Evolution’s Youth Movement

Anthropologists usually don’t find the skeletons of long-dead toddlers when digging into ancient ground. But at Syria’s Dederiyah Cave, they did just that in 1993 and again in 1997.

Excavations at this approximately 60,000-year-old site yielded the partial remains of one small child and the nearly complete skeleton of another. Researchers estimate that both died at around age 2.

The Dederiyah youngsters are part of a growing contingent of fossil kids attracting scientific interest. Fossils of children had previously been treated more as oddities than as beacons of evolutionary insight. Now, however, these ancient youths are revealing aspects of human evolution that had evaded scientists studying fossil elders from various Homo species, the first of which appeared around 2.5 million years ago.

In particular, infant and juvenile fossils hold clues to a critical issue: the evolution of distinctive patterns of growth in modern Homo sapiens and our immediate ancestors. Understanding individual development may for the first time delineate pivotal differences among Neandertals and several Homo species.

Developmental stages

Scientists agree that people grow to adulthood over a longer period and across more developmental stages than apes and monkeys do. A consensus also holds that australopithecines, members of an early genus in the human evolutionary family that lived from around 5 million to 2 million years ago, followed a developmental pattern closer to that of modern apes than that of humans.

Yet relatively little is known about the evolution of individual growth patterns in Neandertals and other members of the Homo genus. Research into growth and development has long taken a back seat to detailed analyses of skeletal traits of adult Homo fossils.

Adult skulls look enough alike to form a hazy continuum among specimens assigned to modern, early, or archaic H. sapiens or to other Homo species. Without clear dividing lines among skulls and teeth, the most abundant fossils, some researchers identify a dozen or more Homo species, while other scientists propose that only anatomically diverse forms of H. sapiens have roamed the Earth during the past 2 million years or so.

Recent discoveries in developmental biology have begun to influence this debate. For instance, in animals ranging from sea urchins to mice, early development may proceed differently for closely related species that end up looking much the same.

Development may also take unexpected turns for members of a species exposed to minor changes in diet or social organization, with dramatic consequences for body shape. Finally, genetically mediated shape changes in certain parts of the skeleton may trigger developmental processes that lead to extensive remodeling elsewhere (SN: 11/25/00, p. 346).

These findings indicate that anthropologists’ small but growing collection of fossil infants, children, and teenagers may hold untapped clues to humanity’s origins, says Steven R. Leigh of the University of Illinois at Urbana-Champaign. Leigh and other like-minded anthropologists presented their latest findings at the annual meeting of the American Association of Physical Anthropologists, held in Kansas City in March.

On the other hand, the twists and turns of individual development can sometimes hide more than they reveal about evolution, Leigh notes. It’s time however, to drop the assumption that evolutionary forces primarily affect the shape and function of adults’ bodies rather than how youngsters grow, he contends.

“There’s a fair amount of chaos right now regarding how to look at individual development and ancestral patterns of growth,” Leigh remarks. It’s clear that developmental patterns evolve much more rapidly and profoundly than has often been assumed, he says.

Growth patterns

Several research milestones have inspired current explorations of growth and development in fossil species, said Barry Bogin of the University of Michigan-Dearborn at the March meeting. In 1917, D’Arcy Thompson used mathematical models to show that different patterns of growth from a common fetal form could produce the contrasting skull shapes of adult chimpanzees and humans. In 1924, Adolph Schultz used eruption times of permanent teeth to mark off three primate life stages: infantile, juvenile, and adult.

Two decades later, Samuel Brody showed that a juvenile phase occurs in people and chimps, but not in cattle and other farm animals. Since then, researchers examining teeth and other characteristics have identified a juvenile stage in wolves, elephants, whales, and other highly social mammals.

Another research milestone was reached in 1975, when researchers began to study tooth growth in australopithecines. The closest parallel today to the pattern of dental development of these ancient, two-legged creatures is not in people but in common chimpanzees.

Since then, the scientific focus has shifted to Homo species. By examining teeth and other body parts of living populations, Bogin identifies five human growth stages after birth: infant, child, juvenile, adolescent, and adult. Childhood and adolescence don’t appear in any other living species, he asserts.

Several researchers have explored the possibility that Homo erectus individuals also passed through childhood and adolescence. This potential ancestor of H. sapiens lived from around 1.6 million to perhaps 100,000 years ago.

H. erectus fossils, such as a nearly complete skeleton of a solidly built boy in his early teens, indeed reflect an extended childhood growth period, says Susan Antón of Rutgers University in New Brunswick, N.J.

Earlier studies had disagreed on whether H. erectus experienced a cardinal feature of human adolescence, the sudden growth spurt. Antón’s latest investigation indicates a teenage growth surge in H. erectus slightly smaller than that in H. sapiens. Jaw and facial heights of H. sapiens typically increase 15 to 20 percent during their teens. Antón’s analysis of seven H. erectus fossils finds that during the teen years, H. erectus increases 80 to 90 percent as much. However, this statistical finding remains tentative.

“We need to use caution in talking about an adolescent growth spurt for Homo erectus and wait for more fossil finds,” she says.

H. erectus also plays into a long-running debate over whether modern humans have evolved so that adults, at least from the neck up, now look like juveniles in ancestral Homo species did. Scientists refer to this phenomenon as neoteny.

Preliminary data indicate that the rounded cranial shape of adult H. sapiens resembles that of juvenile H. erectus, according to Leigh. He compared measurements of 70 modern human skulls from people of various ages with those of two adult and one juvenile H. erectus skulls.

Neoteny in the skull may have been an evolutionary compromise, Leigh theorizes. This process made room for the larger brains in H. sapiens without remodeling many facial and cranial traits from H. erectus, in his view.

