After looking at the fossil tooth remains of close human relatives, Homo naledi, Ian Towle discovers their diet was very different to ours.

Dental fractures, or chips as they are commonly called, are common in nearly all human populations today. However, the chipping frequency in past populations has rarely been explored. In the research described here the teeth of fossil hominins (humans and closely related fossil species) as well as several extant primates and recent human groups were recorded for fractures with the aim of providing information on diet and behaviour.

The size, number and position of chips can all give insight into the lives of fossil individuals, with each fracture created by a hard object contacting a tooth. For example, if a population tends to have the most chips on their anterior teeth then it suggests the cause may relate to processing foods or other cultural behaviours involving the teeth. In contrast, small chips found predominately on posterior teeth and on interproximal areas suggests a dietary component is likely involved.

High rate of tooth chipping

The most striking result of this research is the high rate of chipping on the teeth of H. naledi, with over 40% of teeth affected. This newly discovered hominin species lived sometime between 335,000 and 236,000 years ago, suggesting they may have shared their environment with modern humans and creates intriguing questions into the ecological niche they would have filled.

The chipping is not distributed evenly over their dentitions. Molars are the most fractured, with over half having at least one chip and many having multiple small chips. That said, anterior teeth are still affected much more than other species, with over 30% having one chip or more. These results show H. naledi regularly chewed on small hard objects, with the size, teeth most affected and the position on the teeth suggesting non-masticatory behaviour is unlikely the cause. This is among the first evidence to be presented on the diet and behaviour of this fascinating new addition to the human family tree and suggests they ate a substantially different diet than other fossil hominins.

To put these results into context, H. naledi has over twice the chipping rate of Australopithecus africanus, and four times that of Paranthropus robustus, two fossil hominin species often thought to commonly consume hard foods. This contrast becomes even more marked when compared to living great apes, with gorillas having around 10% of teeth chipped and chimpanzees only 5%. Furthermore, the multiple small chips, sometimes over five on a single tooth, found on H. naledi teeth is not found on any individual in the comparative samples studied, strongly suggesting a unique diet for this species.

Right versus left

The species studied with the most similar rate and pattern of chipping to H. naledi are baboons, with 25% of their teeth having fractures. The chipping in baboons is likely caused by the environment in which they forage, including consuming large amounts of grit and other hard objects. Certain modern human samples also show a similar chipping rate to H. naledi, such as Inuit and Aboriginal Australians as well as many archaeological samples; however, the pattern of chipping is usually substantially different, with the modern samples tending to show the most fractures on their anterior teeth. The few archaeological examples in the literature that do have similar fracture patterns support the conclusion that the chipping in H. naledi relates to diet and not to using teeth as tools, and is likely caused by the consumption of large amounts of grit adhering to foods.

Another intriguing finding is that the right teeth of H. naledi consistently have more chips than left teeth, regardless of the tooth type studied (right: 50% versus left: 38%). It may be possible that this reflects H. naledi individual’s handedness, placing food items more often in the right side of the mouth, but may also be related to a predisposition to chew with one side over the other. Once further samples are uncovered this relationship can be explored further.

Chipping rates

The results of this research highlight the variability in chipping rates between different human populations and species, emphasising the vast array of diets and behaviours represented. Homo naledi may have specialised in eating certain foods such as tubers, with grit adhering to the surface. However, it is also possible that a particularly small hard food item was commonly eaten or H. naledi processed foods in such a way as to only allow small hard food items to be consumed. Further research on the microscopic wear of these teeth, isotopic analysis, and studying plant remains in their dental tartar may provide further evidence for the exact cause of these dental fractures. At present it is possible to conclude that H. naledi consumed a diet significantly different from any other fossil hominin species yet studied, containing large amounts of small hard objects.

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