21 May, 2005 at 09:56 Leave a comment

Teenagers special: The original rebels

  • 05 March 2005
  • From New Scientist Print Edition. Subscribe and get 4 free issues.
  • Lynn Dicks
  • Lynn Dicks is a writer based near Cambridge, UK
The origins of angst

The origins of angst

Growth spurt

Growth spurt

EAST Africa, one-and-a-half million years ago: a group of women sit with their young children. They are heavy-browed with small skulls – not quite human, but almost. Some are checking their children for ticks, others teaching them how to dig tubers out of the ground. Not far off, a gaggle of teenage girls lounge under a tree, sniggering and pointing at some young men who are staging fights nearby. The older women beckon: “Come and help us dig out this root – it will make a great meal,” they seem to say. But the girls reply with grunts and slouch off, sulkily.

Could this really have happened? Our immediate ancestors, Homo erectus, may not have had large brains, high culture or even language, but could they have boasted the original teenage rebels? That question has been hotly contested in the past few years, with some anthropologists claiming to have found evidence of an adolescent phase in fossil hominids, and others seeing signs of a more ape-like pattern of development, with no adolescent growth spurt at all. This is not merely an academic debate. Humans today are the only animals on Earth to have a teenage phase, yet we have very little idea why. Establishing exactly when adolescence first evolved and finding out what sorts of changes in our bodies and lifestyles it was associated with could help us understand its purpose.

We humans take twice as long to grow up as our nearest relatives, the great apes. Instead of developing gradually from birth to adulthood, our growth rate slows dramatically over the first three years of life, and we grow just a few centimetres a year for the next eight years or so. Then suddenly, at puberty, growth accelerates again to as much as 12 centimetres a year. Over the following three years adolescents grow an astonishing 15 per cent in both height and width. Though the teenage years are most commonly defined by raging hormones, the development of secondary sexual characteristics and attitude problems, what is unique in humans is this sudden and rapid increase in body size following a long period of very slow growth. No other primate has a skeletal growth spurt like this so late in life. Why do we?

Until recently, the dominant explanation was that physical growth is delayed by our need to grow large brains and to learn all the complex behaviour patterns associated with humanity – speaking, social interaction and so on. While such behaviour is still developing, humans cannot easily fend for themselves, so it is best to stay small and look youthful. That way you do not eat too much, and your parents and other members of the social group are motivated to continue looking after you. What’s more, studies of mammals show a strong relationship between brain size and the rate of development, with larger-brained animals taking longer to reach adulthood. Humans are at the far end of this spectrum.

If this theory is correct, the earliest hominids, Australopithecus, with their ape-sized brains, should have grown up quickly, with no adolescent phase. So should H. erectus, whose brain, though twice the size of that of Australopithecus at around 850 cubic centimetres, was still relatively small. The great leap in brain capacity comes only with the evolution of our own species and Neanderthals, starting almost 200,000 years ago. Brains expanded to around 1350 cm3 in our direct ancestors and 1600 cm3 in Neanderthals. So if the development of large brains accounts for the teenage growth spurt, the origin of adolescence should be here. The trouble is, some of the fossil evidence seems to tell a different story.

The human fossil record is extremely sparse, and the number of fossilised children minuscule. Nevertheless in the past few years anthropologists have begun to look at what can be learned of the lives of our ancestors from these youngsters. One of the most studied is the famous Turkana boy, an almost complete skeleton of H. erectus from 1.6 million years ago found in Kenya in 1984. The surprise discovery is that there are some indications that he was a young teenager when he died.

Accurately assessing how old someone is from their skeleton is a tricky business. Even with a modern human, you can only make a rough estimate based on the developmental stage of teeth and bones and the skeleton’s general size. For example, most people gain their first permanent set of molars at age 6 and the second at 12, but the variation is huge. Certain other features of the skull also develop chronologically, although the changes that occur in humans are not necessarily found in other hominids. In the middle teenage years, after the adolescent growth spurt, the long bones of the limbs cease to grow because the areas of cartilage at their ends, where growth has been taking place, turn into rigid bone. This change can easily be seen on an X-ray.

