I've just described the way that the outer membrane of a single-celled organism could, by reacting to vibrations in water, act as a form of basic vibration detector - or ear - allowing even this incredibly simple organism to develop the capability of being able to "hear".
The same process was at work for other stimuli too. As well as developing the ability to react to vibrations, simple organisms were developing the capability of reacting to light.
How could such an amazing facility arise - how did creatures harness the ability to react to such a mysterious and immaterial thing as electromagnetic waves? And how, over the eons of evolutionary time, did some creatures eventually expand this ability into the seemingly incredible capability of being able to see? How did some of them end up with eyes? The whole process is much simpler than you may think.
The phenomenon of being aware of light seems mystifying to us partly because we tend to think of light as a rather other-worldly, ethereal and insubstantial thing - however, its ability to affect physical objects is reasonably straightforward.
To us light has a sort of magical quality (to the extent that one of the memorable phrases at the beginning of the Bible is "Let there be light"). This is largely because we have eyes that utilize light in such a complex and wonderful way, and as a result we tend to forget that light is just a normal part of the physical universe like everything else.
Now, to understand how creatures react to light, and how they developed eyes in a relatively straightforward manner, follow this description of how you, as a person, experience the energy that reaches you from the Sun.
When you stand in the Sun you experience solar energy in two very different ways - as light and as heat.
In terms of their basic nature, the heat from the Sun is almost exactly the same thing as the light from the Sun - they are both waves of energy in the form of electromagnetic radiation. The only difference between the heat and the light is the wavelength of the radiation (See Figure 4 near the beginning of the book). The light has wavelengths between 400 nanometres (4 billionths of a centimetre) and 700 nanometres. We see the 400nm light as violet and we see the 700nm light as red, with the wavelengths in-between being seen as the other colours. The radiation that has wavelengths longer than red is invisible to us, but we are aware of the presence of some of it because it is what we experience as heat. This radiation is part of the range of wavelengths known as infrared radiation (infrared meaning below red). The Sun emits radiation at many more wavelengths than those of visible light and infrared radiation, but only these ones, along with radio waves, can penetrate the earth's atmosphere and reach us on the ground.
Now, we as humans don't give heat from the Sun the same semi-mystical status that we give to the light that it emits (despite the fact that heat-seeking holidaymakers are often called Sun worshippers). This is because the main way that we detect the heat from the Sun is as a (usually) rather pleasant but vague physical sensation on our skin, while we experience light via all of the complexity of our sense of vision. Not only does the whole concept of heat seem more mundane than does that of light, but the underlying physical processes that allow us to detect it seem more mundane too.
These are the processes.
When waves of infrared radiation strike atoms, the energy from the waves is transferred to the atoms, making them vibrate. Which makes them hot. That's what heat is: moving atoms. And that's about it. Infrared radiation is generated in the first place by atoms moving rapidly because they are hot - so when that infrared radiation in turn strikes an object and makes the atoms in that object hot in turn the whole process is simply a form of transference of energy.
When we stand in the Sun on a nice day we can tell where the Sun is in the sky even if we've got our eyes covered, purely due to the heat that we feel from it. The infrared radiation from the Sun makes our skin hot - and specifically it makes our skin hot on the side that's facing the Sun, the side that's struck by the infrared radiation. You notice this especially when you keep one side of your body directed towards the Sun for an extended period of time, such as when you're sunbathing.
Our entire skin is like a full-body heat detector. However, and very importantly, the skin isn't a specialized heat detector dedicated to only that one function - heat detection is merely one of the roles that it plays, along with, amongst other things, holding our insides in. (There are strong parallels here with the outer membrane of the single-celled organism described earlier - where the membrane kept the contents of the cell in place and was also capable of detecting vibrations in the water and thus of "hearing".) Now, although your experience of heat when you're out in the Sun is quite generalized, you've probably noticed that the parts of you that truly "face" the Sun are much more subject to the effects of heat than other parts - for instance your shoulders or the back of your neck are affected more than your elbows. The same applies to the top of your head if you're bald. This is because the infrared radiation from the Sun is hitting these areas full on, while it's hitting your elbows at an angle. Heat that strikes a surface at an angle is spread out over a larger area, and thus its intensity is diluted (Figure 62). This is one reason why the heat from the low winter Sun is weaker than the heat from the overhead summer Sun.
When any organism is exposed to the Sun the parts of it that directly face the Sun get hotter than the rest of the organism: areas that are at an angle to the Sun are heated up less intensely, while a large proportion of the organism is exposed to no heat from the Sun at all as it's in the shadow created by the organism itself. As a result, different parts of the surface of any organism that's in the Sun are at noticeably different temperatures. If the organism possesses a means of registering those temperature differences it has a way of detecting the direction that the heat is coming from, or in other words, of detecting the direction of the Sun.
