I’m trying to imagine all of the properties of a human coming into being step by step over time.
How does the eye evolve? I hope this is not a bugaboo question that creationists ask. I’m not one of those.
How do we imagine steps or stages wherein the eye (or other complex functions) evolve? They must be mutations which enhanced survival. What mutations advance an animal towards an eye, when he is without them? What stage or initial conditions must be met, beyond being a one celled amoeba?
What is an example of a possible “first” mutation from “Eyes do not exist” towards an eye?
It is a creationist Greatest Hit, but even Darwin was flabbergasted (cite to Wiki Evolution of eye), although I doubt it prompted any loss of faith in what he was doing.
Basically some proteins showed up that react to light, called opsins, which “react” to light in different ways than “merely” turning into energy for the rest of the organism.
The development of further cellular connections refining that reaction (where is the light, how much, what to do with it for the organism) came later.
And further refining of the nervous system geared just for those little suckers (retina, lenses) took a bit more time.
The refracting, “transparent” portions of the eye are crytsalline molecules sucked into the mix and found good to go.
ETA: I see I didn’t give a simple direct answer to the OP, on what may have been the earliest examples of why these molecules, etc., persisted: it was to carry out the Prime Directive for life: “Find Light,” generally for photosynthesis.
“Skin” didn’t come into the picture for aeons, and in event that part of animal anatomy has no bearing whatsoever then or now on the physical or functional aspect of the visual system.
If you mean, instead of skin, “on the outside,” even there today you have complete eyeballs underneath a semi-transparent skin in some fishes.
Neat cite/photo of one spooky looking fish to come…
Back when tiny little simple organisms were swimming around in the ocean developing a light sensitive spot would let them know which way was up or react when anything blocks the light from above. Even with the simplest means of propulsion a connection to the light sensitive spot would help avoid predators. A few million adaptations later and those light sensitive spots and the connection to motion become complex enough for the little buggers to find food, shelter, and spot their own kind for mating.
I think that sleep is somehow tied to the development of eyes since they would have been useless at night. Organisms couldn’t just run in flight mode all night long because the light from above seemed to be blocked, they would have developed a sleep mode to ignore a lot of stimuli.
And: get a rhythm going to meet up with other organisms and mate, or later, particularly with the subsequent evolution of phototaxis actually get somewhere different for a meet up.
From memory, after development of a patch of light-sensitive cells, one of the subsequent developments is an indentation of the light-sensitive patch to form a small pit. The advantage here is the sides of the pit create shadow zones, allowing a rudimentary system for determining what direction the light is coming from. (The fact that the light sensitive cells are at the bottom of the pit is why the light-sensitive cells in the retina of the human eye are at the back of the eye.)
The next step is that the pit gets deeper and the opening gets smaller, allowing the primitive eye to focus in a manner like that of a pinhole camera.
Eventually the opening gets covered over with a translucent membrane which can be used to focus the light (i.e. a lens).
Pretty fascinating. And the stepwise progression is straightforward enough that the eye has evidently evolved independently several times here on Earth.
It is, but it’s easily rebuffed. Creationists espouse a concept called “irreducible complexity,” which states that some physiological features can’t be explained by evolution, since evolution requires that earlier, more simple versions of the feature must nonetheless provide a survival advantage, and they claimed that such complex physiological features do indeed fail to be useful when you take away any one of thier components.
It boils down to a lack of reative thinking: if you ponder it for a bit, you damn well can imagine more primitive versions of the modern eyeball that are still useful to the organism.
Someone (maybe Steven J. Gould?) answered the creationist question “what good is 1% of an eye?” with “it’s 1% better than having no eye”. Light sensitivity, even without the ability to focus an image, is certainly a survival advantage in detecting the approach of predators.
There are other, simpler, uses for photo-sensitive proteins than vision; for examples: circadian cycles, orientation and, perhaps, magnetoreception. Even some bacteria have a magnetic sense. In higher animals, magnetoreception probably uses cryptochrome in the eye. (I don’t know if cryptochrome is considered a type of opsin. IIUC, cryptochromes are present in human retina but with unknown function.)
Anyway, extremely primitive eyes providing only rudimentary detection of light and magnetism would have survival value; and incremental improvements to those eyes would add value.
in what he was doing.