Why blind snakes have eyes

Why does a blind snake have eyes? No, it’s not a joke, it’s a potentially puzzling scientific question—which an understanding of evolution can help answer.

Blind snakes are small, non-venomous creatures that spend most of their lives underground. They feed on the eggs and larvae of ants and termites, and rely on their sense of smell to get them from A to B. Look at a blind snake embryo, however, and you’ll see that it has eyes—just like a regular snake. The blind snake’s eyes reduce in size over the course of the foetus’s development. By the time it hatches from its shell, the blind snake is pretty much, well, blind.

How do we explain these odd features? They don’t seem to be terribly useful—in fact, eyes can be a point of vulnerability if angry ants decide to defend their nests by attacking them. However, if blind snakes and sighted snakes share the same ancestor, the presence of eyes on the former isn’t such a mystery.

Related organisms go through similar developmental stages as each other, with species sharing common ancestors also sharing the same features as embryos. Evolutionary change can only happen through the gradual modification of existing structures. So it’s not uncommon to see features in the developing embryo of an organism (such as eyes) that, later in its development, become modified (for example, reducing in size) or disappear.

It’s not only in blind snakes that the developmental stages of the embryo resemble the embryonic stages of other closely related organisms. Some ancient characteristics in our fishy ancestors, like gill slits, are present in human embryos but disappear during development. Human embryos even have a tail in the early stages of foetal development, in common with other species with whom we share ancestors and ancient genes. In humans, the protruding tail normally disappears by the time we are born, but occasionally a mutation will result in a baby born with a tail.