Human Errors #1: The Eye

Nathan Lent’s book on the “design defects” in the human body, Human Errors, is endlessly fascinating. But why write a book on errors when there are so many “working” things? For one:

“By exploring human shortcomings, we can peer into our past. Each and every flaw… tells a story about our species’ evolution.”

 

He starts with the most famous flaw that lies in our eyes:

“The photoreceptor cells of vertebrate retinas appear to be installed backward – the wiring faces the light, while the photoreceptors face inward, away from it.”

BTW, it’s not just us – it’s the same for all vertebrates, from fish onwards. But not all species have their eyes wired this way:

“Interestingly, the retina of… octopi and squid – is not inverted.”

This proves that:

“Nature ‘invented’ the camera-like eye at least twice, once in vertebrates and once in cephalopods.”

 

The problems this arrangement creates for vertebrates are many:

Light must travel around the bulk of photoreceptors, a complicated path;

Even worse, the light has to travel through a “thin film of tissue and blood vessels”, which impact the quality of vision;

The twisted “wiring” makes us our eyes prone to retinal detachment;

Lastly, it results in the famous blind spot (since the wiring loops backwards, it’s inevitable that there’ll be an area where there are no photoreceptors at all).

 

So why did all vertebrates end up with such eyes? Well, once the development of the eye started off with the photoreceptors facing backwards in that common ancestor of all vertebrates, it was not possible to flip the entire structure – that’s too big a change, requiring too many genes to change simultaneously, and that’s not how evolution works. Remember, evolution works via incremental changes…