Sinuses Reveal More About The Evolution Of Ancient Humans

Jan Bartek - AncientPages.com - The changing shape of the frontal sinuses is helping to reveal more about how modern humans, and our ancient relatives, evolved.

An international team of researchers led by Antoine Balzeau of the Muséum National d'Histoire Naturelle revealed that the small cavities, located just above the nose, are linked to the size of the frontal lobe.

A replica of the Peking Man skull. Credit: Yan Li - CC BY-SA 3.0

This part of the brain is responsible for processes that make us uniquely human such as speech, emotion and planning, with the sinuses now providing another way for scientists to infer the development of this part of the brain.

The frontal sinuses also offer a new way to investigate the relationships between different species of ancient hominins, with the study providing additional support for Homo naledi belonging to our genus, despite having some prehuman characteristics.

Professor Chris Stringer, an expert in human evolution at the Museum who co-authored the paper, says, "Sinuses are interesting morphological features in fossils but they have been neglected. Many papers that describe new species don't mention them, and they are often only illustrated incidentally to the rest of the specimen."

"This has left what data there are very disparate. To try and rectify that, this paper compiled the biggest selection of fossil sinus data ever from many different sources. It shows that within our genus, Homo, sinuses can be used to tease apart the relationships between different species."

Co-author Dr. Laura Buck, formerly of the Museum but now at Liverpool John Moores University, adds, "In early hominins and non-human apes, the size and shape of frontal sinuses is directly related to the amount of space available for them to grow into."

"The step change we see between these species and later hominins, including ourselves, suggests a shift in the way the skull is organized and develops. It may be relevant that this is happening at the same point as we start to see substantial brain expansion in these taxa."

What are sinuses?

Sinuses are air-filled spaces within the bones of the skull that are lined with a mucous membrane. Humans have four types of sinuses: the maxillary sinuses under the eyes, the ethmoidal sinuses between the eyes and nose, the sphenoidal sinuses to the outside of the eyes, and the frontal sinuses.

While they have been known about for centuries, it is unclear what their role is. Suggestions that they produce mucus and nitrogen oxide to defend against infection or provide thermal and shock protection to the nervous system can help to explain their current function, but not necessarily why they evolved.

Our species, Homo sapiens, has four types of sinus - two of which are only found in humans and our close relatives. Credit: CFCF - CC BY-SA 3.0

Some researchers have even suggested that sinuses are an example of an evolutionary spandrel, a structure that evolved as the by-product of something else and has no initial role. Further adaptation may give it a function later in evolutionary time.

Research on other animals, such as bovids and primates, have shown that sinuses can differ between species, driving interest in whether sinuses could also be helpful to distinguish between species of ancient humans.

The exact path of human evolution is still the subject of heated debate, with many competing theories over how our species came into being, and how many close relatives we have. Examining the sinuses of ancient species could help to address this.

The maxillary and frontal sinuses are of particular interest, as in primates they are only found in humans and our closest relatives, the chimpanzees and gorillas.

This new study examined 94 fossil hominins from over 20 species to gain a greater insight into variation of the frontal sinus, and what that reveals about human evolution.

How can sinuses be used to study human evolution?

The researchers used CT scans of the specimens to create 3D models of the frontal sinuses, allowing them to digitally reconstruct the structures. These models were then used to take measurements which could be compared between the different species.