.02;

read the link for a detailed explanation but briefly, each step is useful. The light sensitive cells lets the organism be aware of something cutting off light. Such as the shadow of a predator.

Then the cells are more efficient if they are grouped in a cavity.

Then a lens can create an image.

and so on.

Each small step is useful and gets selected. Small steps add up to big changes over time.

That's why the statistical argument is false too. It isn't pure chance; selection is involved. Tossing 20 heads on a coin is astronomical but not if you set each head you get aside until you have 20 of them.

Roger