Fossil teeth flesh out ancient kids’ varied growth rates

X-ray technique sheds light on hominid development

Paranthropus robustus jaw and teeth

AGING DOWN  A Paranthropus robustus child, represented by this jaw and teeth, died at age 3.1, according to new evidence. The child was several months younger at death than previously estimated. P. robustus lived in what’s now South Africa between 2 million and 1.5 million years ago.

Tanya Smith

Kids who belonged to now-extinct species in the human evolutionary family grew at unexpected rates, unlike the growth of either present-day people or apes, a new study of their teeth finds. As a result, the researchers can now more accurately estimate ages at death for ancient hominid youngsters whose fossil teeth have been found.

Using X-ray technology to examine microscopic growth lines inside fossil teeth, a team led by Harvard University anthropologist Tanya Smith concludes that researchers can no longer use living apes as developmental yardsticks for ancient hominids. The timing of tooth formation and molar eruption, which denotes the speed of overall physical development, fluctuated from one now-extinct species to another, the researchers report February 18 in PLOS ONE.

“Great ape developmental standards do not necessarily yield accurate age of death estimates” for ancient hominid children, Smith says.

Recent studies have uncovered considerable variation in tooth development rates among modern humans and apes, so the new findings for ancient hominids make sense, says anatomist Christopher Dean of University College London, who was not part of Smith’s team. Dean led initial studies of hominid tooth growth in the 1980s, which analyzed enamel exposed by natural breaks in fossil teeth.

Smith’s group measured tooth formation of two children of the species Australopithecus anamensis, four Australopithecus africanus youngsters, 11 Paranthropus robustus kids, six children from an unidentified Homo species and two kids whose teeth are difficult to classify as either Homo or Australopithecus. Other than A. anamensis, a 4.2-million-year-old East African species, these ancient youngsters lived in what’s now South Africa between about 2 million and 1.5 million years ago.

BETWEEN THE LINES Scanning technology generated virtual views of a fossil tooth from an early Homo child, top. Scans peered inside the tooth to focus on growth lines in enamel, middle. A closer look documented eight daily cross-hatched enamel layers, in white brackets, between successive long developmental lines that form in teeth, denoted by white arrows. Smith et al/PLOS ONE 2015

One developmental marker the researchers focused on was a microscopic line that forms inside teeth at regular intervals during childhood as daily enamel layers accumulate. Knowing the timing of these intervals is essential for estimating the age at which tooth development was complete.

By counting daily enamel layers to establish the timing of those growth intervals,  the researchers found that members of P. robustus, part of a dead-end hominid line, developed the microscopic line every six to 12 days. That’s a range comparable to that of A. africanus and previous estimates for Neandertals, ancient Homo sapiens, and present-day people and gorillas. Previous studies of naturally broken fossil teeth had suggested that the timing of these growth intervals varied by two or three days at most among P. robustus and A. africanus individuals.

Compared to A. anamensis and A. africanus, P. robustus rapidly formed unusually thick tooth enamel and its molar teeth possibly erupted at later ages, although not as late as molar eruption occurs in present-day humans. Delayed molar growth in P. robustus represents an evolutionary shift “more in the direction of modern humans than of living apes,” remarks anthropologist Debbie Guatelli-Steinberg of Ohio State University in Columbus.

Curiously, one early Homo individual displayed extended tooth development similar to that of modern South Africans, while in a second early Homo individual, molar teeth grew at a faster, more chimplike pace. More early Homo teeth would let scientists explore this developmental disparity, Smith says.

New age-at-death estimates for 16 hominid youngsters, generated from measures of growth intervals in their teeth, differed from previous estimates by 0.2 to 1.1 years. A P. robustus child now placed at 3.1 years old was previously estimated using chimps as a developmental gauge to have died at 3.35 to 3.48 years old. An A. africanus kid is now estimated to have died at age 4.35, about a year later than previous estimates.

Reevaluations of age at death can now be conducted for other hominid children, including a child from 3.3 million years ago thought to have perished at age 3 (SN: 9/23/06, p. 195). Accurate age estimates for ancient hominid kids will help scientists untangle the evolution of prolonged human childhoods, Smith says.H. sapiens forms and erupts teeth more slowly than any now-extinct hominid did.

Bruce Bower has written about the behavioral sciences for Science News since 1984. He writes about psychology, anthropology, archaeology and mental health issues.

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