The Birth of Childhood This Pattern of Growth Evolved
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NEWSFOCUS on November 14, 2008 Given that we are unique among mam- mals, researchers have been probing how The Birth of Childhood this pattern of growth evolved. They have long scrutinized the few, fragile skulls and Unlike other apes, humans depend on their parents for a long period skeletons of ancient children and have now after weaning. But when—and why—did our long childhood evolve? developed an arsenal of tools to better gauge how childhood has changed over the www.sciencemag.org Mel was just 3.5 years old when his mother children from Kathmandu to Rio de Janeiro past 3 million years. Researchers are died of pneumonia in 1987 in Tanzania. He do not survive on their own unless they are at scanning skulls and teeth of every known had still been nursing and had no siblings, least 6. “There’s no society where children juvenile with electron microscopes, so his prospects were grim. He begged can feed themselves after weaning,” says micro–computed tomography scans, or weakly for meat, and although adults gave anthropologist Kristen Hawkes of the Uni- powerful synchrotron x-rays and applying him scraps, only a 12-year-old named versity of Utah in Salt Lake City. By con- state-of-the-art methods to create three- Spindle shared his food regularly, protected trast, “chimpanzees don’t have childhoods. dimensional virtual reconstructions of the him, and let him sleep with him at night. They are independent soon after weaning,” skulls of infants and the pelvises of mothers. Downloaded from When Spindle took off for a says anthropologist Barry Bogin They’re analyzing life histories in traditional month, another adolescent, Pax, of Loughborough University in cultures to help understand the advantages came to Mel’s rescue, giving Online Leicestershire, U.K. of the human condition. In addition, some sciencemag.org him fruit and a place to sleep Humans are also the only ani- new fossils are appearing. On page 1089 of Hear more until Spindle returned. Mel sur- about childhood’s mals that stretch out the teenage this issue, researchers report the first nearly vived to age 10. beginning in a podcast years, having a final growth complete pelvis of a female Homo erectus, Fortunately for Mel, he was with author Ann Gibbons. spurt and delaying reproduction which offers clues to the prenatal growth of an orphan chimpanzee living in until about 6 years after puberty. this key human species. the Gombe Stream National Park rather On average, women’s first babies arrive at All of this is creating some surprises. than a small child living in the slums of a age 19, with a worldwide peak of first babies One direct human ancestor, whose skeleton big city. With only sporadic care from older at age 22.5. This lengthy period of develop- looks much like our own, turns out to have children, a 3-year-old human orphan would ment—comprised of infancy, juvenile years, grown up much faster than we do. The life not have survived. and adolescence—is a hallmark of the human histories of our closest evolutionary Mel’s story illustrates the uniqueness of condition; researchers have known since the cousins, the Neandertals, remain controver- one facet of human life: Unlike our close 1930s that we take twice as long as chim- sial, but some researchers suspect that they cousins the chimpanzees, we have a pro- panzees to reach adulthood. Even though we may have had the longest childhoods of all. longed period of development after wean- are only a bit bigger than chimpanzees, we The new lines of evidence are helping ing, when children depend on their parents mature and reproduce a decade later and live researchers close in on the time when child- to feed them, until at least age 6 or 7. Street 2 to 3 decades longer, says Bogin. hood began to lengthen. “Evidence suggests GEOGRAPHIC; M. NEWMAN/LPI/GETTY IMAGES; PHOTOS.COM; BRIAN WOOD CREDITS (LEFT TO RIGHT): SARAH LEEN AND BECKY HALE/NATIONAL 1040 14 NOVEMBER 2008 VOL 322 SCIENCE www.sciencemag.org Published by AAAS NEWSFOCUS Changing face of childhood. Childhood has more than doubled in length in modern humans as com- Female age at pared to chimpanzees and the Dikika baby australop- first breeding ithecine (reconstructed in lower left). Delaying child- Childhood Age at (years) Average Age at birth allows for bigger, stronger mothers who can give eruption of (estimated by maximum weaning first molar 3rd molar eruption life span birth more frequently, as seen for example in tradi- Stages (years) (years) in fossils) (years) tional hunter-gatherer societies (upper right). Chimpanzees, Pan troglodytes 4.0 4.0 11.5 45 that much of what makes our life history Lucy, Australopithecus afarensis 4.0? 