Letter: Whose eyes are they?

From Terry Bollinger

Gleefully and unrepentantly quoting out of context, I note from the section on the eye in “Evolution’s greatest inventions” (9 April, p 28) that “It was once believed that the eye had evolved independently on up to 65 different occasions. But new genetic evidence suggests that it only happened once,” and “The first eyes appeared…in a group of trilobites”.

All these years of mourning the extinction of the trilobite, only to discover that I’m a direct descendant of one.

The editor writes:

• It does seem contradictory to state that the genetic evidence suggests a single origin for eyes, and then to say that the first animals to evolve eyes were trilobites. However, the two claims can be reconciled. It all depends on your definition of “eye”.

A few years ago geneticists discovered that eye development in animal embryos is almost always initiated by a gene called pax6, which like many developmental genes is found in many different lineages. That led to the idea that eyes all date back to a common ancestor with a prototype eye. That is probably true, but the prototype was very simple – one photoreceptor cell and one pigment cell. Various descendants of this organism – including trilobites – went on to discover unique ways of improving on this prototype to create fully fledged eyes with lenses. So in a sense, eyes evolved both once and on numerous occasions.

Ashburn, Virginia, US