down of fat on the hips, buttocks and thighs, the growth of
more body hair, the adolescent growth spurt and, finally, menarche – the
onset of menstruation. Menarche is usually followed by a period of one to
three years of adolescent sterility, in which menstrual cycles occur but
without ovulation. So it is often not until a young woman is 14 or more
years old that she becomes fertile.

Fertility, however, does not necessarily imply reproductive maturity.
Becoming pregnant is only a part of the business of reproduction. Maintaining
the pregnancy to term and raising offspring to adulthood are equally important.
In Western countries today, the risk of spontaneous abortions and complications
of pregnancy for girls under 15 years old is more than twice as high as
that for women of 20 to 24 years. Babies born to American mothers under
15 years of age are more than twice as likely to be of ‘low birth weight’
than infants born to women aged 25 to 29.

A mother’s age, of course, is not the only factor affecting her baby’s
survival prospects: low socioeconomic status, smoking, a failure to put
on weight during pregnancy and ethnic origins are important, too. However,
holding all these other factors constant still leaves the teenage mother
and her infant at risk.

Why? Part of the answer lies with basic biology. A decade ago Marquisa
LaVelle, a physical anthropologist now at the University of Rhode Island,
uncovered evidence of a physical reason for the high percentage of small
babies born to teenage mothers. Examining pelvic X-rays from a group of
healthy girls, LaVelle noticed that their pelvic inlets – the bony opening
of the birth canal – reached adult size only when the girls were 17 or 18
years old, four or five years after menarche. The implication was as unexpected
as it was profound. The adolescent growth spurt does not influence the size
of a girl’s pelvis. Rather, the pelvis has its own slow pattern of growth
which continues for several years after a girl has reached adult stature.

The fact that women must wait up to a decade from the time of menarche
to reach full reproductive maturity suddenly begins to make sense. So, too,
does the observation that the average ages at which women in cultures as
diverse as the Kikuyu tribe of Kenya and urban North America marry and
have their first child all tend to cluster around 19 years. Yet the slow
development of the female pelvis raises questions as well as answering them.
Why should evolution have selected a developmental trait that hinders successful
childbirth for teenage girls?

The answer may lie with the need to learn social skills. A mother-to-be
must acquire information about pregnancy and experience in adult sociosexual
relations and child care. And this, in my view, is where adolescence comes
into play.

The dramatic physical changes that girls experience during adolescence
serve as efficient advertisements for their sexual and social maturation
– so efficient that they stimulate adults to include adolescent girls in
their social circles and encourage the girls themselves to practise adult
social interactions: male-female bonding, ‘aunt-like’ caring for children
and so on. As anthropological research shows, girls in every human culture,
on reaching adolescence, display a surge of interest in the sexual behaviour
of adult women.

In our female ancestors, then, adolescence evolved because it enabled
girls to learn how to be more reproductively successful as young women.
But is there any direct evidence for this? Some support comes from the fact
that first-born infants of monkeys and apes are more likely to die than
those of humans. Studies of yellow baboons, toque macaques and chimpanzees
show that between 50 and 60 per cent of their firstborn offspring die in
infancy. By contrast, in hunter-gatherer human societies, such as the !Kung
of southern Africa, only about 44 per cent of first-born children die in
infancy.

Life-saving experience

Furthermore, studies of wild baboons by Jeanne Altmann of the University
of Chicago show that although the rate of infant mortality for the first-born
is as high as 50 per cent, it drops to 38 per cent for the second infant,
and 25 per cent for the third and fourth infants. This improvement in infant
survival is in part due to the experience the mother gains with each birth
– experience girls accumulate during adolescence. The initial human advantage
may seem small, but it means about 16 more people than baboons or chimpanzees
survive out of every 100 first-born infants – more than enough over the
vast course of evolutionary time to make human adolescence an overwhelmingly
beneficial adaptation.

A further evolutionary advantage may accrue from female adolescence.
By priming girls to help their older siblings rear children, adolescence
enables women to give birth to more infants than primates. The primatologist
Jane Goodall finds that female chimpanzees have their first baby at about
13 years and must wait an average of 5.6 years between successful births,
because each infant is totally dependent on its mother. As a result, few
chimpanzee females produce more than three offspring. In traditional human
societies, by contrast, women usually have their first baby at 19 and then
go on to produce an infant every two to four years. This means they can
easily have six or more children.

