Cumulative cultural evolution in humans implies a life-history schedule that facilitates the early weaning, slow growth, and delayed onset of reproduction underlying the transmission of cognitively complex skills and the development of extensive networks of cooperation. These life-history benchmarks, which are correlated with dental eruption patterns, can be studied in fossils through the age at emergence of the first molar. Project 8 addresses our poor knowledge of the timing of first molar emergence in our closest living relative, the chimpanzee, which is the most appropriate yardstick for assessing the trajectory of human life-history evolution.
This project aims to identify the specific ecological context that drives alterations in growth and life history, as read through the data on molar emergence. It will help us understand how and why hominins made the important transition towards a prolonged juvenile growth period and delayed first reproduction, that underpin the complex cognitive and social learning skills that are fundamental attributes of human uniqueness.
Detailed Project Description
Uniquely expanded levels of human social cooperation resulted from a synergy between selection for our exceptionally large brains and a complex social fabric. In particular, the evolution of early weaning, slow growth, and a delayed onset of reproduction resulted in modern human offspring that are highly dependent on parents and their social group for a lengthy childhood. This prolonged dependency created opportunities for the transmission of complex social, cognitive, and behavioral skills that are necessary for survival and reproductive success.
These aspects of growth are tightly correlated with the pace of dental development, providing a probe to reconstruct the pace of life periods in our hominin ancestors. These same aspects of growth are also sensitive to variation in local demographic and ecological conditions, such as resource availability, mortality profiles, and fertility schedules. A detailed understanding of how variable demography/ecology may manifest as alterations in the pace of dental development will shed substantial light on when during the course of human evolution the unique pattern of modern human growth and development first appeared—which can provide clues about when uniquely human patterns of social learning and cooperation first arose.
Currently, the only means of probing life history in the human fossil record is by exploiting the link between dental development and overall maturation rates. How reliable this relationship is in modern humans and our closest living relatives, the great apes, is not entirely clear. This project will establish the precise linkage among rates of body/brain growth, dental development, and the overall pace of life and in so doing, refine our methodology for interpreting the evolution of growth throughout the human evolutionary story.
Mammalian life-history profiles lie along a spectrum between two end points referred to as “live fast, die young” and “live slow, die old.” The modern-human life history profile is unique, as it incorporates elements of both “fast” and “slow” schedules: highly dependent offspring, slow maturation, protracted childhood, and long lifespan suggest a “live slow” strategy, whereas a relatively short gestation, early weaning, and short interbirth interval with overlapping offspring suggest a “live fast” schedule. As the pace of dental development—in particular, the age of first molar (M1) emergence—is the hard-tissue feature most tightly correlated with a species’ life-history profile, much of the evidence for reconstructing early hominin life-history comes from estimating the timing of this important dental developmental event.
Much is known about the variation in the timing of molar emergence and in select life-history variables in modern human populations and information is gradually accumulating on variation in great ape life histories. However, effectively nothing is known about variation in M1 emergence ages among species or populations of wild great apes, particularly for our closest living relative, chimpanzees (Pan troglodytes). Life history is a local phenomenon, mediated by complex interactions among extrinsic and intrinsic factors, including resource availability, diet, birth rates, and risk of death. Understanding how sometimes subtle changes in these aspects of ecology and demography produce shifts in life history among closely related populations, and the extent to which these differences are manifest in the timing of dental developmental events such as molar emergence, is a crucial first step for refining our reconstructions of the evolution of modern-human life history from the fossil hominin record.
Our goal in this proposal is to evaluate the extent of population-level variation in molar emergence ages across free-ranging chimpanzee populations. We will compare these estimates with a growing database of ecological, demographic, reproductive, and behavioral variation across great ape species.
What we know about ages at molar emergence in chimpanzees is based on captive populations, often of unknown geographic provenience, and place mean M1 emergence at ~3.2-3.3 yrs.. Until recently, the only reliable M1 emergence age for wild chimpanzees came from an analysis of a single P. troglodytes verus individual from Taï Forest, Ivory Coast, which placed this important temporal event at ~3.8-4.0 yr. However, new observations from a free living population of wild chimpanzees (P. troglodytes schweinfurthii) from Kanyawara, Kibale National Park, Uganda, place this event at ~3.2 yr, much earlier than in the one individual of P. t. verus and nearly identical to that for captive chimpanzees. Local ecology, resource availability, mortality/survivorship profiles, and social group dynamics differ quite markedly between Kanyawara and Taï Forest chimpanzees, raising the possibility that different populations evolved somewhat divergent life history schedules as a function of small-scale habitat heterogeneity. This may be true of molar emergence schedules as well but the data are presently inadequate to reliably demonstrate this. Within the context of general life-history theory, life-history variation across populations is not surprising, but the extent to which this phenomenon is expressed as variation in molar emergence is only hinted at as well integrated dental and ecological data are available only for the Kanyawara population.
