Abstract
Discovery of a new hominin (Homo naledi) in the same geographical area as Australopithecus africanus creates the opportunity to compare developmental dental stress in higher latitude hominins with low that in latitude apes, among whom repetitive linear enamel hypoplasia (rLEH) recurs seasonally at about 6 or 12 months. In contrast to equatorial Africa, a single rainy/dry cycle occurs annually in non-coastal southern Africa. It is predicted that LEH will recur annually but not differ in duration between ancient and more recent hominins. Data were collected from epoxy casts of anterior teeth attributed to H. naledi (18 incisors, 13 canines) and A. africanus (29 incisors, 8 canines) using a digital microscope, surface scanner and scanning electron microscope. The location, number, width, depth and distance between defects (including perikymata counts and spatial measurements) of 136 LEH events were compared among crown moieties (deciles 4–6 and 7–9), tooth types and taxa. Enamel defects are concentrated in the cervical half of anterior crowns, and in similar numbers in each taxon. Contrary to expectations, H. naledi show bimodal LEH durations reconstructed at about 2 and 8 weeks compared to just 4 weeks in A. africanus. Both taxa show bimodally recurrent episodes of LEH centring on 2 and, more commonly and severely, 6 months. A combination of two independent annual stressor types, one disease and one seasonal, could explain the observations. These estimations of duration and recurrence of developmental stress require evaluation using actual perikymata periodicity for H. naledi and more refined understanding of palaeoenvironments for both taxa.
 Significance: 
 
 Seasonal stress is a central concern in the biological and health sciences. Because of the innate way that enamel is deposited, the timing of stress in the childhood of apes, modern humans and their fossil ancestors can be measured with a precision of about 1 week.
 Application of this method to South African Pliocene Australopithecus africanus and Mid-Pleistocene Homo naledi reveals that, unexpectedly, both forms show semi-annual stress – a finding that is tentatively attributed to two independent annual stressors, possibly disease and malnutrition.
Highlights
Support for the inference of independence can be found in the observation that exactly 50% of long interval recurrences (13/26) link with each of short and long duration LEH events
The discovery of H. naledi in the same geographical area as A. africanus has created an opportunity to evaluate the temporal patterning of developmental dental stress in higher latitude hominins with low latitude apes among whom LEH tends to recur on average at intervals of 6 months, or multiples thereof, linked it is thought to moisture cycles influencing the likelihood of disease and/or malnutrition
H. naledi shows bimodal durations of stress centred on 2 and 8 weeks while A. africanus shows unimodal duration of stress centred on 4 weeks
Summary
The great attraction of enamel hypoplasia studies in biological anthropology is that comparative studies of developmental well-being can include modern, ancient and even fossil assemblages.[1,2,3,4] Previous studies of enamel hypoplasia in single teeth from large apes (gorillas, chimpanzees and orangutans) found that, generally speaking, several enamel furrows are often observable, especially on the canine teeth, representing a time span of ‘felt stress’ of 5 or more years[5], and that the average interval between successive furrows is in the order of 6 months or multiples thereof; i.e. 12 or 18 months.[6,7,8,9] Both western African apes (chimpanzees and lowland gorillas from Cameroon) and orangutans from Indonesia live in low latitude contexts, where commonly there are alternating twice yearly rainy and dry seasons driven by semi-annual passage of the inter-tropical convergence zone in Africa and twice yearly moisture-carrying monsoonal winds passing over the islands of Borneo and Sumatra.[6,7] By contrast, chimpanzees from Senegal, who experience only one wet season alternating with one very long dry season, show a reconstructed average interval between repetitive episodes of enamel hypoplasia of just under a year.[9,10]The assertion that repetitive linear enamel hypoplasia (rLEH) shows an average recurrence of 6 or 12 months, is not without challengers[11,12], but see Smith et al.[13]. In central southern Africa, of a relatively warm, wet austral summer and cold, dry austral winter (i.e. only a single moisture cycle annually)[14,15,16], it is predicted that South African hominins will show rLEH with an average recurrence of about 12 months (null hypothesis). Rejection of this hypothesis will weaken any direct connection between rLEH and seasonality
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
