Some snakes, like cobras and vipers, have fangs positioned in the front of their upper jaws, much like the position of our canine teeth. Many other snakes have fangs positioned in the rear of their upper jaws, much like the position of our wisdom teeth. How are the different type of front and rear fangs evolutionarily related? The most common hypothesis was that front and rear fangs evolved independently. But, from an evolutionary point of view, it seems unlikely that different families of snake have miraculously invented a structure so complex as the fang on separate occasions.

Ever since the advent of Darwinian theory scientists have wondered about this question, and it has thus been a major evolutionary puzzle. We turned to embryonic development in order to find an answer.

We looked for expression of a gene – sonic hedgehog – that is a tell-tale marker for embryonic teeth. To our surprise, we found that the front fangs begin their development in the back of the embryonic mouth and gets displaced forward to the front of the mouth by the rapidly-growing embryonic jaw bones. This is highly suggestive for an evolutionary origin of the front fangs in the rear of the mouth.

Furthermore, we found that in rear-fanged snakes, the fang develops from a separate tooth-forming tissue (dental lamina). In humans, the teeth in our upper and lower jaw develop from one continuous tooth-forming tissue. The teeth in front of the rear fang in snakes develop from another, separate tooth-forming tissue, so the rear fangs and front teeth are developmentally independent. We made 3D microscopic reconstructions of the development of rear and front fangs and found it to be strikingly similar.

After we mapped these findings over a molecular phylogenetic tree, our data suggest that fangs evolved just once, in the rear of the upper jaw, from their own separate tooth-forming tissue. This means that, during the evolution of advanced snakes, some rear teeth were uncoupled from the front teeth, after which the rear teeth were allowed to evolve independently and without constraint from the front teeth. They evolved into fangs in close association with the venom gland. It was as if our wisdom teeth would suddenly start to develop from their own tissue, separate from the rest of our dentition.

From that point on, different families of snakes evolved different lifestyles and ways of attacking prey. Some prey animals are best dealt with using fangs at the front of the mouth, others by using rear fangs. Front-fanged snakes, like cobras and vipers, independently lost the teeth in front of the fang and evolved a front-fang venom delivery system. But, as we show, these apparently different systems have a common developmental and evolutionary origin.

So, instead of going to all the trouble of re-inventing the fang in a new position, snakes just tinkered with it in the easiest way possible: by changing its position in the jaw during embryo development. In this way, the fang could be deployed in the best location to function as a weapon.

Snakes are very succesful animals. During the Cenozoic era (which covers the past 65 million years) they have colonized virtually the whole planet. Snakes inhabit almost every conceivable ecological niche; from the tops of trees, to the oceans, the deserts, and even temperate climate countries like The Netherlands. The uncoupling of the rear from the front teeth early in the evolution of advanced snakes may have contributed to this major snake radiation, resulting in the diversity of snakes seen today.

Understanding evolutionary processes is important if we wish to understand the natural world that we live in. It contributes to our knowledge of how and why organisms have become the way they are, and to what changes and adaptations have been used in the processes. People only protect what they love, and only love what they understand. So understanding the natural world and how it evolved can have a good positive influence on conservation efforts. Snakes have done some amazing things during their evolution, none the least the elongation of their bodies, and the evolution of venom glands and fangs, and explaining how these events might have happened will hopefully result in a better appreciation of snakes. If this results in better conservation efforts, we might be saving some new medicines, since each snake population might have a usefull venom compound. You never know. But once a population goes extinct, their venom is lost for eternity. So, protecting snakes can also have a positive influence on mankind.

Our study shows how natural selection can remodel an ancestral pattern into something more sophisticated and specialized. Natural selection changes the adult body plan by tinkering with embryonic development. In a sense, therefore, the control panel for morphological evolution lies in the embryo. Some snakes have modified their embryonic development by tinkering with the ancestral rear part of their dentition, and came up with one of nature’s most sophisticated natural bio-weapon systems. Our study also shows how natural selection can cause major phenotypic change by uncoupling events in the embryo. The uncoupling of structures may be a perfect way for evolution to free structures in the embryo from constraints that would otherwise hold it back, providing it with more freedom to adapt and evolve separately from other structures. It is as though the control panel in the embryo has aquired a new set of control switches for modifying the rear teeth.

A good analogy would be with computers, where the embryo is the hardware and the genetic controls are the software. Evolution produces a software upgrade. So pyhons have version-1 software which has a single menu bar to control all the teeth at once. Advanced snakes could be thought of as having upgraded to version-2 software which includes a new menu bars for controlling the development of the front and rear teeth independently.

The rear-teeth in snakes would have had a major constraint from the front teeth, since they developed originally from one tooth-forming tissue (as in pythons and boas, non-fanged snakes, and as seen in our own dentition). By having the rear-teeth develop from their own tooth-forming tissue, they were allowed to evolve separately and in close association with the venom gland, finally forming the fang-gland complex.