So much for adolescence in girls; what of boys? Boys become fertile
well before they assume adult size and the physical characteristics of men.
The little fertility research done on boys suggests that they begin producing
sperm at an average age of 14.5 years. Yet the cross-cultural evidence is
that few boys successfully father children before their late teenage years.
When Carol Worthman, now at Emory University in Atlanta, Georgia, lived
with the Kikuyau tribe in Kenya, she found that it is customary for the
men to defer marriage and fatherhood until the age of about 25, though
they become sexually active following their circumcision rite at around
18. The National Center for Health Statistics in the US reports that only
four per cent of all births in the US are fathered by men under 20 years
old.

Why the lag between sperm production and fatherhood? One explanation
may be that the sperm of younger adolescents do not swim well enough to
reach an egg cell in the woman’s fallopian tubes. But it could be that
the average boy of 14.5 years old is only beginning his adolescent growth
spurt. In terms of physical appearance, physiological status and psychosocial
development, he is still more a child than an adult. Young men display
the opposite developmental trend to that of girls in that they experience
a delay between reaching reproductive maturity and later advertising this
maturity with an increase in physical size, body hair, muscularity and other
secondary sexual characteristics.

To trace the evolutionary advantage of this delay, one must turn to
the subtle psychological effects of testosterone and the other androgen
hormones released from the male gonads during early adolescence. In effect,
these hormones ‘prime’ boys to be receptive to their future roles as men.
Over the past three decades, studies on a cross-section of youths in Europe,
North America and Japan have established that as blood levels of testosterone
begin to increase, but before the growth spurt reaches its peak, there
is an increase in psychosexual activity. Nocturnal emissions begin and
masturbation, dating and infatuations all intensify, as do feelings of guilt,
anxiety, pleasure and pride. At the same time boys become more interested
in adult activities, adjust their attitude to parental figures, and think
and act more independently. In short, they begin to behave like men.

However – and this is where I believe the survival advantage lies –
they still look like boys. Because their adolescent growth spurt occurs
late in sexual development, young males can practise behaving like adults
before they are actually perceived as adults. The sociosexual antics of
young adolescent males enable boys to fine-tune their sexual and social
roles before either their lives, or those of their offspring, depend on
them. In many traditional societies, for example, competition among males
for women can be fierce, even fatal, and older men usually come off best.
In such circumstances, the ‘cute’, childlike appearance of an adolescent
male may be life saving.

My argument, then, is this. Girls best learn their adult social roles
while they are infertile but perceived by adults as mature; boys best learn
their adult social roles while they are fertile but not yet perceived as
such by adults. Without the adolescent growth spurt this unique style of
social and cultural learning could not occur. And this is why adolescence
deserves to stand alongside our large cerebral cortex, bipedality and unique
sexual behaviour as a factor defining us as human. Indeed, all these characteristics
stem from the same underlying biological trait: our uniquely human pattern
of growth.

Barry Bogin is professor of anthropology at the University of Michigan,
Dearborn, MI 48128, USA. He is author of Patterns of Human Growth (Cambridge
University Press, 1988) and ‘The evolution of human childhood’ (BioScience
January, 1991).

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Growing evidence against chimpanzee adolescence

Do apes experience adolescence? Thirty or more years ago the matter
looked settled: the answer was no. By monitoring how fast chimpanzees grow
up in captivity, James Gavan of the University of Missouri acquired a set
of growth data in the 1950s which appeared devoid of any evidence of a spurt.
But now the case has been reopened, and some biologists are attempting to
revive belief in the notion of the adolescent ape.

The initial analyses of Gavan’s data revealed a growth pattern similar
to that of monkeys, the only difference being that the chimpanzees experienced
longer periods of infancy and juvenility. This interpretation was backed
up later by field work. Jeanne Altmann, a primatologist at the University
of Chicago, found that in the wild, olive baboon mothers wean their infants
between the ages of 12 and 18 months, and the juvenile baboon reaches puberty
at about four years in females and six years in males. Research on chimpanzees
by the primatologist Jane Goodall showed that in apes weaning takes place
at about four-and-a-half years and females reach puberty at about 10 to
11 years. Neither baboon nor chimpanzee showed signs of passing through
adolescence.

