The smallest eyes we’ve discovered are about three microns in diameter. If you stacked 35 of them on top of each other, you’d have a pile about as thick as a sheet of paper.
Synechocystis PCC 6803 is a strain of small, photosynthetic, freshwater bacteria which serves as a model organism for studying a wide range of phenomena. One of its remarkable behaviours is its habit of moving towards light. If a cell finds itself near a light source, it pushes out an extension towards the light and drags itself forward. Precisely how Synechocystis knows which way the light is has been a puzzle for some time.
Many kinds of bacteria move towards or away from a chemical; they know which way to go by measuring the gradient of the chemical across the length of their body. A similar strategy might work for sensing light; the cell could sense the difference in the intensity of light passing through it and so judge which way the light is. An international team of researchers estimated how strong this effect would be and found that a single cell would absorb at best 20% of the light going through it — in other words, the light would be at 100% intensity on one side of the cell and 80% intensity on the other. That might be enough for the cell to work out a direction, but it’s not what the researchers think is going on.
When the team looked at Synechocystis cells under a microscope with light coming from the side (rather than the bottom), they noticed an intense light spot on the side of the cell opposite the light. This led to the hypothesis that the spherical cell acts as a microscopic lens, focusing light on the opposite side from the light source. They tested this idea in several different ways and found good evidence that the cell focuses incoming light to create a strong signal and then moves away from the focus point (or towards the light). (The paper is open access, so have a look at it if you’d like more details about the experiments.) In their model, “essentially the Synechocystis cells functions as a microscopic eyeball, with the spherical cell body as the lens and the cytoplasmic membrane as the retina” — a unicellular eye 3µm across.
In the Origin of Species, Darwin famously referred to the complexity of the eye, an organ of “extreme perfection and complication” which seems to have become a favourite of creationists. The intricacy of the eye makes it difficult for some people to imagine how it could have emerged by the gradual accumulation of small changes. From the outset, Darwin argued that the case depended on showing the existence of “numerous gradations” — eyes of varying complexity, from simple light sensors to complex organs with lenses. Not only have these numerous gradations been discovered, but we also know that eyes evolved several times in different lineages, from the familiar human eye to the “camera” eyes of octopuses and the bizarrely beautiful eyes of mantis shrimp.
Lack of imagination isn’t the only problem afflicting conceptions of evolution. Far too many people, whether laypersons or biologists, are drawn to “just so stories” about evolution. These stories are logical, compelling, and memorable, so they spread readily and gain lots of credibility in popular intellectual culture. The problem is that these stories aren’t as well supported as they are compelling, so we end up with a general idea of evolutionary processes and evolutionary history that is often based on what seems sensible rather than what has been conclusively demonstrated. Of course, what “seems sensible” depends on a range of social factors, from the cultural to the personal; as a result, “just so stories” can easily end up serving and reinforcing prevailing ideologies, world views, or social arrangements.
Reference
Schuergers, N. et al. (2016) Cyanobacteria use micro-optics to sense light direction. eLife 5:e12620. doi: 10.7554/eLife.12620
