I was recently made aware of a structural adaptation in the eyes of nocturnal mammals that has me awed. Similarities in this system remind me much of what I’ve read on trilobite vision.
What I mean by structural adaptation is one that is more or less morphological, rather than histological. Within the primates, its a common trend that the most nocturnal ones tend to possess only rods in the back of their eyes, rather than cones. Rods, which we also possess, are useful for low-light vision and cones are essential for color discrimination. Humans and Old World Monkeys have three types of cones (they are trichromats), New World Monkeys save for howlers (trichromats) and owl monkeys (monochromats) generally have two or three types depending on their sex and luck of the draw, and the prosimians are generally dichromatic with a number of monochromats interspersed (tarsiers may actually possess the ability to see ultraviolet in their peripheral field, but they’re weird).1
Lemurs and lorises, to the exclusion of tarsiers, monkeys, and apes also possess what is called a tapetum lucidum, which is a reflective layer at the back of the eye which bounces back onto the retina and amplifies what little light there is. This is the shining light you see when you look at your cats at night. As far as vision goes in most biological anthropology classes, this is typically the end of the story, but it turns out there is more.
The evolutionary trajectory described here is somewhat similar to what happened in trilobites. Trilobites, which lived 500-250 million years ago, possessed a unique visual system unlike anything else seen since. Although their eyes were quite variable, as the trilobites represent a diverse group of arthropods, they were generally compound as in insects, but were actually solid and made of calcium carbonate. This feature has allowed their visual apparatus to preserve pretty well in the fossil record, and hence we know a lot about trilobite visual systems.
Generally, trilobite vision worked like this: many cylindrical calcite lenses arranged hexagonally typically formed each eye. These lenses were able to focus light through their structural arrangement straight down each column which eventually hit a visual receptor at the end. The structure of the column was such that even if light did not hit it straight on, it could still efficiently be focused directly onto the receptor with ease. Since they could not change the shape of their eyes to focus them, each lens was constructed in such a way that they allowed objects to be focused at both near and far ranges (some of our advances in modern optics have in fact converged on the trilobite pattern which developed over a time span of millions of years).2
Although not entirely the same, nocturnal mammals developed a similar system. Like any other cell, the rod cells of each vertebrate contains tightly packed DNA in its nucleus. Within the nucleus, your chromosomes are packed into at least two forms: heterochromatin (very tightly packed) and euchromatin (less tightly packed). The function of euchromatin is in DNA expression, while heterochromatin for the most part is important for the maintenance and preservation of DNA. In nocturnal mammals, though, heterochromatin has taken on another role.
The heterochromatin in nocturnal rods is tightly packed in the center of each nucleus, acting as a microlens through which light can pass and be additionally focused, and euchromatin has been pushed to the walls of the cells.3 This structural adaptation is present in most nocturnal mammals such as mouse lemurs, mice, and even your cat, but is absent in diurnal primates and animals like cows. It’s quite clear, that like the calcite eyes of trilobites, that in an attempt to get as much light as possible into the retina, that the eyes of nocturnal mammals have adapted to take advantage of the natural physics of light.
Is this convergent evolution? Not necessarily, although the idea that the mammalian eye, composed entirely of non-mineral tissue, would find a creative way to structurally focus light in such a way using only what it has available to it is, in evolutionary terms, really cool. Although research in this area is continuing (this was only really found in 2009), it may help to elucidate the natural history of the bizarre tarsier (a nocturnal primate which evolved from diurnals) and the owl monkey. Similarly, perhaps there is something to learn for modern optics in the structural layout of nocturnal eyes, much like what may have been figured out by observing trilobite lenses.
1Hendrickson, A., Djajadi, H.R., Nakamura, L., Possin, D.E. and Sajuthi, D., 2000. Nocturnal tarsier retina has both short and long/medium‐wavelength cones in an unusual topography. Journal of Comparative Neurology, 424(4), pp.718-730.
2Fordyce, D. and Cronin, T.W., 1993. Trilobite vision: a comparison of schizochroal and holochroal eyes with the compound eyes of modern arthropods. Paleobiology, 19(3), pp.288-303.
3Solovei, I., Kreysing, M., Lanctôt, C., Kösem, S., Peichl, L., Cremer, T., Guck, J. and Joffe, B., 2009. Nuclear architecture of rod photoreceptor cells adapts to vision in mammalian evolution. Cell, 137(2), pp.356-368.