From jaw to nose: mammals owe their noses to the jaws of reptiles

Tokyo, Japan – Humans – and other mammals too – owe their noses to the evolution of reptile jaws? Well, that’s the conclusion of an evolutionary study of animal skeletons and embryos by Japanese scientists – who discovered how mammals developed their flexible, protruding noses.

A team from the University of Tokyo has also discovered the origin of the strong sense of smell in mammals. Their findings create the potential for new animal models, like chickens or frogs, that lab experiments often use to study facial developmental disorders like cleft palate.

The traditional scientific understanding of facial evolution is that the jaws of mammals and reptiles develop in almost the same way. Even though mammals have a unique nose, the evolution of their bone structure was unclear until now.

“Existing fossils of four-legged animals, reptilian and mammalian ancestors, have the same number of upper jaws. It’s very easy to think that the bones are the same, but now we can study embryos and follow cell development to study these bones in much greater detail,” says postdoctoral researcher Dr. Hiroki Higashiyama in a university outing.

Evolution has moved the jaw to the nose

UTokyo researchers used cell studies and re-examinations of fossilized skulls to develop a new and improved understanding of how mammals evolved separate noses and upper jaws. This reorganization of the premaxillary and septomaxillary bones probably allowed mammals to form flexible noses capable of “sniffing”. (© Hiroki Higashiyama)

The study is the first of its kind to examine the evolution of facial structures using cell studies comparing multiple embryos from multiple species to each other. Dr Higashiyama and colleagues worked in Prof Hiroki Kurihara’s lab, designing experiments to track facial development in the embryos of different species of birds, reptiles and mammals – focusing specifically on chickens, geckos and mice.

The researchers then focused on a group of cells known as facial prominences that are responsible for creating the physical structures of the face in embryos. The team stained the cells in order to track them as they moved and grew to maturity.

According to scientists, there is a group of cells called the frontonasal prominence, which forms the tip of a reptile’s jaw, which develops into a nose protuberance in mammals.

The tips of mammalian jaws, on the other hand, arise from a separate group of cells called the maxillary prominence. Using this new perspective on their experiments, the researchers moved on to examining fossilized specimens.

As the ancestors of species collected more physical and genetic differences, the bone at the end of a reptile’s upper jaw, known as the premaxillarybecame smaller and migrated upward, while the bone behind it, the septomaxillarygrew larger and moved forward to become the tip of the mammalian jaw.

Mammals have developed the ability to “sniff”

The researchers say that the facial bones of egg-laying mammals, like the Australian platypus and the echidna, are living examples of how bone structures evolved from the older evolutionary reptile model to mammalian structure. more recently evolved. This separation of nose and jaw gives mammals their unique ability to “sniff”, using muscles to flare the nostrils, deeply inhaling odors from the environment.

“This discovery is a key innovation in the evolution of our mobile noses and those of other mammals, which contributes to the highly sensitive mammalian sense of smell,” adds Dr Higashiyama.

The study authors believe that being able to distinguish between different smells may have helped mammals develop larger and more complex brains than earlier ancestor species. This research has provided physical evidence for evolutionary change in premaxillary and septomaxillary arrangements, but further research is still needed to identify genetic causes.

“Now we know the composition of facial prominences and embryonic development in several species, so we can compare facial development disorders in chickens or frogs to humans. We have mainly just improved textbook knowledge for now, but in the future, these animal models will be a practical application of our studies,” Dr. Higashiyama concludes.

Mice are currently the only animals researchers use to study cleft lip and cleft palate conditions. However, they are an expensive scientific resource to maintain, and as a species mice are slower to develop. Therefore, research on other potential animal models of facial developmental disorders could become a valuable research tool.

The study is published in the Proceedings of the National Academy of Sciences.

South West News Service writer Georgia Lambert contributed to this report.

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