Humans, along with their teeth, take a very long time to grow. A puppy does not stay little for long, and her first adult teeth come in around five to six months of age. Contrast this puppy with a human baby, who will not get her first adult teeth until she is around six years of age.

A protracted growth period is considered one of humanity's uniquely derived features relative to other primates. One view is that over the course of our evolutionary history, human juvenile growth was extended by the insertion of a novel developmental phase – childhood – which has been defined as a period after weaning during which a juvenile continues to depend on others for survival (180). Whether or not human childhood is unique, it is clear that subadult growth spurts begin at much later ages in humans than they do in African apes (181). Humans reach sexual maturity at comparatively later ages as well (181).

Viewed through the lens of “life history” theory, the scheduling of key life benchmarks such as age at weaning, age at sexual maturity, age at first reproduction, intervals between births, and natural life span, is shaped by natural selection. For different species experiencing different environments and ecologies, natural selection will result in different life history solutions – or adaptations – to the problems of survival and reproduction. Natural selection is thought to have optimized the amount of energy organisms devote to growth versus reproduction at different points in their lives. There is a trade off between the two because energy is limited and so can be devoted to either growth or reproduction, but usually not simultaneously to both (182).

According to life history theorists and biologists Eric Charnov and David Berrigan (182), the optimal age at which a primate species reaches sexual maturity is largely determined by adult mortality – the risk of death due to “extrinsic factors” such as predation, disease, or accidents.

A book about fossil teeth would have been unthinkable before 1669. Three years earlier, a group of Tuscan fishermen caught a colossal white shark. As Brian Switek tells the story in his engaging book, Written in Stone: The Hidden Secrets of Fossils and the Story of Life on Earth, that shark excited the imagination of Medici Grand Duke Ferdinando II, a great patron of the sciences (1).

The shark's body was too large to transport and had begun to decompose anyway, so its head was cut off and sent to the Grand Duke, who chose his most talented resident anatomist for the privileged job of dissecting it (Figure i). Danish-born Nicolaus Steno (aka Niels Steensen) turned out to be the man for the job (1). As he poured over his dissection, Steno was struck by the uncanny similarity of the shark's teeth to what were then popular triangle-shaped stones called glossopetrae or “tongue stones” (2, 3) (Figure 1).

At the time, glossopetrae were used for all sorts of purposes: as antidotes to snake venom, treatments for epilepsy, amulets, and when ground into a fine powder, as toothpastes (2, 3). Roman philosopher Pliny the Elder thought glossopetrae dropped from the sky on moonless nights (4). Inspired by his shark dissection, Steno published his own explanation in 1669. He suggested that the corpuscular theory, which held that matter was made of tiny corpuscles, could explain how shark teeth turned to stone (5). When the teeth were buried in sediments, the corpuscles of minerals gradually replaced the corpuscles that made up teeth, transforming them into stone. This explanation is not so far from our modern understanding of how fossils form. In the case of fossil teeth though, most of the original mineral remains while mineral from surrounding sediments fills in tiny pore spaces within them in the process of permineralization.

That fossil teeth played such a starring role at the dawn of paleontology is no coincidence. To be sure, sharks lose a lot of teeth. But it is the fact that teeth are hard, compact, and composed of mineralized tissue that makes them prime candidates for preservation and fossilization. Indeed, most of the fossil record consists of teeth and that is also true of the human fossil record. This book is about what fossil teeth tell us about human evolution.

So ends Lord Jacques's soliloquy “All the World's a Stage,” summing up human life history from infancy and childhood to toothless old age and death. In Chapter 4, we saw that some australopith species may have had juvenile growth periods shorter than or as long as those of some modern apes, but none were quite equal to modern humans in this regard.

Anthropologist Barry Bogin defined human childhood as the period between weaning and puberty during which offspring continue to depend on others for survival (180). For fossil hominins, the existence of childhood as so defined would be hard to determine. Applying Bogin's definition would require information about ages at weaning, which are difficult to acquire in fossil hominins, though not impossible (see Chapter 10). It would also be necessary to ascertain the length of time over which growing juvenile hominins actually depended on others for survival, and that would be even more difficult to determine. Nevertheless, through the study of teeth, the extension of juvenile growth periods, if not childhood as Bogin defined it, can be traced over the evolutionary history of the genus Homo. So too can the evolution of “second childishness” – old age.

