Whether dog, horse or human: the face of mammals differs from that of all other animals on earth. Because we all have a protruding and sometimes movable nose – and this mammalian snout is a unique reinvention of evolution, as researchers have found out on the basis of detailed comparisons. According to this, two bones in the jaw of our early ancestors changed their place and function and thus created the typical mammalian snout.
At first glance it may hardly be noticeable, but the facial area of mammals differs in one decisive detail from that of reptiles, birds and other terrestrial vertebrates: In all these animal groups, the upper jaw forms the tip of the snout. The nose follows slightly behind and usually consists of fewer than two holes.
Movable nose instead of rigid jaw tip
It is different with us mammals: in most mammals, the nose is prominently developed, can sniff, twitch and perform other movements and often protrudes beyond the jaw. It was this flexible nose that made it possible for our mammalian ancestors to significantly improve their smell and touch and thus to develop completely new ways of life and ecological strategies. But how the typical mammalian snout and nose came about in the course of evolution and what anatomical origin it has has so far remained largely unexplained.
“Until now, science has viewed the development of the faces of reptiles and mammals as relatively comparable,” explains co-author Ingmar Werneburg from the University of Tübingen. This was due to the fact that the tip of the snout in both groups of animals is formed by the intermaxillary bone (premaxillary) – and thus a bone structure that is homologous to the west. However, the anatomists have been puzzled for a long time that the mammalian snout is supplied by other branches of the facial and jaw nerves than in reptiles.
Comparative look into embryonic development
In order to get to the bottom of the matter, Werneburg, first author Hiroki Higashiyama from the University of Tokyo and her colleagues have now examined the development of the upper jaw and nose in detail. On the one hand, they compared the bony structures of reptiles, mammals and the fossils of mammalian ancestors. On the other hand, however, they followed the embryonic development of these structures down to the cellular level.
“Twenty years ago, this level of detail would have been technologically unthinkable,” says Ingmar Werneburg. For their study, the researchers marked the different cell groups involved in snout formation in the embryos of chickens, ant urchins, geckos and mice and were thus able to understand their movement and growth. They also compared the nerve supply to the snout areas.
The astonishing result: the snout of mammals and their prominent nose are the result of a profound transformation of the jawbone in the course of evolution. In reptiles and other non-mammals, the premaxillary forms the tip of the upper jaw. It also gives hold to the incisors of geckos, frogs and others. Another bone, the septomaxillary, is only small and lies behind the intermaxillary bone.
But as the ancestors of mammals evolved, the location and proportions of these two bones began to change. The septomaxillary grew larger and advanced, while the premaxillary moved upward. In the first mammals, there was a change in function: the premaxillary that had previously carried the incisors was replaced by the septomaxillary. This fused with the upper jaw bone and has since formed the tip of the jaw and the holder for the incisors in all mammals.
The premaxillary was thus free to take on a new function: in all mammals it forms the support structure for the nasal area. In connection with this, this part also received a separate nerve connection. “This is what makes sniffing anatomically possible in the first place – nostrils can be moved, smells can be sucked in,” explains Werneburg.
New look at our evolution
These results change the way we look at the evolution of mammals. Because contrary to what we thought, the typical snout and ultimately our nose did not emerge directly from the similar-looking structures of the reptiles. Instead, the places and functions of the bones involved were exchanged. “The mammal-specific face, the snout, is therefore an evolutionary novelty,” states the research team.
Only this decisive transformation makes it possible for mammals not only to develop their agile noses and thus a highly developed sense of smell. The facial muscles also changed and enabled new facial expressions – from sniffing to sniffing. (Proceedings of the National Academy of Sciences, 2021; doi: 10.1073/pnas.2111876118)
Source: PNAS, Senckenberg Research Institute and nature museums