In many primates, the males have significantly longer canine teeth than the females – a trait associated with competition and aggressiveness. Humans are an exception. As far as the dentition is concerned, there are hardly any differences between men and women. But when in our evolution did the sexual dimorphism of the canines recede? A study has now come to the conclusion that our ancestor Ardipithecus ramidus already had small canines 4.5 million years ago with no pronounced gender differences. One reason could have been that the females preferred less aggressive males.
In many great apes, the canines are an important weapon in disputes with conspecifics. When it comes to winning over females – for example through direct combat against competitors or through successful intimidation – large canines offer the males a selection advantage. In the case of fossil species, the dentition can therefore also offer conclusions about social behavior: the more pronounced the sexual dimorphism of the canines, i.e. the larger the male canine teeth are compared to the female ones, the more the individuals living together were likely to be of competition and aggressiveness between the males embossed.
Sex differences in the dentition
In humans, on the other hand, the canines of men and women are similar in size. A team led by Gen Suwa from the University of Tokyo in Japan has now investigated when this peculiarity developed in our evolution. To do this, they calculated the size ratio of male to female canines for numerous fossil and contemporary species from human relationships. The problem with this is that, particularly in the case of species with less pronounced dimorphism, it is often unclear whether the fossil concerned was a male or a female individual.
In previous studies, it was assumed, for the sake of simplicity, that all teeth that were larger than the average belonged to males and all teeth that were smaller than the average belonged to female individuals. “However, in species in which the sizes of male and female canines overlap, this method leads to an overestimation of the dimorphism,” the researchers explain. Instead, they used a new statistical method that includes, among other things, the probability of the gender distribution, the so-called pdPeak method.
Small canines as early as 4.5 million years ago
Using this method, Suwa and his colleagues calculated the canine tooth proportions for various extinct and now living monkeys, for various Australopithecus and Homo species, and for Ardipithecus ramidus, the oldest known human ancestor, who lived around 4.5 million years ago. The result: “Our analyzes show that the dimorphism of the canines of Ardipithecus ramidus is significantly less pronounced than that of the bonobo, the least dimorphic and least aggressive of the living great apes in terms of behavior,” the researchers say. The upper canines of an Ardipithecus man were therefore on average only about 1.06 times larger than those of an Ardipithecus woman. “This is within the range of variation in modern humans,” explain Suwa and his colleagues.
Later representatives such as various Australopithecus species also showed a very low dimorphism of the canines. “Our results suggest that the sexual dimorphism in Australopithecus was very similar to that of modern humans,” the researchers write. “Taken together, our results suggest that even the early members of the human clade were characterized by a greatly reduced dimorphism of the canines.”
Less aggressive males
“Our results support the thesis that the reduction in size of male canines took place very early in human evolution and essentially coincided with the advent of bipedalism,” the authors explain. Former scientists, including Charles Darwin, believed that the canines receded because they were replaced by tools and weapons. “Complex tool technology that goes beyond that known from non-human primates has only been known for around 2.5 million years,” the researchers say.
With Ardipithecus 4.5 million years ago, other causes must have been decisive. “This evolutionary pattern points to a profound change in behavior that was accompanied by a comparatively low level of male aggression at the beginning of human evolution, a pattern that was later shared by Australopithecus and Homo,” explain Suwa and his colleagues. “A lower general level of aggression can be seen as an important evolutionary prerequisite for the later achievement of increased inter-individual cooperation and complex prosociality – characteristics that are the hallmarks of our ancestry.”
Source: Gen Suwa (University of Tokyo, Japan) et al., Proceeding of the National Academy of Sciences, doi: 10.1073 / pnas.2116630118