This orthodoxy is being challenged by Elizabeth Watts, a zoologist at
Tulane University of Louisiana, New Orleans. Reanalysing Gavan’s data from
the 1950s, she found that as chimpanzees reach sexual maturity growth of
their leg and arm bones suddenly quickens. As a result, their limbs end
up 5 millimetres longer than would otherwise be expected. Could this be
evidence of adolescence?

Most biologists are sceptical, and for three reasons. First, unlike
the easily detectable human growth spurt, the chimpanzee growth ‘spurt’
is revealed only by sophisticated mathematical analysis. Secondly, the human
growth spurt affects virtually the whole body, not just the arms and legs.
Finally, according to calculations by Holly Smith, a colleague of mine at
the University of Michigan, Ann Arbor, by the time a chimpanzee’s arms and
legs begin their growth spurt the animal has already completed 86 per cent
of its skeletal growth. At the onset of adolescence, by contrast, boys and
girls have completed, on average, only 77 per cent of their growth.

* * *

The first teenager

Anthropologists and biologists have long assumed that the adolescent
growth spurt has been part of our life cycle ever since Homo sapiens evolved,
between 40 000 and 125 000 years ago. But the question remains: is adolescence
unique to our species, or did our three hominid ancestors – Australopithecus,
Homo habilis, and Homo erectus – also experience it?

The number of known fossil remains of immature and adult hominids has
increased in recent years, allowing scientists to tackle this question with
greater accuracy.

In 1985, New ÐÓ°ÉÔ­´´ published an article by Roger Lewin arguing that
it may be misleading to think of ‘early hominidsas diminutive humans’.
Following this admonition, Timothy Bromage and Christopher Dean, then at
University College London, developed a method to estimate developmental
growth rates of hominids from the fossil record.

The method is based on micro-structural features of incisor tooth crowns,
and measures lines of incremental growth of the enamel of the tooth. It
is assumed that these lines represent regular intervals of growth, akin
to tree rings. Bromage and Dean found that Australopithecus and Homo habilis
may have grown at rates closer to those of apes than to people. This was
confirmed in 1987 by Glenn Conroy and Michael Vannier of Washington University
in St Louis, Missouri, who used computed tomography (CT) images of the dental
development of the Taung child fossil, an australopithecine who died at
three to four years of age.

The CT images reveal the state of formation of the permanent teeth that
were still encased within the jaws at the time of death. That formation
is consistent with ape-like rates of development, which are about twice
as fast as human rates.

Holly Smith, a colleague of mine at the University of Michigan, Ann
Arbor, has estimated the rates of hominid development using a third method,
based on rates of formation of tooth crowns and roots. She concludes that
early hominids developed at rates between those of chimpanzees and people,
but closer to those of apes. Based on these studies it seems unlikely that
either Australopithecus or Homo habilis experienced adolescence.

Another candidate for the hominid species that first evolved adolescence
is Homo erectus. In 1985 Frank Brown, John Harris, Richard Leakey and Alan
Walker (from the University of Utah, the Los Angeles County Museum, the
National Museums of Kenya, and John Hopkins University, respectively) announced
the discovery of a nearly complete Homo erectus skeleton (catalogued as
KMN-WT 15 000) 1.6 million years old. The fossil boy was unusually tall
for an immature specimen.

Walker and his colleagues at Johns Hopkins estimate his height at death
to be 160 centimetres, equivalent to that of an average 15-year-old today.
The skeleton’s size and certain other characteristics have prompted speculation
that it may be the remains of one of the ‘first adolescents’.

Smith has scrutinised the evidence for this claim. Basing her interpretation
on the formation of molar teeth, the state of development of epiphyses
(the ‘growing ends’) of long bones, and size she describes the skeleton
as ‘too advanced’ to have followed a human pattern of growth. ‘Early Homo
erectus life cycle and life span were intermediate to those of extant apes
and humans,’ writes Smith. ‘His dental age, skeletal age, and body size
are quite consistent with the idea that the adolescent growth spurt had
not yet evolved in Homo erectus.’

However, with existing techniques it is difficult, if not impossible,
to say for sure. The problem is quite fundamental: detecting the adolescent
growth spurt in living people requires repeated measurements on the same
individual; but fossils provide only one measurement per individual. Even
so, Smith concludes that ‘the unique skeleton of KNM-WT 15 000 stands at
a point near the very beginning . . . of the evolution of human life history’.