This brain growth may have sparked changes in diet and social life that enabled humans to exploit diverse habitats, Leigh suggests. For instance, hunting techniques may have changed to provide more meat to pregnant and nursing mothers of large-brained babies.

H. erectus apparently had a narrower repertoire of behaviors, Leigh adds. This fossil species exhibits cranial evidence of slower brain growth than that found in modern humans. It also shows marked changes in brain shape during development, Leigh says. The rounded brain cases of H. erectus youngsters become longer and narrower in adults.

Excavations in Spain

Intriguing developmental evidence about two other ancient Homo species comes from excavations in Spain’s Atapuerca Mountains.

As early as 800,000 years ago, a Homo species living in this region displayed a prolonged pattern of tooth growth, as seen in modern humans, says Jose M. Bermudez de Castro of the National Museum of Natural Sciences in Madrid (SN: 4/3/99, p. 212). Bermudez de Castro and his coworkers unearthed nearly all the teeth for three children. The team assigns these ancient fossils to a new species, Homo antecessor. Other researchers, however, regard the finds as representing an undetermined Homo species.

Bermudez de Castro’s group also excavated teeth from a child at another, approximately 300,000-year-old Atapuerca site. This child comes from a collection of fossils that Bermudez de Castro assigns to Homo heidelbergensis, a species first discovered in Germany.

For each of the four Atapuerca fossil children, the Spanish team noted the maturity of teeth at the front and the back of the mouth. These areas follow different patterns of dental growth. The researchers then compared the data with patterns of dental maturity in modern apes and people, as well as australopithecines.

“Both Homo antecessor and Homo heidelbergensis shared with modern humans a prolonged pattern of dental maturation,” Bermudez de Castro says. This suggests that both species were direct ancestors of H. sapiens, in his opinion.

Expanded tooth development of this magnitude indicates that the life histories of these ancient species included an adolescent phase, he adds.

Elsewhere in their bodies, Neandertal kids also show signs of having grown in a humanlike way. These observations give ammunition to those who suggest that, in essence, Neandertals were us, rather than a separate species.

Preliminary comparisons indicate that Neandertal and prehistoric H. sapiens children have similarly shaped hipbones, report Tona Majo and Anne-Marie Tillier, both of the University of Bordeaux in France. Moreover, their fossil hipbones look much like those of today’s children. Where differences from the modern human bone occur, they’re shared by Neandertals and ancient H. sapiens, the researchers say

Majo and Tillier examined pelvic remains of three Neandertal children from La Ferrassie, a roughly 70,000-year-old French site, and of three H. sapiens youngsters from Qafzeh, a 90,000-year-old Israeli site. Fossil individuals ranged from infancy to age 6 at the time of their deaths.

The length and thickness of the Dederiyah toddlers’ leg bones also fall within the range of the H. sapiens, says Osamu Kondo of the University of Tokyo. Kondo and his coworkers had excavated these Neandertal youngsters’ remains.

Finally, the upper arm and upper leg bones of a Neandertal baby found in Israel’s Amud Cave appear nearly human, says Hartley Odwak of University College London. He compared computed tomography scans of the Amud limb bones and those of modern infants’ bones. Like the Dederiyah toddlers, the infant’s bones show no evidence of having been as thick, relative to body size, as those of adult Neandertals.

Skull growth

Only one or two pivotal changes in skull development may distinguish modern H. sapiens from closely related predecessors such as the Neandertals, suggests Daniel E. Lieberman of George Washington University in Washington, D.C. Even if development looks similar in Neandertals and modern humans, small but critical changes could still mark them as separate species, he argues.

Lieberman analyzed data on skull growth from infancy to adulthood in 12 Denver-area residents. A defining feature of human anatomy is that the floor of the skull flexes up near the front as a child grows, Lieberman says. This opens up the vocal tract and enables people to make a variety of speech sounds. As the skull flexes, the cranium becomes rounder and the face shortens and retracts until it’s directly under the brain case.

Flatter skull bases and brain cases combine with longer, more protruding faces in adult Neandertal and archaic H. sapiens fossils, he notes.

A preliminary comparison of skulls from Neandertal and human children, conducted by Gail E. Krovitz of George Washington University, finds that the shape differences described by Lieberman appear by age 3.

Subtle, genetically mediated shifts in the development of the skull base and perhaps the cranium may have been enough to trigger the evolution of modern H. sapiens from its most recent ancestors, such as Neandertals, Lieberman and Krovitz theorize.

While their scenario is possible, comparisons of fossil youngsters with modern children are difficult to make with any precision, according to Bogin. As recently as 15,000 years ago, the hunting and gathering way of life sparked developmental processes that yielded adults of markedly larger stature and with denser bones than observed in people today, he says.

“Lifestyle greatly shapes how people grow and develop,” Bogin says. “We need to know a lot more about the lifestyles of creatures such as Neandertals and Homo erectus before we can understand how they developed and compare them to us.”

Another problem, says Leigh, is that developmental differences between species sometimes hide more than they reveal about evolutionary relationships. For instance, Leigh finds that within the papionin family of monkeys, only a baboon undergoes the bulk of its brain growth early in life. Mangabeys, which display many genetic similarities to baboons, grow their brains at a slower pace and bear little resemblance to baboons as adults.

Yet adult mandrills, which possess fewer genetic similarities to baboons than mangabeys do, have an extended muzzle and a relatively large brain case that gives them a baboonlike look. This convergence of skull shapes occurs because mandrills, after a slow period of mangabeylike brain growth, experience rapid brain expansion before entering adulthood, Leigh says.

The moral of this tangled developmental tale is a cautionary one: Give fossil kids due respect when investigating human origins.

Just don’t expect them to blurt out any precocious evolutionary insights.