Growing up slowly 160,000 years ago

For centuries, it has been recognized that, in the absence of a birth certificate, the best way to estimate the chronological age of a growing child is from his/her teeth. Some of the earliest studies of dental development were carried out to document the variability of tooth eruption with respect to chronological age at a time when, for example in England, the Factory Act of 1833 made it legal for children aged 9–13 years to work a 9-h day in textile mills. Data for dental eruption in 1837 showed that between their eighth and ninth birthdays, 524 of 530 children had both lower incisors erupted (1). Since that time, the trend to grow to greater heights and weights has increased dramatically in line with better nutritional and social conditions. However, over the same period, there has been little change in the timing of tooth eruption (2). In this issue of PNAS, Smith et al. (3) use novel techniques to estimate the chronological age of an ancient modern human from Jebel Irhoud, Morocco, dated to 160,000 years before the present (ybp). They show that the age of lower incisor tooth eruption was much the same then as it is today.

The very first modern humans that were anatomically indistinguishable from ourselves appear ≈200,000 ybp, but Jebel Irhoud 3, described by Smith et al. (3), is one of the earliest known anatomically modern human juveniles. Earlier fossil hominins from the Plio-Pleistocene had shorter periods of growth and development, more similar to living great apes than to modern humans, and for a long time, the hunt has been on to determine exactly when a major shift in life-history strategy occurred. The trend has always been to assume that this shift was earlier rather than later in our evolutionary history, but increasingly the evidence …

*E-mail: ucgacrd{at}ucl.ac.uk