This week’s episode of The Wonders of Life, fronted by Professor Brian Cox, covered the evolution of the senses, including the evolution of the eye.
I first read about the evolution of the eye in the 1980s in one of Richard Dawkins books – probably the appropriately ocularly titled The Blind Watchmaker.
It’s a fascinating subject, because the eye is so awe inspiringly sophisticated (and frankly, quite odd). How could its complex parts have come together to form such a singular apparatus?
I found Brian Cox’s explanation unsatisfactory. He didn’t actually describe how the eye evolved, how one thing led to another. He essentially just demonstrated how the eye worked. If you already knew the story of the evolution of the eye you could follow the reason why he mentioned the aspects of the eye that he mentioned, but if you knew nothing you’d still be in the dark. (And obviously, if you already knew you didn’t need to be told anyway).
My one criticism of the series is the inadequate explanations of many of the principles and processes involved, and this was a good example of that (although his explanation of the migration of the bones of the inner ear from the jaw was excellent!).
So how did the eye evolve?
Well, the answer is that the parts didn’t start out being particularly complex, and they didn’t all come together at the same time.
The eye evolved from something simple into something complex (as indeed did life itself). Here’s that evolution in five easy stages.
1: The first, primeval ‘eye’ was probably nothing more that a few molecules that reacted to light in some way, randomly positioned on the surface of an organism – not really an eye at all, just a few molecules that by chance allowed the organism to react to whether it was light or dark, day or night: not much, but enough to give the organism an edge over its contemporaries that lacked such molecules. Because of that edge the organism that had the light sensitive molecules had a higher chance of survival than its contemporaries that lacked the molecules, and thus it prospered.
2: The next stage in the evolution of the eye was probably the clumping of light sensitive cells in the base of a depression on the surface of an organism. This would give the organism a more refined sense of the direction that any light was coming from. The ‘eye’ wouldn’t be able to see shapes. It would just be able to sense light and shade, a bit like the sensation you get when you close your eyes and pass your hand in front of them to block off any light that’s filtering through your eye lids – you can sense the presence of your hand but you can’t see what it is.
3: The deeper the depression that contained the light sensitive cells, the more sensitive the ‘eye’ was to the direction that any light was coming from. As a result, organisms with deeper depressions prospered in comparison to less deeply endowed organisms, and deep eye depressions became the norm.
Gradually the eye depressions became so deep that they became ‘eye holes’.
‘Eye holes’ that were relatively deep and wide but that had only got a small pin-prick entrance hole on the surface would, by luck, be capable of forming an image in the inner surface of the cavity, using the same principle as the pinhole camera.
4: All that was needed then was a film of transparent skin over the hole to make a nice self contained sealed unit.
5: All that was needed then was for the film of transparent skin over the hole to act as a lens and…
VOILA – THE EYE!
How simple is that? No mystery at all!
Further to that ridiculously brief explanation of the evolution of the eye, here’s more on the subject of the evolution of the eye (and other senses) taken from my snappily titled book, Where Are We, What Are We, Why Are We? (And Why Do We Want To Know?) on the subject of the nature of our existence.
The Evolution of the Senses
As life gradually became more complex, so too did its capability for reacting to its surroundings – with the emergence of what we would regard as fully fledged senses.
I’ve just described [in the previous chapter of the book] 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 our way of thinking, 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 [in the book] – 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 (See figure below). 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 the figure below is a hypothetical, simple spherical organism. 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 the figure above 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 (enhanced by the fact that 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 below).
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.
Hopefully that sounds reasonably straightforward, and with any luck you can relate to it due to your own experiences in going out into the Sun (and possibly getting a bit too much of it on your nose or shoulders).
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 spherical 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).
If you’re finding this interesting you can keep reading here.