What evolved first, sight or hearing?
While getting tangentially involved in an unrelated conversation about the science behind evolution yesterday, I ended up getting this great question from J. William Runnells yesterday:
@BehavEcology I have a question. Do we know what evolved first: The eye or the ear?
— PrometheusEpimetheus (@JWRunnells) September 12, 2013
Now, I got all excited and gave it some thought and read a bunch of stuff and even went to the library 1 to respond to this, but before I do, a disclaimer is in order:
This is a complex question, and not something I’ve done work on myself. I welcome additions, corrections, and even complete overhauls from people who work in this field.
That said, let’s give it a whirl.
The first issue is, what do we mean by the question? I asked for some clarification.
@JWRunnells Depends on what you mean by "eye" and "ear", to start. The the first "eye/ear-like" structures? Final versions? Physical form…
— Steven Hamblin (@BehavEcology) September 12, 2013
@JWRunnells … or the sensory ability to process light/sound?
— Steven Hamblin (@BehavEcology) September 12, 2013
He wrote back and said that sensory ability is what he was interested in. This complicates things a little, in a greatly interesting way. Why? Well, animals have many ways to sense, process, and use information from the environment in the form of light or sound. For eyes, this ranges from a few sensory cells situated in a small depression to the elaborate camera-style eyes that we have as humans (and which are greatly exceeded by many animals).
Hearing is similarly complex, and can range from simple hair cells that are triggered by the pressure of sound waves, to the complex ears we use to navigate the world today (the internal machinery of which got its start, among other places, as a gill structure in fish).
All things being equal, to answer the question of when the ability to use or respond to light or sound first evolved is going to be difficult to answer because the earliest and simplest forms of sensory processing apparatus wouldn’t leave much in the way of a fossil record (especially since it was likely to have been in simple uni- or multi-cellular life forms). So, I’m going to spoil the ending right here and say that I have no idea which came first. I even asked around my department and got a variety of head scratchings and tentative, off-the-record opinions. I welcome more informed opinions on this. But, in the meantime, that’s not to say that we’re completely without knowledge.
Putting aside the use of sunlight in photosynthesis, a good candidate for the first photo (light-sensitive) receptors in the tree of life hinges on a family of proteins known as the opsins, which actually change shape when hit by light, forming the basis for proton-pump mechanisms that move protons around in response to light (a common biochemical trick). In particular, as Ivan Schwab notes in his fantastic book Evolution’s Witness2, bacteriorhodopsin or proteorhodopsin (or a predecessor of these proteins) are strong candidates for the first photoreceptive machinery. This wouldn’t have been an eye, by any means, at least not in the way we think of it. It would probably have been a membrane protein in some unknown member of Archaea, shuttling protons around in response to errant photons.
Another candidate for this early photoreceptor is a class of enzymes known as photolyases, which are used to repair DNA. These enzymes repair damage in response to ultraviolet light, which as anyone who’s paid attention to the warnings about sun exposure in many years, can causes cellular damage which would be catastrophic to the early forms of life. A very closely related set of proteins known as the cryptochromes get the nod as a potential for an early photoreceptor because of their widespread adoption in pretty much every kingdom and their use as a regular of photoperiodicity (which you probably know as circadian rhythm).
How long ago did these proteins and enzymes arise? Obviously we won’t find tiny fossilised proteins any time soon, but innovative work in molecular biology can at least put some sensible age ranges on these things. For instance, this paper by Rivera et al. from last year in the Journal of Experimental Biology pegs the origin of one of the blue-light-receptive cryptochromes as being at least 650-800 million years ago, putting a lower bound on earliest receptors. Feuda et al., also from last year in PNAS, clock the diversification of opsins at about 700 million years ago. Given that these age ranges are in line with the first members of Metazoa, it’s not too surprising to see action in the evolution of these proteins around then. But the first ‘witness’, as Schwab describes it, could have been anywhere before that time, in the early members of Archaea or Bacteria stretching back up to about 3 billion years ago.
What about the use of sound by early life? I’m a lot more in the dark here3. As this piece from the Max Planck Institute for Neurobiology discusses, the use of simple hair receptors that would be sensitive to sound waves is phylogenetically old, back to at least the earliest vertebrates some 500 million years ago. But while cilia take my vote as the best candidates for early sound receptors, I can’t find anything great on how the use of sound might have arisen in, say, ciliates. It seems likely that the use of cilia to translate sound pressure into electrical signals let early life forms respond to sound in an adaptive fashion, but I can’t find anything on whether this has been studied in any great detail; most of the work on the evolution of hearing begins with the early vertebrates and invertebrates when specialised structures for sound become common and elaborate.
So … I don’t know.
But that’s not something to be upset about, it’s something to be celebrated. We already know an awful lot about the use of light and sound in early life forms and how both hearing and sight evolved over hundreds of millions of years, but there’s also so much more to learn! And learning more about how sight and hearing evolved is a great way to expose yourself to the beautiful power and endless tinkering of evolution as it jury-rigs, invents, and engineers4 its way to the diversity of sensory apparatuses that we see in the wild today.