Rods can respond to individual photons, that is pretty much accepted. The issue is what can we perceive. The first nerve synapse screens out about 50% of isol

Rods can respond to individual photons, that is pretty much accepted. The issue is what can we perceive. The first nerve synapse screens out about 50% of isolated photons (ones that are not correlated from other rods) which has been estimated from modeling. Which means that some of those singular photons are getting through (perceived) and can be isolated from the background noise through careful experiments. I don't think we evolved to see single photons, but that we evolved to be very sensitive to light while reducing the noise and that sweet spot means that we can quite possibly, under ideal conditions, see individual photons slightly more than we think we see non-existent photons.