From a life history and ecological perspective, African ape populations are quite variable, and our new chimpanzee data will clarify the extent to which ecology drives the timing of critical life-history events (such as weaning age, the age at first reproduction, and the spacing of births, which are packaged in humans to produce periods of prolonged growth and childhood dependency). Exploring how these factors influence the timing of tooth emergence—the one growth parameter that can be established accurately in early human fossils—will enhance the confidence with which we can directly probe the fossil record for evidence of the acquisition of those life history attributes that characterize human uniqueness.
We will use the collection curated in the Naturmuseum Senckenberg in Frankfurt am Main, Germany. It represents a local population of >300 central Liberian chimpanzees, (P. troglodytes verus) ranging in age from newborns to adults. As one of our collaborators (F. Schrenk) curates this collection, permission for working on the material has already been granted. The collection has already been surveyed by two of the PIs, who have scored the dental developmental status of all individuals. Specimens of appropriate stages (i.e., at some stage of either at M1, M2, or M3 eruption) were flagged for the extraction, embedding, and sectioning of molars. We will target acquiring ages at death (and thus ages at molar emergence) for 3–5 individuals at either ca. M1, M2, or M3 emergence. Methods regarding tooth extraction and sectioning are well established in the paleoanthropological literature and have been used before with great success by the IHO researchers in studies of molar emergence in museum specimens of wild-shot great apes. All molars will be photographed, digitally scanned in 3D, molded, and casts made immediately post extraction. Embedding and sectioning will be carried out using an automated Buehler sectioning set-up (budgeted within the Advanced Hominin Imaging Lab) under the supervision of GS.
As is well known, enamel and dentine components of tooth crowns and roots preserve a set of growth lines that occur on a daily cycle. Thus, from sections of teeth that are somewhere in the process of formation it is possible to determine the exact number of days from the initiation of M1 growth to the last day of growth (at death). Since M1s initiate prior to birth, they preserve a birth, or neonatal, line; counting the number of tooth formation days from the neonatal line until the end of enamel formation, and adding that to the number of days from the start until the last day of root development, yields the age at death, and thus the age at M1 emergence, for that individual. Adjustment is made for the stage of eruption to determine the time window for gingival emergence. Emergence ages will be evaluated within the context of proxies for relevant ecological data (such as rainfall records) and compared directly to the rich ecological and developmental database for P. t. schweinfurthii specimens from Kanyawara, Uganda.
Reconstructing ages at molar emergence in this sample of wild-shot individuals is ideal, as it will provide an opportunity to integrate data on climate, rainfall, and resource availability (from historical records in the Senckenberg archives) with dental developmental data to provide a more complete reconstruction of the ecology and life history of a wild population of P. t. verus chimpanzees and allow detailed comparisons of the linkages among growth, dental development and life history between this population and P. t. schweinfurthii from Uganda and those attributes in our expanding database on gorilla development. Our pilot data from emerging M1s of two individuals from the Senckenberg P. t. verus collection suggest substantially later M1 emergence ages than in P. t. schweinfurthii, highlighting differences in how dental development and life history covary across chimpanzee populations embedded in quite different ecologies.
Advancing our understanding of the effect of local ecology and life history on variation in dental development through the design of natural experiments—comparisons across closely related, living populations—will provide a more reliable framework in which to infer life history shifts in the human fossil record based on changes in molar emergence schedules. For example, current data suggest subtle differences in the timing of molar emergence between contemporaneous Neanderthal and early modern H. sapiens populations, at ~150-50 ka. Do these differences imply growth-related distinctions in life-history schedules? In the absence of reliable contextual data on great ape development, it is impossible to infer accelerations or decelerations in growth and resulting social, ecological, or demographic shifts in human evolution.