Here in Figure 63 is a hypothetical, simple spherical organ-ism. It could be a tiny single-celled organism or it could be a larger though essentially simple multi-cellular organism - perhaps one that for some bizarre reason was the size and shape of a football. It's easy to see that even an organism with such a simple structure may be able to detect the direction of the Sun quite precisely if it has the means to translate the temperature differences on its surface into a useable sensation.
Although the simple spherical life-form depicted in Figure 63 is potentially capable of detecting the direction of the Sun, life-forms that are more complex in shape than this have an even greater potential for doing so, as the complications in the shape provide more opportunities for the organism's surface to be exposed to different amounts of heat.
Such organisms don't have to be that much more complicated in shape though. Take, for instance, an organism that is spherical but that has a single bump on it. The bump provides a second set of curved surfaces that are presented to the Sun at different angles, thus increasing the organism's sensitivity to the position of the Sun. On top of this the bump also creates a shadow area that would help to reinforce the effect. As a result, any organism with a bump on its surface will be better at detecting the direction of heat than one without bumps (This doesn't only apply to simple organisms such as the hypothetical one described here - it applies to you and me too. A protrusion such as your nose would be a good example of such a bump).
The principle that extra curves on an organism's surface make it more sensitive to the direction of the Sun doesn't only apply to bumps and protrusions - it applies to dents as well (Figure 65).
A low bump or dent would improve the accuracy of detecting the direction of the heat from the Sun, while a more pronounced bump or dent, with a greater curvature, would improve the accuracy further, as the greater the curvature the more localized the effect of heat on the surface, and the greater any shadow area, thus the greater the sensitivity.
Although I'm talking about living organisms here, it's important to realize that for an object to be affected by heat the object doesn't have to be alive. After all, anything that's exposed to the Sun heats up. The surface of an inert object that's the same shape as the living organisms just described, with all of the attendant bumps and dents, would heat up in exactly the same way, with different parts heating up to different extents. The crucial difference between the way that an inert object and a living one react to heat is in the potential complexity of the reaction, in that the living organism may register the effect as a physiological sensation.
So far I've been talking about the way that an organism is affected by, and is sensitive to, heat - however, the very same principles apply to the way that it is affected by light. This isn't surprising: light is, after all, almost exactly the same thing as radiant heat except that it has a different wavelength.
Whenever light strikes an organism (or indeed anything else for that matter) it agitates the molecules on the surface, in very much the same way that infrared radiation does. The molecules will be agitated even if the organism doesn't "notice" the fact that they are being agitated.
For an organism to become sensitive to the agitation caused by light it needs to evolve a method of registering it as a sensation, very similar to the way that the agitation caused by infrared radiation is registered as the sensation of heat.
Some of the molecules on an organism's surface will happen to react to light in a manner that is relatively easy for the organism to notice, and these molecules will be the ones that are "monitored" for their effect - let's call them light-sensitive molecules.
Bear in mind that these suitably light-sensitive molecules on the surface of the organism are not there because they are noticeably affected by light - they just happen to be there, and their reactivity to light is just a random property. There may indeed be similar light-sensitive molecules scattered throughout the whole organism, and indeed throughout the surrounding environment, but the light-related properties of these particular molecules is redundant and is not harnessed. In primitive organisms these light- sensitive molecules would possibly be distributed randomly and evenly over the surface of the organism (because they are just there). Therefore, when the molecules are affected by light hitting them the resulting "monitored" sensation that they produce could be a vague, rather undefined, sensation that's analogous to the vague, undefined sensation that we experience as heat.
In many situations the ability of an organism to detect light even in this vague manner would give the organism a distinct advantage over its fellow organisms, and it would thrive, thus creating offspring that were also capable of detecting light in this way.
The more accurately an organism could tell which direction the light was coming from, the more of an advantage it would have. I'm still talking about relatively low levels of accuracy here, possibly analogous to the level of accuracy that you yourself have when detecting the direction of an electric heater by using your heat-detecting skin alone (with your eyes closed). This level of accuracy would allow organisms to detect the direction of approach of a possible predator, due to the moving "shadow" that the predator would produce (similar to the way that, using the electric heater analogy, you can detect the presence of a person if they move between you and the heater).