In this chapter, we will see that within the genus Homo, changes in juvenile growth periods and longevity parallel each other in direction, though it is not clear that these changes in growth periods and longevity occurred in lockstep with each other. A common trajectory toward lengthening both juvenile growth periods and lifespans, but with possible differences in their rates of change, is consistent with the concept of “modular” life history evolution (186). The concept of “modularity” as applied to life history recognizes that different aspects of a species life history can respond to selection independently to some degree. This would be the case if the genes and developmental pathways that affect length of juvenile growth periods, for example, are not the same as those affecting longevity – or at least if not all of those genes and developmental pathways are the same.

Chapter 4 left off on evidence of dietary divergence between Paranthropus and early Homo, evidence that lends support to Robinson's Dietary Hypothesis. The present chapter considers how dental evidence contributes to our understanding of dietary change within the genus Homo, prior to the evolution of Neanderthals and anatomically modern humans (AMHS). During this period of human evolution, archaeological evidence becomes a crucial source of information for dietary reconstruction. Faunal remains with evidence of human alteration (e.g., butchery), stone tools and eventually evidence of controlled use of fire supply crucial pieces of the dietary puzzle in the Homo lineage. But teeth, too, yield important clues. This chapter focuses on those clues within their archaeological context.

In addition, this chapter reviews evidence and explores arguments for the possibility that changes in culture – the use of tools and fire – had an effect on dental evolution in Homo habilis, Homo erectus, and Homo heidelbergensis. While a major reduction in the size of the canine apparently occurred at or near the base of the hominin clade (Chapter 3), posterior teeth did not reduce until much later, arguably not until the appearance of Homo erectus (251, 324). There were also changes in anterior teeth. Relative to estimated body size, incisor teeth are larger in Homo habilis than they are in any australopith, but then decrease in size with Homo erectus (325).

The big question is why posterior teeth (and later anterior teeth) reduced in size in the first place. Anthropological legend C. Loring Brace and colleagues found that major reductions in tooth size during the Pleistocene occurred relatively recently, during the last 200,000–300,000 years (commentary in Current Anthropology on Wrangham and colleagues’ cooking hypothesis [52]). Brace argued that the degree of dental reduction in different regions of the world is proportional to the length of time people have been cooking in these regions (326). Cooking would have had the effect of softening food and thus relaxing the selection pressure on maintaining large teeth to process hard and/or tough foods.

Charles Darwin's analysis of the size and shape of human canine teeth convinced him that at some point, human male ancestors possessed large and projecting canines, similar to those of male gorillas or orangutans. In Darwin's view, male canine teeth, like other male armaments such as horns in mammals or spurs in birds, were the result of sexual selection: the advantage that members of the same sex have over one another in “exclusive relation to reproduction.”

Darwin conceived of sexual selection happening in two ways: first, through competition among males for mating opportunities with females, and second through differences among males in their ability to attract females. These two sides of the sexual selection coin are called intrasexual and intersexual selection, respectively. In the first case competition occurs among males and in the second, females select males who they find most attractive (or on the basis of other traits that would presumably enhance the survival of their offspring). Thus ensues the evolution of armaments, like the large curved horns of male bighorn sheep, and ornaments, like the peacock's tail. Slashing canine teeth, in Darwin's view, were a male armament which evolved through time because males with larger and sharper teeth were able to win more contests for females and thus pass these traits to their offspring.

Darwin unfortunately did not have a satisfactory answer for why there is a general pattern in nature of male-male competition for mates and female choice of them, but later work (116, 117) offered an explanation in terms of sex differences in investment in offspring. In mammals, it is usually females that make an enormous investment in offspring through gestation and lactation. Males, who by comparison invest much less, compete with one another for access to females.

In 1871, in the “Descent of Man,” Charles Darwin speculated that human third molars, or “wisdom teeth,” were evolving into rudimentary structures (115). Such structures, he explained, tend to be highly variable and reduced in size or even “wholly suppressed.” Wisdom teeth fit this definition well in that they show great variability in size, shape, and development. Furthermore, although the congenital absence of third molars is extremely rare in great apes (456), it is quite common in humans, with a worldwide frequency (at least one missing third molar) of 22.63% according to a new meta-analysis (260).

The reduction or absence of third molars can be viewed as part of the larger trend of dental reduction in recent human evolution, one of several aspects of modern human biology that the study of ancient teeth illuminates. Dental studies also give us insight into the phylogenetic origins and patterns of dispersal of anatomically modern humans (AMHS) out of Africa, our long developmental periods, and the evolutionary history of our dental diseases. In detailing these insights, I develop two main themes. The first is that some of these aspects of modern human biology are interrelated. As one example, our present long lifespans may give us a reproductive payoff in offspring number and/or offspring survival, but longer lifespans require teeth to function for longer periods of time, increasing the risk of dental disease.

