How We Came to See Clearly
Both Near and Far
Piqued by the current polemic over Darwinian evolution versus “intelligent design,” I consulted texts on ophthalmology to refine my understanding of the visual mechanism of accommodation. Its evolution, I thought, might seem more complex than nature alone could explain. Was it divine guidance? Surely there cannot be a convincing natural scenario.
My inquiry led to no such conclusion. I believe it can be shown how accommodation may have evolved in plausible steps, given billions of years and genetic changes―mutations, induced by reproductive errors and random radiation.
The human eye in repose is focused on the horizon. For close vision, the lens is adjusted by certain structures and functions. In other words, the eye accommodates to closer distance by self-focusing. How did this remarkable mechanism come about?
In the beginning, mutations at a nerve-ending on the head of a sightless aquatic creature, say a “sea worm,” engendered a light-sensitive area. This contained cells equipped to initiate a signal that enabled the brain to see blurred shapes. The creature could avoid obstacles but could hardly tell prey from foe. Still, it was fitter than its unchanged kin.
The next “uphill” mutations (most are “downhill,” useless and eventually lost) resulted in a gelatinous lens with a pocket behind it. These allowed a clear image to fall on the light-sensitive cells. After many eons, an eyeball formed; and the light-sensitive cells became embedded in a membrane, a retina, lining the back of the eyeball and connected to the brain by an optic nerve. As to the order of other additions―muscles to move the eye, a protective cornea, an iris to control the admission of light―embryology may offer clues. “Ontogeny recapitulates phylogeny.”
Since our primeval creature lived in the murky sea, distance focus would have been amiss. The lens, to elucidate the visible, had to be more convex (more magnifying) than distance vision would later require.
In time the aquatic creature gained lungs and legs and took to the land. Ability to see far objects would now be advantageous. There was need for a way to change the power of the lens. Further mutations yielded a lens-enclosing capsule and surrounding muscles to stretch it. The lens could now be flattened, making distant as well as near vision possible.
A more efficient mechanism, however, would at length win over. Our terrestrial’s eye, to see distant objects (now the greater need), had to be in tension. Subsequent mutations corrected this. A muscular ring, the ciliary muscle, formed around the entire lens structure. This could override the capsule-flattening muscles, which in time became mere fibers (zonules). Contraction of the ciliary releases tension on these fibers, letting an elastic lens thicken for near vision; whereas relaxation of the ciliary draws on the fibers and flattens the lens for distance. This change, which survives in humans today, allowed mammals to watch the terrain constantly without eye fatigue.
One is tempted to point out that intelligent foresight might have resulted in a simpler mechanism. Suppose the lens itself mutated, assuming a shape suited to distance vision. A ciliary muscle would need only squeeze to enhance near vision, making other structures (capsule, fibers) superfluous. Either such a mutation never occurred or somehow proved inefficient. Perhaps evolution simply missed an opportunity.
A flaw in the present mechanism shows up in most humans at about age forty, when accommodation fails and glasses are needed. Elastic lens implants may soon be available to correct this. Or evolution may solve the problem within a few million years.
Speculation on evolution, far from idle, aids scientists today. Witness the discovery of endorphins: analgesic hormones secreted by the brain. Scientists asked why evolution would produce brain receptors for opiates? Answer: it wouldn’t. So there must be natural substances that go to those receptors when pain threatens survival. The search for such substances disclosed a new group of hormones and may lead to new drugs.
“Intelligent design” is religion’s latest assault on science. The appeal to divinity in explaining natural phenomena recalls the state of alchemy, where superstition muddled research for centuries. The accelerating progress of science in modern times has been a major force in raising the human condition. To undermine science teaching by interjecting religion could only blunt this force and augment the confusion and mischief of “creationists.”