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By Ian Randall
Deputy Science Editor
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The story of how us humans—and other mammals—got our noses may have just gotten more complicated.
This is the conclusion of a new study by researchers from Japan who have studied how the face develops in embryonic mice.
"Our findings challenge conventional views on upper jaw evolution," the researchers said in a statement.
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Compared with our distant cousins the reptiles, the faces of mammals are made more complex by a protruding nose that is separate from the upper jaw.
While mice are commonly used as a model organism, much has been unknown about how their craniofacial structures develop, making it difficult to compare mice with other vertebrates—human included—the researchers explained.
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To address this limitation, in the study, Hiroki Higashiyama and colleagues at the University of Tokyo mapped out how the faces of mice develop from the mid-embryonic stage to just before birth.
They did this by taking sections of the developing mice, staining them to best pick out the bones, cartilage and nerves and then reconstructing the sections into 3D models.
The team found that, in mice, the early development of the head and the peripheral nerves (those outside of the brain and spinal column) follows common patterns seen in other jawed vertebrates.
Similarly, the early formation of the cartilage at the base of the skull was found to follow a pattern typical for mammals—and comparable to that seen in reptiles.
When they got to the upper jawbone, however, the researchers said that they found evidence that challenged conventional theories of development and evolution.
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"Traditionally, it was believed that mammalian jaw bones were directly inherited from reptiles," the researchers explained. "However, our results strongly support the holo-maxillary hypothesis, which we previously proposed."
In this hypothesis, the noses of mammals like mice and humans were repurposed from a lost, ancestral jawbone known as the "premaxilla," which separated from the upper jaw.
At the same time, the team explained, a new jawbone (which scientists have dubbed the "incisivum") emerged, incorporating elements from the septomaxilla (a delicate bone associated with the nostrils) and the vomer (a plough-shaped bone that, among other things, forms the septum in reptiles).
"This implies that the mammalian upper jaw underwent a major structural reorganization, setting it apart from that of reptiles," the researchers said.
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According to the researchers, their conclusions about the facial development of embryonic mice also appears to apply to us humans.
"In human fetal development, a temporary upper jawbone (incisivum) forms and later fuses with the maxilla, though reports on its ossification [bone formation] sites vary," the researchers explained.
"Our hypothesis, which suggests that the incisivum consists of both septomaxillary and palatial elements, helps explain these discrepancies."
This insight, the team concluded, may help doctors better understand such craniofacial disorders as cleft lip and cleft palate, in which there is a split in the upper lip.
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Reference
Higashiyama, H., Kuroda, S., Iwase, A., Irie, N., & Kurihara, H. (2025). On the Maxillofacial Development of Mice, Mus musculus. Journal of Morphology, 286(3). https://doi.org/10.1002/jmor.70032
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