Natural selection only works in complete living things. It cannot account for the adding of eyes, blood and brains etc in premeval life.
I hope you understand that primordial life was complete and living, and therefore the subject of natural selection.
Also, no one (that I know of) is suggesting that a full brain or eye sprung out of a pool of unicellular organisms - that is ridiculous.
But some unicellular organism do have organelles for detecting light. And nematode have a few cells that act as a very rudimentary eye.
I think one problem here, and with the IC arguments, is that the proponents ask for a mechanism whereby an eyeball could assemble from nothing. That is not what evolution predicts - but rather a progressive specialization of a structure like the eyespot on a nematode...
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