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Comment by wolfhumble | Hacker News Reader

He starts his lecture assuming: "suppose we start with an ancestor who didn't really have an eye at all but just a single sheet of light sensitive cells . . ."

This seems like a very basic condition, and that going from 0 to 'just a single sheet of light sensitive cells' is almost nothing. But of course that is not the case. Before that you would need (+++):

1) Photoreceptor Proteins 2) Functional nervous system or signal processing pathways 3) A machinery to translate the absorption of light into an electrical signal 4) A system for coordinating these signal with other parts of the organism . . .

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Single cell organisms can be light sensitive. Can move away or towards light. The signal cell erythropsidinium even has an "eye":

https://en.wikipedia.org/wiki/Erythropsidinium

Yes, these organisms are really complex even though they are unicellular.

I'm not sure that's correct for very basic multicell organisms. I'm no biologist, but I believe primitive systems wouldn't need these types of advanced pathways when light sensitive cells could just produce signaling compounds to drive the behaviour of the rest of the organism without necessarily requiring a centralised nervous system, akin to how single cell organisms orient themselves in their environment.

This is correct. Rhodopsin, for example, is a G-protein coupled receptor, which means that activation of the receptor triggers G-proteins inside the cell which can activate or inhibit any number of cellular processes.

I'm having trouble figuring out what your objection is.

If we're going to study an evolutionary process, we have to pick a starting point. "How an eye evolves from a single sheet of light sensitive cells" is particularly relevant because creationists like to claim that "half an eye isn't useful, so it couldn't have evolved".

"How does a single cell evolve light sensitivity" is not a particularly hot button topic. Creationists don't drone on about plants.

I'm not sure about that. At least in my experience there are two types of creationists. The ones who have integrated a non-belief of evolution into a core component of their religious beliefs. This type isn't going to listen to any argument for evolution so I don't know why time should be spent making arguments for them.

The other flavor are the ones saying "wait a second microbiology is really complex, I'm not sure the standard evolutionary process cuts it here."

Saying, "well let's just assume microbiology isn't a problem", isn't going to be compelling to them.

It is an interesting endeavor, but with respect to the whole creationism vs evolution discourse it feels like a complete waste of time.

Creationists don't argue things are generically "too complicated", they argue that there are structures in nature that aren't discoverable by progressive mutations. As Dawkins explains in the video, eyes are an extremely commonly cited example. I've never heard anyone argue that cells couldn't evolve photosensitivity, but I have heard plenty of people argue that eyes only work when they're fully functional, which is what Dawkins is explaining as untrue here (as is TFA). I don't find the idea of debating creationism especially likely to be fruitful overall, but answering the question "How does evolution explain the structure of eyes?", e.g., for an adolescent who is grew up in a fundamentalist household, seems entirely reasonable to address.

Aha! But you have neglected steps as well, fetid ignoramus!

You portray such a +++ animal as simple and obvious, but indeed in fact you would need (++++++):

1) adherins and cadherins to bind this sheet together 2) an organelle to produce and regulate the cellular proteins 3) a method to encode those proteins 4) a method to replicate and propagate that encoding so that new proteins can be encoded 5) start and stop codons and a method of duplication or recombination errors to allow for new copies of protein encodings to be created separately from old copies...

You could have 2 - 4 for other sensory cells, like heat or touch, and then be a single mutation away from sensing light.

Iirc, some snakes have some heat sensing cells arranged in pits to give them a directional heat sense. If you did that with visible light instead of IR those pits would be what halfway to an eye would look like

> sensory cells, like heat or touch

Like light sensitive cells these would need a similar 1 - 4 +++, so the point would really be the same.

Not at all... If the question is "how could an eye evolve" it's reasonable to start from a baseline of a complex organism that has sensory cells but lack eyes or even light sensitive sensory cells.

A cell sensing light can use the same type of nerves etc as other sensory cells, so there is no need to explain how a cell sensing light + a nerve + a central nervous system evolves in one step.

If you wonder how a nerve could evolve for example, that is a different question. If you have two cells and they can exchange chemical signals, which is useful in itself, you have the start of a nerve.

In the book that talk is based on (I think it might be The Blind Watchmaker?) he says (paraphrasing) that he's using the example to illustrate a process, and the exact start and end points don't really matter. If you prefer an earlier starting point, that's fine. Start with a single cell that can respond to light, or even earlier. The process still applies.

If you're going to make that objection, you might as well head straight up the chain and aim for the biggest example of all: how does the first self-replicating molecule appear?

The point still stands that evolving an eye is going up a long gentle slope not climbing a tall cliff.

I disagree with your presumptions.

For exmaple, for #3, you say electrical signal. But cells normally use chemical, not electrical signals.

For #1- nope, just need an enzyme or other thing that makes a pigment molecule, and something that can detect a chemical (re-use of an existing protein)

For #2- no nervous system, and cells already have many internal signal processing systems.

For #4- those already existed.

Biology mostly just copies and re-uses systems that already exist. It's still incredible there was a path of mostly random events that led to fully formed eyes, though.

You practically get the nervous system for free. If those photo sensitive cells serve some other purpose, you get them for free as well.

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