Teenagers (and their teeth) - then and now

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December 08, 2007

Teaser: Human offspring take more than twice the time to reach adulthood than do our closest living relatives, chimps and gorillas. This period of delayed maturation results in what we call the teenage years and is a characteristic of modern humans. Paleoanthropologists wonder how far back in the record of bipedal existence this delayed maturation commenced.

Image: /sites/default/files/thelittleiceage.jpg

Two papers, one in the journal Science the other in the Proceedings of the National Academy of Sciences, shed light on this phenomenon. They discuss the results of analyses on teeth and crania of two early relatives of our species, separated widely in both time and geography, and they reach intriguing conclusions.

Working on the pattern of dental wear and facial size displayed in an extensive collection of fossilized skulls and jawbones of Paranthropus robustus (from cave sites in South Africa and dated at between 1.5 and 2 million years ago), a team led by Charles Lockwood concluded there was pronounced sexual dimorphism (males larger than females) and that males matured more slowly than females. This pattern of extended male growth, termed bimaturism, is common in apes that possess a harem-like group structure, with a single, dominant adult male, many adult females, juveniles and infants. This implies that there would have been struggles among males for dominance and access to females. Lockwood and his colleague's work implies that Like some modern apes juvenile P. robustus males left their birth group to live on their own and that when older and larger, had to fight to gain a harem of their own. Science commentator Ann Gibbons says, "The males' facial features were on average 17% larger than females', suggesting a harem-like mating strategy. These robust australopithecines chose a risky mating strategy: Top males invested energy in bodybuilding in order to possess a harem of females, much like silverback gorillas do today."

It has been recognized for some time by paleoanthropologists that sexual dimorphism characterized several species comprising the geographically diverse genera of Australopithecus and Paranthropus. That is not new. Says William Kimbel, Science Director at the Institute of Human Origins, Arizona State University, " ..the importance of the paper is not as much in the finding of high dimorphism — this has been the conclusion of many studies for virtually all australopith species — but the inference of bimaturism and its implications for social structure."

Other scientists, working on a Neanderthal individual (Homo neanderthalensis) from Belgium, conclude that Neanderthal juveniles, both males and females, matured at a slightly faster rate than do we (Homo sapiens).

The author of an accompanying commentary, speaking of the lead researcher, says "She found, for example, that the second molar erupted a few years earlier in this 8-year-old Neandertal than in H. sapiens, suggesting that Neandertals grew up faster than we did."

The ability to trace the pace of an individual's growth from structures preserved within ancient teeth was the subject of a recent Becoming Human podcast, a link to which is found on this page, below, under the title "Fossil Teeth Speak."

Together these studies highlight the important lessons about growth and development among our earliest and even fairly recent extinct relatives that are available from often fragmentary fossil remains.

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