The second theme is the dynamic relationship between culture and biology in the evolution of modern human dental reduction and dental disease, both of which have been influenced by changes in diet, food processing, and eating habits. Nevertheless, much remains to be understood about the extent to which differences among cultures over time and space can be linked to population patterns of dental reduction and dental pathology. In Chapter 7, I discussed the case of dental reduction in the Sima de los Huesos Homo heidelbergensis hominins, which José Bermúdez de Castro's analysis suggests has more to do with genetic drift than with culturally-induced relaxed selection on tooth size.

Ever since Neanderthals were discovered more than 150 years ago (see Chapter 5), scientists have debated just about every aspect of their biology. Based on the distinctive features of Neanderthals (Chapter 4) is easy to see why King considered Neanderthals to be a different species from humans, christening them Homo neanderthalensis. Like other scientists of his time, King also inferred mental capacity from the shape of the cranium, considering Neanderthals to be low-brows in more than one sense of the word. Between 1911 and 1913, paleontologist-artist Marcelin Boule published illustrations depicting Neanderthals as stooping hairy beasts with grasping toes. This too did not help Neanderthals with their image problem. Boule's conception held sway until 1957, when Straus and Cave showed that Boule had misread the evidence, and that Neanderthals walked upright just as we do (281).

Ideas about Neanderthals began to change in the 1950s, in part because the field of physical anthropology itself was transforming. In the wake of World War II, physical anthropologists repudiated the idea that there are races or types of humans with distinct physical and mental attributes. Instead, the consensus view was, as it is today, that there are no clear physical or mental divisions among human populations along racial lines, that mental attributes are not linked to physical attributes, and that there is no evidence that human populations differ in intelligence. Racist thinking, it was argued, had led to a serious misunderstanding and Neanderthals were welcomed back into our species. Our modern understanding of species rests on evolutionary biologist Ernst Mayr's concept of reproductive isolation – species evolve independently of one another and become distinct if they do not exchange genes. As part of our species, the implication was that when and where Neanderthals and anatomically modern humans coexisted, they could have interbred to produce fertile offspring. By extension, today's humans (though in need of a time machine to do it) could have mated with Neanderthals and had children who themselves were fully capable of reproducing.

As understood today, human evolutionary history involves some twenty species of fossil hominins, give or take a few depending on the taxonomic scheme to which one subscribes. In the field of paleoanthropology, taxonomic questions – how to classify fossil species – are famously contentious. As Richard Dawkins comments in the Ancestor's Tale (8), there is more than one paleontology book in print entitled “Bones of Contention.” Names given to species carry implications for how species are related to one another. A species placed in the genus Homo for example, is recognized as sharing a more recent common ancestor with modern humans than a species placed in the genus Australopithecus.

Resolving species relationships is no straightforward task. That is one reason (among others) why there are so many different points of view about evolutionary relationships among hominin species. Such relationships are summarized in diagrams called phylogenies or phylogenetic trees. All methods of constructing phylogenies rely on comparing the characteristics of species, from their DNA sequences to their dental features to any aspect of their anatomy and even behavior. The most direct means of genetic comparison is through DNA, but, only in more recent hominins has it thus far been possible to extract adequately preserved DNA (see Chapter 5). For earlier species, the characteristics of teeth and bones must suffice for assessing evolutionary relationships. As noted, teeth figure prominently in such assessments: they are extremely well-preserved in the fossil record and much of the variation in their morphology is due to genes (rather than environment).

There are different ways to assess evolutionary relationships among species. According to the method of phenetics, all available traits should be used to construct phylogenies. Species with greater similarity are considered more closely related. This method does not take into account two big problems. The first is the problem of homoplasy, when two species have similar traits but not because they inherited them from a common ancestor. One way that homoplasy can occur is when species independently evolve similar solutions to similar problems. The wings of dragonflies and seagulls were not inherited from their common ancestor but evolved independently as adaptations for flight in their separately evolving lineages.

With these words, Louis Leakey and colleagues named the species Homo habilis. In so doing, they widened the definition of the Homo genus to include a species with smaller average brain size than that of Homo erectus, which was up until then the earliest known species of our genus. To include this species in the genus Homo, Leakey and colleagues revised the so-called “cerebral Rubicon” dividing Australopithecus from Homo down to 600 cubic centimeters from the 750 cubic centimeters used up until that point (240). This is what Leakey et al. meant by revising the “diagnosis of the genus.” Indeed, over the history of paleoanthropology, definitions of the genus Homo have expanded further, encompassing the varied features of fossils assigned to the genus (177, 241). Thus, how to define the genus Homo – and indeed how to define a genus in the first place – are matters of long-standing and vigorous debate (47, 177).

