Older age becomes common late in human evolution

Abstract

Increased longevity, expressed as number of individuals surviving to older adulthood, represents one of the ways the human life history pattern differs from other primates. We believe it is a critical demographic factor in the development of human culture. Here, we examine when changes in longevity occurred by assessing the ratio of older to younger adults in four hominid dental samples from successive time periods, and by determining the significance of differences in these ratios. Younger and older adult status is assessed by wear seriation of each sample. Whereas there is significant increased longevity between all groups, indicating a trend of increased adult survivorship over the course of human evolution, there is a dramatic increase in longevity in the modern humans of the Early Upper Paleolithic. We believe that this great increase contributed to population expansions and cultural innovations associated with modernity.

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.

Fig. 2.

Probability of finding observed OY ratios for three hominid groups in distributions of OY ratios generated from hominids from an earlier time period by resampling. Each OY ratio is created by randomly drawing an adult sample of the same size as the test (later) group from the earlier one and calculating the OY ratio in it (sample sizes are shown in Table 1). This procedure was performed 10,000 times in each case, and the generated samples describe the probability of observing the OY ratio from the test (later) sample in the earlier one. For example, each one of the 10,000 OY ratios in A is a random sample of 208 australopithecine adults, because the actual Homo sample size is 208. (A) The observed early Homo OY ratio (vertical bar at 0.25) compared with ratios generated from the australopithecine sample. The observed Homo ratio was never reproduced in 10,000 OY ratios in draws of 208 generated from the australopithecine sample. (B) Observed Neandertal OY ratio (0.39) compared with ratios generated from the Early and Middle Pleistocene Homo sample in 10,000 draws of 133. OY ratios of 0.39 and larger occurred in 2.26% of the generated distribution. (C) The observed Early Upper Paleolithic OY ratio (2.08) compared with ratios generated from the Neandertal sample in 10,000 draws of 74. The observed ratio is far outside this distribution, representing a major increase in the number of adults over the age of 30 in the death distribution

Fig. 3.

The effect of mortuary practices on the increase in Early Upper Paleolithic longevity that is shown in Fig. 2C. Although the OY ratio of Early Upper Paleolithic burials is slightly higher than that of nonburials, both are significantly larger than the largest OY ratios generated from the Neandertal sample. (A) The observed OY ratio of Early Upper Paleolithic nonburials (2.06), compared with the ratios generated from the Neandertal sample in 10,000 draws of 49. (B) The observed OY ratio of Early Upper Paleolithic burials (2.13), compared with the ratios generated from the Neandertal sample in 10,000 draws of 25.