Fig. 1.

Three australopithecine dentitions of different ages shown to the same approximate size: MLD 2 (Right), with a deciduous molar, permanent M1, and unerupted M2; SK 34 (Center), a younger adult; and STS 36 (Left), an older adult. We use these dentitions to illustrate how specimens were placed into younger and older adult categories. M3 eruption was considered to indicate the age of reproductive maturation, and older adults were defined as twice the age of reproductive maturation, the age at which one could theoretically first become a grandparent. As shown, the wear on the M3 of SK 34 is comparable with that on the M1 of MLD 2, indicating ≈5 years of wear, by using the human model discussed in the text. This finding would indicate an age of ≈20 years (15 plus 5), within our younger adult category. Also as indicated, the M3 of STS 36 exhibits more wear than the M1 of SK 34 (i.e., >14 years), indicating a probable age of >30 years, within our older-adult category. The use of different eruption schedules produces the same categorical assessment. If the australopithecine molars in this illustration erupted at 3, 7, and 11 years of age for M1, -2, and -3, respectively, to use one chimpanzee model, the wear would represent less time (≈3 years of wear on the M1 of MLD2), but adulthood and older adulthood would begin at ages 11 and 22, respectively. SK 34 would remain in the young adult category (11 plus 3), and STS 36 would remain in the older adult category (>11 years of wear on the M3). See text for further details.