What’s the good of half an eye?
A common argument, among those who wish to debunk evolution, is the argument from irreducibly complexity. The idea is that, if there is a complex organ – like an eye – and that there is no value in having a slightly less complex version of the same organ, then it cannot have evolved and must have been designed. The eye is the most often quoted organ, partly because Darwin himself, in The Origin of the Species, said
…to suppose that the eye… could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree…
But most critics, when they quote that line, forget to read the rest of the chapter where he describes in detail how the eye could have evolved. If anyone is still wondering what use an extremely primitive eye might have, consider this recent experiment from the science journal Nature via the excellent Neurophilosophy blog.
In todayâ€™s issue of Nature, former collaborators of Boyden report on a similar method, based on another photosensitive protein, called NpHR. The authors cloned the NpHR gene from N. pharaonis. The gene product is a protein called NpHR, which works in the same way as ChR2 – it is also a light-activated chloride channel. Photoactivation opens the channel, allowing an influx of chloride ions into the cell. Like ChR2, it can be used to knock out single action potentials in cultured cells. But the authors of the new study also describe experiments in which they expressed the protein in living nematode worms (Caenhorhabditis elegans), and used it to control the wormsâ€™ behaviour.
This film clip shows the use of the system to control swimming behaviour in a nematode worm. When a pulse of light is directed at the worm, it activates the NhPR protein; this hyperpolarizes the motor neurons and inhibits their activity. The swimming movements cease about 600 milliseconds of illumination. Switching off the light pulse inactivates the protein, so that the motor neurons resume sending action potentials to the muscles. Illumination with yellow light is indicated by a yellow dot