In a 2014 review paper in the journal Science, paleoanthropologist Susan Antón and colleagues’ employed a broad definition of the genus that included specimens traditionally assigned to the species Homo habilis (e.g., KNM-ER 1813). In their view, these specimens shared with later genus members a suite of characteristics that contributed to their evolutionary success. Namely, these are: “… increases in average body and brain size and changing dental size coupled with increased toolmaking and stone transport” which they contend “suggest dietary expansion, developmental plasticity, cognitive evolution and social investments” (2014: 1236828–8). The present chapter gives an overview of species that have been considered to be members the genus Homo, that to varying degrees partake in the package of adaptations Antón and colleagues describe. Key dental features and possible phylogenetic relationships to anatomically modern human are integrated into this overview.

Australopiths were not likely to have gone on diets but they certainly had them. Diets provide the energy and nutrients that enable survival and reproduction. Natural selection favors those individuals who are best at transforming their diets into surviving offspring, who will, in turn pass on the genes that made them and their parents successful. For this reason, diets fundamentally shape species’ biology, from how they distribute themselves in relation to resources to the contours of their molar cusps.

Because what a species eats is a keystone adaptation, scenarios of human evolution often begin with changes in diet. In the 1950s, the South African paleoanthropologist John T. Robinson (1923–2001), presented an argument about early human evolution (48, 49), now referred to as the “Dietary Hypothesis.” According to Robinson, the cranial and dental differences between South African hominins Paranthropus robustus and Australopithecus africanus pointed to a major dietary difference between them. Robinson reasoned that the large muscles of mastication (as inferred from bony anatomy) and expanded molars of Paranthropus robustus were adaptations to a tough vegetarian diet, including roots and bulbs, that required a great deal of chewing. His analysis of craniodental morphology in Australopithecus africanus, however, led him to conclude that this species was the founder of a new line of more omnivorous, tool-using hominins that could respond more flexibly to changing environmental conditions. This is the line, he argued, that eventually gave rise to us.

Raymond Dart's ideas (50) about early human evolution were likely to have influenced Robinson. Dart's study of South African cave sites led him to believe (incorrectly, it turned out [51]) that hominin hunting was responsible for the animal bones accumulated there. In his view, hunting transformed our peaceful ape-like ancestors into intelligent, weapon-wielding, blood-thirsty predators. More recent ideas about the importance of meat in human evolution de-emphasize the act of hunting as a prime mover of human evolution, focusing instead on the nutritive benefits of meat in facilitating human evolution. Humans and chimpanzees do differ in their meat-eating habits.

So lamented Don Quixote when shepherds bearing slingshots took out several of his “grinders.” He had attacked their flock of sheep, which he mistook for an army (easy mistake). However misguided, the noble knight-errant clearly understood the value of teeth.

To paleoanthropologists, the value of teeth lies in the wealth of information they preserve about our past. Morphological analyses of fossil teeth elucidate biological relationships among species and populations within them. Studies of the chemical composition of enamel and of dental wear inform us about the diets of our ancestors and close relatives. And, investigations of dental growth and development enlighten us about their life history modes. These are some of the more vital contributions of teeth to understanding our evolutionary history. Here, I will synthesize dental insights into human evolution discussed throughout this book and consider what's next for what can be called “dental paleoanthropology,” the subfield of paleoanthropology that makes use of fossil teeth to understand hominin biology. Finally, I will reflect on how an evolutionary perspective on our dental past helps us understand how we view and treat teeth today.

Putting in All Together: The Whole Tooth

If we were to draw up a list of differences between humans and our chimpanzees relatives, that list would include – among other things – our differences in brain size, language ability, ability to adapt to new environments, our reliance on cultural solutions to problems of survival and reproduction, the length of our periods of juvenile growth and dependency, and the length of our lifespans. Fossil teeth give us a record of the origins and evolution of several of these and other unique human attributes.

The origins of human adaptability can be traced back to the teeth of our australopith ancestors. Stable carbon isotope analysis of their dental enamel (Chapter 2) reveals that, unlike modern chimpanzees whose diets even in savanna environments are limited to C3 plants, australopith species were in most cases incorporating both C3 and C4 resources into their diets (83).