This is a fun and pretty accurate Youtube video on how different animals (vertebrates and invertebrates) visually process the world. There is so much to learn from the evolution of the eye and we here on Webvision are always happy to see basic science and the science of vision being communicated to the wider public.
Trilobites were one of the most successful marine arthropods that lived from the Early Cambrian throughout the Devonian, finally going extinct in the Permian ages, a run of over 270 million years. They are well represented in the fossil record and even the earliest forms had complex compound eyes much like modern arthropods. These eyes had elongated lenses composed of calcite that modeling has revealed to provide excellent optical properties with good depth of field and little to no spherical aberration. These lenses brought light to photoreceptor cells at the base of the lens, but we’ve never before had an understanding of what that structure or anatomy looked like.
The problem of course with the fossil record is that very little internal structure remains in fossilized specimens. However, a very cool new study that examines trilobite eyes through X-ray tomography by Brigitte Schoenemann and Euan N. K. Clarkson reveals how these cells looked, down, perhaps to the cellular level. Followup work with μct-scanning and synchrotron radiation analysis reveals that the sensory structures (like rod or cone outer segments) are arranged in flower petal like structures around a central, diamond shaped photoreceptor cell body with pigment granules packed in-between. Its kind of like a modern limulus eye (image here).
It will be interesting to see if they can image other species of trilobite to get an evolutionary look at how eyes and perhaps primitive retinas developed over 500 million years ago.
Image Credit: Bryan William Jones, Ph.D.
We at Webvision have a fascination with the evolution of the eye and are always looking for interesting papers that help describe from where, how and when vision came. This paper, Molecular analysis of the amphioxus frontal eye unravels the evolutionary origin of the retina and pigment cells of the vertebrate eye by Pavel Vopalenskya, Jiri Pergnera, Michaela Liegertova, Elia Benito-Gutierrez, Detlev Arendt, and Zbynek Kozmika attempts to answer the question of where the vertebrate eye came from. While the amphioxus has for many years been recognized as a viable candidate for the earliest vertebrate eye, the retinal structure is different from that of other vertebrates. Specifically, the photoreceptors of amphioxus are simple ciliated cells as opposed to the more sophisticated elaborations of structures on top of cillia in other vertebrate retinas. This paper describes in molecular terms, gene coexpression and structural features the different cell types of amphioxus in an attempt to define neuronal circuitry. Its a very cool paper that provides additional detail into the evolutionary origins of vision and we encourage you to have a look.
Image Credit: Hans Hillewaert from Wikipedia