The function of our sinuses – small, air-filled cavities in the skull, lined with mucous membranes – is a subject of continued debate: common hypotheses have included thermoregulation and the dispersal of chewing strain, while others suggest that they have no particular purpose. However, new research suggests that although we may not know exactly why they evolved, the study of the development of sinuses over time can offer previously untapped opportunities to understand human evolution.
Homo sapiens have four types of sinuses, two of which are found only in humans and our close relatives: the maxillary and the frontal sinuses. The frontal sinuses, which are situated in the frontal bone, in the lower part of the forehead, are the focus of the recent research carried out by an international team of researchers led by Dr Antoine Balzeau of the Muséum National d’Histoire Naturelle in Paris. The way these features changed in size and shape across ancient human species can provide important information about how their skulls changed over time, but until now the frontal sinuses have largely been overlooked in studies of human evolution. The new study set out to fill these gaps in our understanding. The team compiled the largest selection of fossil sinus data ever put together, comprising 94 fossil hominins from more than 20 species. These specimens underwent CT scanning in order to create 3D models, which could then be used to measure the sinuses and compare their size, shape, and position within and between different species, in the hope of understanding more about their relationships to each other.
The results determined that, although sinus size could not be used to identify specific early species of the human lineage, as they fall within the same ranges, later species including Homo erectus, Homo neanderthalensis, and Homo sapiens have distinct ranges of sinus size and, in some cases, particular shapes, which can be used to distinguish these species from each other and from other fossil groups. Also among the species examined was Homo naledi, whose categorisation is still the subject of debate due to their combination of human and non-human traits. Significantly, researchers found that the sinus size of Homo naledi was similar in range to that of Homo erectus, further supporting arguments for their human status.
The research investigated, too, the connection between the frontal sinuses and the frontal lobe, the part of the brain responsible for processes like emotion, speech, and planning. In chimpanzees, bonobos, and gorillas, as well as earlier hominins such as Sahelanthropus tchadensis, and members of the Australopithecus and Paranthropus genera, the shape and size of the frontal sinuses seem to be simply directly proportional to the amount of space available for them to grow into. But around 2 million years ago there appears to have been a shift in the way the skull was organised, with the face and the braincase developing more independently of one another, resulting in much wider variation in the size and shape of the frontal sinus of later hominins. Dr Laura Buck, one of the paper’s authors, suggests that this may be related to the significant brain expansion that occurred in these species around this time. The link between sinuses and frontal lobe size in species from Homo erectus onwards offers researchers a new way to explore the development of the frontal lobe and the behaviours associated with this.
In addition to offering insights into human evolution, the data also shed new light on the function of the sinuses, undermining suggestions that their development was directly connected to adaptation to climate or chewing. In particular, the researchers argue that the popular hypothesis that Neanderthals’ frontal sinuses were an adaptation to cold climates should be firmly rejected. Instead, they propose, the specific shape of different species’ frontal sinuses is the result of genetic heritage, connected to the form of the brain and the skull, and is, to a certain degree, random.
-The research, which has been published in Science Advances (https://doi.org/10.1126/sciadv.abp9767), is the most in-depth study of ancient hominin sinuses carried out to date and demonstrates their valuable potential in discussions about human evolution.