You need as many of these developmental markers as possible to get an estimate of age. The Turkana boy did not have his adult canines, which normally erupt before the second set of molars, so his teeth make him 10 or 11 years old. The features of his skeleton put him at 13, but he was as tall as a modern 15-year-old. “By human standards, he was very tall for his dental age,” says anthropologist Holly Smith from the University of Michigan at Ann Arbor. But you get a much more consistent picture if you look at Turkana boy in the context of chimpanzee patterns of growth and development. Then, his dental age, bone age and height all agree he was 7 or 8 years old. To Smith, this implies that the growth of H. erectus was primitive and the adolescent growth spurt had not yet evolved.

Susan Anton of New York University disagrees. She points to research by Margaret Clegg, now at the University of Southampton in the UK, showing that even in modern humans the various age markers often do not match up. Clegg studied a collection of 18th and 19th-century skeletons of known ages from a churchyard in east London. When she tried to age the skeletons blind, she found the disparity between skeletal and dental age was often as great as that of the Turkana boy. One 10-year-old boy, for example, had a dental age of 9, the skeleton of a 6-year-old but was tall enough to be 11. “The Turkana kid still has a rounded skull, and needs a lot of growth to reach the adult shape,” Anton adds. Like apes, the face and skull of H. erectus changed shape significantly between youth and adulthood. Anton thinks that H. erectus had already developed modern human patterns of growth, with a late, if not quite so extreme, adolescent spurt. She believes Turkana boy was just about to enter it.

If she’s right, and small-brained H. erectus went through a teenage phase, that scuppers the orthodox idea linking late growth with development of a large brain. Anthropologist Steven Leigh from the University of Illinois at Urbana-Champaign is among those who are happy to move on. He believes the idea of adolescence as catch-up growth is naive; it does not explain why the growth rate increases so dramatically. He points out that many primates have growth spurts in particular body regions that are associated with reaching maturity, and this makes sense because by timing the short but crucial spells of maturation to coincide with the seasons when food is plentiful, they minimise the risk of being without adequate food supplies while growing. What makes humans unique is that the whole skeleton is involved. For Leigh, this is the key. Coordinated widespread growth, he says, is about reaching the right proportions to walk long distances efficiently. “It’s an adaptation for bipedalism,” he says.

According to Leigh’s theory, adolescence evolved as an integral part of efficient upright locomotion, as well as to accommodate more complex brains. Fossil evidence suggests that our ancestors took their first steps on two legs as long as six million years ago. If proficient walking was important for survival, perhaps the teenage growth spurt has very ancient origins. Leigh will not be drawn, arguing that there are too few remains of young hominids to draw definite conclusions. While many anthropologists will consider Leigh’s theory a step too far, he is not the only one with new ideas about the evolution of teenagers.

A very different theory has been put forward by Barry Bogin from the University of Michigan-Dearborn. He believes adolescence in our species is precisely timed to improve the success of the first reproductive effort. In girls, notes Bogin, full adult shape and features are achieved several years before they reach full fertility at around the age of 18. “The time between looking fertile and being fertile allows women to practise social, sexual and cultural activities associated with adulthood, with a low risk of having their own children,” says Bogin. When they finally do have children, they are better prepared to look after them. “As a result, firstborns of human mothers die much less often than firstborns of any other species.”

In boys, you see the opposite. They start producing viable sperm at 13 or 14 years of age, when they still look like boys. The final increase in muscle size that turns them into men does not happen until 17 or 18. In the interim boys, who feel like men, can practise male rivalries without being a threat to adult men or an attractive option to adult women. When boys do become sexually active, they have practised and are more likely to be successful without getting hurt.

Bogin’s theory makes totally different predictions to Leigh’s. If the timing of adolescence is related to uniquely human cultural practices, our species should be the first and only one to have a teenage phase. “H. erectus definitely did not have an adolescence,” he asserts. Such strong and opposing views make it all the more necessary to scour the fossil record for clues.

