Evolution's Youth Movement

Article excerpt

Fossil children may harbor clues to humanity's origins

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.

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.

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. …