Episode 68 – Evolution of Eyes

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Vision is so important in the lives of animals around the globe that it’s difficult to imagine a world without it. But sight has a long and mysterious evolutionary history, from simple light-sensitive cells to the complex organic cameras we humans have in our faces. In this episode, we discuss some of the most incredible innovations and unanswered questions in the Evolution of Eyes.

In the news
In New Zealand: the world’s largest parrot!
Also in New Zealand: the world’s largest penguin!
The world’s oldest discovered RNA!
In the Jurassic, crinoids (sea lillies) could spend years rafting across the ocean.
(For a video version of the parrot and RNA news, check out this episode of SciShow News)

Eyes

The evolution of sight transformed the animal world. Armed with functioning eyes, animals could interact with their environment in whole news ways. This may even be one of the evolutionary innovations that fueled the evolutionary arms race of the Cambrian Explosion.

Eyes are specialized organs that detect light. Cells called photoreceptors receive light, causing proteins called opsins to send electrical signals to the brain, which interprets the signal. Eyes come in many forms, from simple patches of photoreceptive cells that can tell light from dark, to “cup eyes” that can capture light from many directions to form a vague image of the world, to “camera eyes” that are equipped with lenses to capture high-resolution visual data.

There are two kinds of camera eye. The first, called simple eyes, are what we generally see in cephalopods and vertebrates, including humans! These consist of a retina (patch of light-receiving cells), cornea (covering over the eye), and a lens (to focus light). The other kind, called compound eyes, are found in arthropods, and include multiple lenses and corneas and sometimes multiple retinas all working together to create an image. Image modified from Wikimedia Commons, originals by Rhcastilhos and Jmarchn (left), Bugboy52.40 (middle), and Kils (right).

Eye evolution

Early in the history of evolutionary study, some people considered eyes to be too complex to have evolved through natural selection. But in modern organisms, we see an amazing spectrum from simple to complex eyes, which may serve as a model to investigate stages of eye evolution.

This image shows the hypothetical evolutionary steps that could have led to the modern camera eyes we see in most active animals. Image by Matticus78 from Wikimedia Commons.

Eyes don’t fossilize often, especially our squishy vertebrate eyes, so there is little fossil evidence for eye evolution. The earliest eyes in the fossil record date to the early Cambrian, and it seems both compound eyes and simple eyes had already evolved by then in arthropods and early vertebrates. This means early eye evolution may have largely taken place in the Precambrian, where fossils are extremely rare. Some research suggests that fully-functioning camera eyes may have evolved very quickly, in under a million years, which would make it even less likely for the fossil record to capture.

Genetic research does suggest that the genetic foundation for animal eyesight evolved very early on in animal evolution. Eye development in many animal groups is heavily controlled by the Pax genes known as Pax6.

Occasionally fossils are preserved well enough to actually show details of their eyes. Here are some examples to explore:
Early Evolution of the Vertebrate Eye—Fossil Evidence
Fossil insect eyes shed light on trilobite optics and the arthropod pigment screen.
Mineralized rods and cones suggest color vision in a 300 million-year-old fossil fish.
An exceptionally well-preserved Eocene dolichopodid fly eye
400 million-year-old sensory structures in the compound eyes of trilobites

The topic of eyes is a vast one. For more information feel free to check out these extra links:
Animal Eyes – An amazing source of information. This text book provided most of the notes for this episode.
15 Crazy Animal Eyes – Gives a lot of great examples of the variety of animal eyes.

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If you enjoyed this topic and want more like it, check out these related episodes:

• Episode 70 – Convergent Evolution
• Episode 121 – Brains
• Episode 88 – Evolution of Teeth

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