4.0? 11.5 45 unique took shape during the evolution of the genus Homo and not before,” says Homo erectus ? 4.5 14.5 (est.) 60? (est.) anthropologist Holly Smith of the Univer- Modern humans, Homo sapiens 2.5 6.0 19.3 70 sity of Michigan, Ann Arbor. Milestones. Key events show that modern humans live slower and die later than our ancestors did. Live fast, die young Back in 1925, Australian anatomist Raymond immune from those risks, given that there are tors to share many key elements of the mod- Dart announced the discovery of that rarest of 6.6 billion of us on the planet. “When did we ern human body plan, with a brain consider- rare specimens, the skull of an early hominin escape those constraints? When did we ably larger than that of earlier hominins. And child. Dart estimated that the australop- extend our childhood?” asks biological unlike the petite australopithecines, this ithecine he called the Taung baby had been anthropologist Steven Leigh of the University Turkana youth was big: He weighed 50 kilo- about 6 years old when it died about 2 million of Illinois, Urbana-Champaign. grams, stood 163 centimeters tall, and years ago, because its first permanent molar The Taung baby and the other australop- looked like he was 13 years old, based on had erupted. As modern parents know, the ithecine children, including the relatively modern human standards. Yet two independ- first of the baby teeth fall out and the first per- recent discovery of a stunning fossil of a ent tooth studies suggested ages from 8 or manent molars appear at about age 6. Dart 3-year-old Australopithecus afarensis girl 9 to 10.5 years old. assumed that early hominins—the group from Dikika, Ethiopia, show that it happened Now a fresh look at the skeleton con- on November 14, 2008 made up of humans and our ancestors but not after the australopithecines. So researchers cludes that, despite the boy’s size, he was other apes—matured on much the same have zeroed in on early Homo, which closer to 8 years old when he died. Dean and schedule as we do, an assumption held for appeared in Africa about 2 million years ago. Smith make this case in a paper in press in an 60 years. Growing up slowly was seen as a Unfortunately, there are only a few jaw edited volume, The First Humans: Origin of defining character of the human lineage. the Genus Homo. The skeleton and tooth Then in 1984, anatomists Christopher microstructure of the boy and new data on Dean and Timothy Bromage tested a new other members of his species suggest that he method to calculate the chronological ages attained more of his adult height and mass www.sciencemag.org of fossil children in a lab at University Col- earlier than modern human children do. lege London (UCL). Just as botanists add up Today, “you won’t find an 8-year-old boy tree rings to calculate the age of a tree, they with body weight, height, and skeletal age counted microscopic lines on the surface of that are so much older,” says Dean. teeth that are laid down weekly as humans He and Smith concluded that the boy did grow. The pair counted the lines on teeth of not experience a “long, slow period of australopithecine children about as mature growth” after he was weaned but grew up ear- as the Taung child and were confounded: lier, more like a chimpanzee. They estimate Downloaded from These hominin children were only about the species’ age at first reproduction at about 3.5 years old rather than 6. They seemed to 14.5, based on the eruption of its third molar, be closer to the chimpanzee pattern, in which in both humans and chimpanzees which the first permanent molar erupts at erupts at about the age they first reproduce. about age 3.5. “We concluded that [the aus- This 8-year-old Turkana Boy was probably tralopithecines] were more like living great more independent than a 13-year-old modern apes in their pace of development than mod- human, the researchers say, suggesting that ern humans,” says Dean. H. erectus families were quite different from Their report in Nature in 1985 shook the ours and did not stay together as long. field and focused researchers on the key The new, remarkably complete female questions of when and why our ancestors Big for his age. The 8-year-old Turkana Boy, recon- pelvis described in this issue, however, sug- adopted the risky strategy of delaying repro- structed here, grew up faster than modern humans do. gests that life history changes had begun in duction. Many other slow-growing, large- H. erectus. Researchers led by Sileshi bodied animals, such as rhinos, elephants, bits of early Homo infants and young chil- Semaw of the Stone Age Institute at Indiana and chimpanzees, are now threatened with dren to nail down their ages.