16 — Comparative Analysis of Gene Expression for Convergent Evolution of Camera Eye Between Octopus and Human

Octopuses are amazing, and you should read this book by Sy Montgomery. (Thanks Gracie for suggesting it!)

One amazing thing that makes octopuses so amazing is their amazing eye biology. Their eyes are “camera-eyes” — just like humans’. But the human evolutionary lineage split from the octopus lineage during the Precambrian period, back when we were still mostly-squishy swimming things, which means that human camera-eyes and octopus camera-eyes (and complex brains, for that matter!) arose independently.

One confounding factor is the lack of a camera-eye in animals that are closer-related to humans than octopus, or more closely related to octopus than humans. For instance, no insects at all have a camera eye, even though octopus and humans are phylogenetically more distant. And some molluscs lack a camera eye even though they are very closely related to octopus and very unrelated to humans.

This paper takes a deep dive into the genetics of the camera-eye, and establishes that many of the genes responsible for constructing such a complex eye probably existed long before the eye itself existed. In that way, both of our branches of the Tree of Life had access to these genes, but only some species, such as vertebrates and octopus (plus some other sorts of mollusc) wound up using those genes (for still unknown reasons).

Not mentioned too much in this paper is how dramatically better-designed the octopus eye biology is than humans’: You’re probably aware of the existence of the human retinal blind-spot, but octopus eyes don’t have that flaw: Whereas human photoreceptors (the cells that actually receive the light) paradoxically point backwards, and receive light only after it passes through a thick layer of tissue, octopus photoreceptors point outward, and receive a direct light-source. This means their optic nerve lives behind their retina, while human optic nerve fibers aggregate in front of the retina and punch a hole directly through it (blind spot!) to reach the brain.