One approach, which has produced a surprising result, relies on the minute analysis of tooth growth. Every nine days or so the growing teeth of both apes and humans acquire ridges on their enamel surface. These perikymata are like rings in a tree trunk: the number of them tells you how long the crown of a tooth took to form. Across mammals, the speed of tooth development is closely related to how fast the brain grows, the age you mature and the age you die. Teeth are good indicators of life history because their growth is less related to the environment and nutrition than is the growth of the skeleton. Slower tooth growth is an indication that the whole of life history was slowing down, including age at maturity.

Back in the 1980s Christopher Dean, an anatomist at University College London, was the first to measure tooth growth in fossils using perikymata. He found that australopithecines dating from between 3 and 4 million years ago had tooth crowns that formed quickly. Like apes, their first molars erupted at 4 years old and the full set of teeth were in place by 12. Over the years, Dean’s team has collected enough teeth to show that H. erectus also had faster tooth growth than modern man, but not so fast as earlier hominids. “Things had moved on a bit,” he says. “They had their full set of teeth by about 15.” Modern humans reach this stage by about age 20. The change in H. erectus seems to imply that the growth pattern of modern humans was beginning to develop, with an extended childhood and possibly an adolescent growth spurt. Dean cautions, though, that the link between dental and skeletal development in ancestral hominids remains uncertain.

These findings could equally support Leigh’s or Bogin’s theories. A more decisive piece of evidence came last year, when researchers in France and Spain published their findings from an analysis of Neanderthal teeth. A previous study of a remarkably well-preserved skeleton of a Neanderthal youth, known as Le Moustier 1, from south-west France had suggested that, with a dental age of 15 and the frame of an 11-year-old, the kid was about to undergo an adolescent growth spurt. But the analysis of his perikymata reveals quite a different picture. Rather than continuing the trend towards slower development seen in H. erectus, Neanderthals had returned to much faster tooth growth (Nature, vol 428, p 936) and hence, possibly, a shorter childhood.

Does this mean they didn’t have an adolescence? Lead researcher Fernando Ramirez-Rozzi, of the French National Centre for Scientific Research (CNRS) in Paris thinks Neanderthals died young – about 25 years old—primarily because of the cold, harsh conditions they had to endure in glacial Europe. Under pressure from this high mortality, they evolved to grow up quicker than their immediate ancestors. “They probably reached maturity at about 15,” he says, “but it could have been even younger.” They would have matured too fast to accommodate an adolescent burst of growth. He points to research showing that populations of Atlantic cod have genetically changed to mature more quickly under the intense fishing pressure of the 1980s. Others contest Ramirez-Rozzi’s position. “You can’t assume, just because Neanderthals’ teeth grew faster, that their entire body developed faster,” says Jennifer Thompson of the University of Nevada, Las Vegas, one of the researchers involved in the Le Moustier 1 study.

Controversy rages, but these latest findings at least highlight one aspect of adolescence that most scientists can agree on. Whatever the immediate purpose of the late growth spurt, it was made possible by an increase in life expectancy. And that being so, one way to work out when the first teenagers originated is to look at the lifespan of a species. This is exactly what Rachel Caspari of the University of Michigan at Ann Arbor has been doing. Her most recent study, published in July 2004, shows an astonishing increase in longevity that separates modern Homo sapiens from all other hominids, including Neanderthals (Proceedings of the National Academy of Sciences, vol 101, p 10895). She categorised adult fossils as old or young by assessing whether they had as much wear on their last molar, or wisdom tooth, as on other molars. “In modern humans we see a massive increase in the number of people surviving to be grandparents,” she says. The watershed comes as recently as 30,000 years ago.

On this evidence, Neanderthals and H. erectus probably had to reach adulthood quickly, without delaying for an adolescent growth spurt. So it looks as though Bogin is correct — we are the original teenagers. Whether he is right about the purpose of adolescence is another matter. He admits we will never know for sure. “Fossils will never give us growth curves,” he says, “and we should not expect our ancestors to grow like we do.”

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