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Authors: Richard Leakey
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was erupting.
In the late 1980s, these assumptions were shattered by the work of several researchers. Holly Smith, an anthropologist at the University of Michigan, developed a way of deducing life-history patterns in fossil humans by correlating brain size and the age of eruption of the first molar. As a baseline, Smith amassed data for humans and apes; she then looked at a range of human fossils to determine how they compared. Three life-history patterns emerged: a modern human grade, in which first-molar eruption occurs at six years of age and life span is sixty-six years; an ape grade, with first-molar eruption at a little over three years and a life span of about forty years; and an intermediate grade. Later Homo erectus —that is, individuals who lived after about 800,000 years ago—fit the human grade, as did Neanderthals. All the australopithecine species, however, slotted into the ape grade. Early Homo erectus , like the Turkana boy, was intermediate: the boy’s first molar would have erupted when he was a little more than four and a half years old; had he not met an early death, he could have expected to live about fifty-two years.
Smith’s work showed that the australopithecines’ pattern of growth was not like that of modern humans; instead, it was apelike. She further showed that early Homo erectus was intermediate between modern human and ape in its growth: we now conclude that the Turkana boy was about nine years old when he died, and not eleven, as I’d initially supposed.
Because these conclusions were contrary to a generation of anthropologists’ assumptions, they were highly controversial. There was a possibility, of course, that Smith had made some kind of error. In these circumstances, corroborative work is always welcome, and in this case it came quickly. The anatomists Christopher Dean and Tim Bromage, both then at University College, London, devised a way of directly determining the age of teeth. Just as tree rings are used to calculate how old a tree is, microscopic lines on a tooth indicate its age. This method of calculation is not as easy as it sounds—not least because of some uncertainty about how the lines are formed. Nevertheless, Dean and Bromage initially applied their technique to an australopithecine jaw identical to the Taung child’s in terms of tooth development. They found that the individual had died at a little over three years of age, just as his first molar was erupting—right on cue for an apelike growth trajectory.
When Dean and Bromage surveyed a range of other fossil human teeth, they, like Smith, found three grades: modern human, ape, and something intermediate. Once again, the australopithecines were squarely in the ape grade, late Homo erectus and the Neanderthals were in the modern human grade, and early Homo erectus was intermediate. And once again the results stirred debate, particularly over whether australopithecines grew like humans or apes.
That debate was effectively ended when the anthropologist Glenn Conroy and the clinician Michael Vannier, at Washington University in St. Louis, brought high technology from the medical world into the anthropological laboratory. Using computerized axial tomography—the three-dimensional CAT scan—they peered into the interior of the Taung child’s petrified jaw and essentially confirmed Dean and Bromage’s conclusion. The Taung child had died when it was close to three years old, a youngster on an apelike trajectory of growth.
The ability to infer biology from fossils through research in life-history factors and tooth development is enormously important to anthropology, because it allows us metaphorically to put flesh on the bones. For instance, we can say that the Turkana boy would have been weaned a little before his fourth birthday and, had he lived, would have become sexually mature at about fourteen years old. His mother probably had her first baby when she was thirteen, after a nine-month pregnancy; and
In the late 1980s, these assumptions were shattered by the work of several researchers. Holly Smith, an anthropologist at the University of Michigan, developed a way of deducing life-history patterns in fossil humans by correlating brain size and the age of eruption of the first molar. As a baseline, Smith amassed data for humans and apes; she then looked at a range of human fossils to determine how they compared. Three life-history patterns emerged: a modern human grade, in which first-molar eruption occurs at six years of age and life span is sixty-six years; an ape grade, with first-molar eruption at a little over three years and a life span of about forty years; and an intermediate grade. Later Homo erectus —that is, individuals who lived after about 800,000 years ago—fit the human grade, as did Neanderthals. All the australopithecine species, however, slotted into the ape grade. Early Homo erectus , like the Turkana boy, was intermediate: the boy’s first molar would have erupted when he was a little more than four and a half years old; had he not met an early death, he could have expected to live about fifty-two years.
Smith’s work showed that the australopithecines’ pattern of growth was not like that of modern humans; instead, it was apelike. She further showed that early Homo erectus was intermediate between modern human and ape in its growth: we now conclude that the Turkana boy was about nine years old when he died, and not eleven, as I’d initially supposed.
Because these conclusions were contrary to a generation of anthropologists’ assumptions, they were highly controversial. There was a possibility, of course, that Smith had made some kind of error. In these circumstances, corroborative work is always welcome, and in this case it came quickly. The anatomists Christopher Dean and Tim Bromage, both then at University College, London, devised a way of directly determining the age of teeth. Just as tree rings are used to calculate how old a tree is, microscopic lines on a tooth indicate its age. This method of calculation is not as easy as it sounds—not least because of some uncertainty about how the lines are formed. Nevertheless, Dean and Bromage initially applied their technique to an australopithecine jaw identical to the Taung child’s in terms of tooth development. They found that the individual had died at a little over three years of age, just as his first molar was erupting—right on cue for an apelike growth trajectory.
When Dean and Bromage surveyed a range of other fossil human teeth, they, like Smith, found three grades: modern human, ape, and something intermediate. Once again, the australopithecines were squarely in the ape grade, late Homo erectus and the Neanderthals were in the modern human grade, and early Homo erectus was intermediate. And once again the results stirred debate, particularly over whether australopithecines grew like humans or apes.
That debate was effectively ended when the anthropologist Glenn Conroy and the clinician Michael Vannier, at Washington University in St. Louis, brought high technology from the medical world into the anthropological laboratory. Using computerized axial tomography—the three-dimensional CAT scan—they peered into the interior of the Taung child’s petrified jaw and essentially confirmed Dean and Bromage’s conclusion. The Taung child had died when it was close to three years old, a youngster on an apelike trajectory of growth.
The ability to infer biology from fossils through research in life-history factors and tooth development is enormously important to anthropology, because it allows us metaphorically to put flesh on the bones. For instance, we can say that the Turkana boy would have been weaned a little before his fourth birthday and, had he lived, would have become sexually mature at about fourteen years old. His mother probably had her first baby when she was thirteen, after a nine-month pregnancy; and
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