Plato was the first to posit the existence of sunlight. Observing that 1) it was dark when the sun was below the horizon and 2) it was light when the sun was above the horizon, he hypothesised that sunlight was a fluid located underground, which is drawn upwards as high as eye level by the sun's massive gravity. Aristotle disputed this theory, claiming that Plato was a dick. Socrates argued a case between these two extremes, saying that sunlight was caused by gravity and that also Plato was a dick. Plato is reported to have responded to these criticisms by gritting his teeth, muttering "Fine!" and going off to sulk.
Preoccupied with conquering the known world, the Romans were far too busy to muck about with sunlight, and copied the entire concept wholesale from the Greeks, changing the names as necessary to sound less, you know, Greek.
In the middle ages, Plato's views lost popularity. Medieval philosophers argued that sunlight was actually the light of burning vampires who'd been caught outdoors at dawn. This view is currently being revived by the SCA.
When Galileo argued that sunlight was in fact light coming from the sun, the Pope ordered him to investigate this personally. The Vatican thus built a giant catapault to hurl Galileo into the sun. Galileo then announced that he'd come up with a new theory, namely that the Pope was a dick. This theory was taken up by Martin Luther, with disastrous consequences.