As with an organism's abilities to detect heat, it's more accurate to detect the direction of light on curved surfaces - and the more curved the surface the greater the accuracy. Organisms that had bumps or pits on their surface would be better at detecting the direction of light and shadows than their smoother surfaced cousins (as in Figures 63 to 66 above), and would thus thrive relative to them.
Let's look more closely for a moment at the organisms that sport pits on their surfaces, ignoring for now those that have bumps.
An organism with a deep pit (as on the right in Figure 66 above) would be able to detect the direction of light much more accurately than would an organism with a shallow pit (Figure 65) - resulting in deep-pitted organisms thriving in comparison to shallow-pitted ones.
Over time and over generations the pits would get deeper and more recessed, as in Figure 67 below, purely because deeper pits make for more acute and directional light detection and are thus more propitious to survival.
Yes, that deep pit in Figure 67 does look suspiciously like an eye, doesn't it? However, this "eye" is nothing more than a recess in an organism's body: it doesn't, for instance, have a lens.
The recess would differ from the rest of the organism's surface in that it would contain a relatively high concentration of light-sensitive molecules compared to the rest of the surface.
Why would this be? It sounds somewhat fortuitous or contrived to say the least. How could such a seemingly deliberate accumulation of light-sensitive molecules just where they are needed come about? It's because in nature, as in most things, nothing is uniform, and there would be a natural tendency for the light-sensitive molecules on any organism to be distributed at least slightly unevenly over the organism's surface, whether the organism had a recess or not. Over the course of the evolution of the recess, as it got deeper over the generations, those organisms that had more light-sensitive molecules inside the recess (due to pure random distribution) would thrive more than organisms that had fewer, because they would be more sensitive to light: they would fair better and would reproduce more, and thus the tendency for a concentration of light-sensitive molecules in the recess would be reinforced.
Eventually an organism would exist where the recess was so deep and round that it was practically closed over, leaving only a small hole through which the light could enter. This would provide the greatest directional accuracy of all for detecting which direction light (and shadow) was coming from, so creatures with this feature would thrive.
Very usefully, when light passes through a very small hole such as the opening in this recess it doesn't simply fall on the surface behind as a diffuse glow. The restriction caused to the light by the small hole is so great that light from any given direction only hits quite a small area on the inside of the recess, with the cumulative effect being that an actual image is mapped out on the surface. This happens without the benefit of a lens, using exactly the same principle that's at work in a pinhole camera. The resulting recess is thus a basic, image-forming eye - fashioned purely by an accretion of minor variations that reinforce each other.
From here it's only a matter of time before the descendants of the organism develop the "circuitry" that could analyse the image that's formed on the inside surface of the recess. Similarly, it's only a matter of time before the "pinhole" at the top of the recess becomes covered by a thin, transparent membrane that would usefully act as protection against contaminants entering the cavity and clogging it up. It would then be only a matter of time before this transparent membrane acquired a variable thickness - and became a lens. The recess changes from being a pinhole camera into a digital camera.
And voila - the eye! The eye is such a useful and surprisingly simple adaptation that it has developed in different creatures along completely independent pathways. Each path possibly diverged from the same light-sensitive patch of molecules on a very early, shared ancestor, but from that point they followed different routes. One such route was that taken by the insects, with their alien-like compound eyes, while another route was taken by the cephalopods, the family of aquatic creatures that includes the squid. Cephalopod eyes work in a very similar way to our own, even though the route by which they were arrived at was different, as can be deduced by anatomical differences between their eyes and ours (In fact the squid's eye is in some ways superior to ours, as ours have various things such as blood-vessels inconveniently interfering with the path of the light).
You may remember that at the beginning of this description of the development of the eye I mentioned that an organism would be able to detect the direction of light more effectively if it had light-sensitive molecules that were either in a pit or on a bump. You've probably noticed that I've just described the evolution of the eye from the pit - so what about the organisms that had light sensitive molecules on bumps? The truth is that although at the very basic level bumps were better than smooth surfaces for detecting the direction of light (in very much the same way that they were better for detecting the direction of heat), they were also, unfortunately, something of a dead end in terms of potential further development. Crucially, there was no way that a bump could develop into a pinhole camera (and then into a camera with a lens). The bump route to true vision was very much a blind alley. As a result no organisms that used bumps for light detecting developed truly functioning eyes from their bumps.
So it is that a few light-sensitive cells could evolve, through simple stages, into an organ as amazingly sophisticated as the eye. Partly as a result of its seemingly extraordinary (though strangely ordinary) evolutionary pathway, the eye has recently developed an interesting extra function that lies beyond its light-sensing capabilities - that of being the perfect model of the process of evolution by natural selection at work, as attested by its inclusion in numerous popular science books and internet sites.
