[books] snippets from today's chapter of An Immense World

There are thousands of different animal opsins, but they are all related.* Their unity creates a paradox. If all vision relies on the same proteins, and if those proteins all detect light, then why are eyes so diverse? The answer lies in light’s distinct properties. Since most light on Earth comes from the sun, its presence can hint at temperature, time of day, or depth of water. It reflects off objects, revealing enemies, mates, and shelter. It travels in straight lines and is blocked by solid obstacles, creating telltale features like shadows and silhouettes. It covers Earth-scale distances almost instantaneously, offering a fast and far-ranging source of information. Vision is diverse because light is informative in a multitude of ways, and animals sense it for myriad reasons.

* In 2012, evolutionary biologist Megan Porter compared almost 900 opsins from different species, and confirmed that they share a single ancestor. That original opsin arose in one of the earliest animals and was so efficient at capturing light that evolution never conjured up a better alternative. Instead, the ancestral protein diversified into a wide family tree of opsins, which now underlie all vision. Porter draws that tree as a circle, with branches radiating outward from a single point. It looks like a giant eye.