New bite to fossil analysis?

Evidence suggesting that early ancestors of modern humans grew in a distinctively nonhuman way--characterized by a short period of intensive growth much like that observed in modern great apes--has come, literally, from the mouths of babes. When examined with an electron scanning microscope, the tooth enamel of 3.5-million- to 1.5-million-year-old infants also indicates that they were significantly younger at the time of their deaths than has previously been estimated, say anthropologist Timothy G. Bromage and anatomist C. Christopher Dean of University College in London, England.

Other scientists are also finding signs that human ancestors who lived over 1.5 million years ago were more apelike than humanlike, notes Bromage, "but there has not been a means of demonstrating growth and developmental stages directly until now."

However, some researchers involved in the study of human origins, an endeavor that often provokes heated debate, contend that too little is known about growth patterns in the enamel of modern humans and apes to make comparisons with fossil teeth.

Bromage and Dean compared electron microscope data for 10 unworn teeth from modern-day youngsters with dental impressions of nine fossil teeth from Africa. The fossils include teeth from the Australopithecines, the earliest known humanlike creatures, and early Homo, a precursor of modern Homo sapiens. The researchers estimated the fossils' ages at the time of death by counting the coarse lines separating enamel layers on the tooth surface. Studies of teeth from modern humans and other mammals indicate, they say, that each layer of enamel forms over an average of seven to eight days. Youngsters' teeth must be analyzed, the researchers explain, because the lines separating enamel layers wear off with age.

Their calculations show that the creatures represented by the fossil teeth were markedly younger than previous estimates of 4-1/2 to 7 years old, based on tooth eruption, maturation and wear. Revised ages range from about 3 to 5 years old at the time of death. Furthermore, growth occurs over a shorter time span in the fossil teeth than in modern human teeth, report the investigators in the Oct. 10 NATURE; fossil tooth growth is similar to that observed in modern great apes.

"What appears to be going on is mosaic evolution," Bromage observes. "Australopithecines walked on two legs and early Homo had a slightly larger cranial capacity, but this didn't necessarily correlate with prolonged growth and development. But I'd bet Homo erectus [which appeared about 1.5 million years ago and had a much larger brain than its predecessors] will show longer, more modern growth periods in its enamel." The teeth of young Homo erectus individuals are hard to come by, says Bromage, but preliminary data from Neanderthals, which lived around 125,00 years ago, indicate that their enamel growth was similar to that of modern humans.

Also hard to come by is consensus on the value of analyzing enamel. "This is not a new idea," says anthropologist Alan Mann of the University of Pennsylvania in Philadelphia, "But there is a lot of evidence that enamel formation is too variable [between individuals] to be of any use in estimating age." Mann has analyzed early human fossil teeth along more traditional lines and concludes that signs of growth are more humanlike than apelike.

Adds anthropologist Fred Grine of the State University of New York at Stony Brook, "There is no good evidence that enamel layers are formed on a daily basis and no solid evidence [on enamel formation] from modern populations of humans and apes to compare to fossils."

Alan Boyde, a pioneer of enamel analysis with electron scanning microscopes, says his studies of modern human teeth support Bromage and Dean's findings. "Their data is unquestionably better than anything that has gone before," says Boyde, also of University College. "They can't be far wrong."