how did earth change 2.5 billion years ago when organisms began using photosynthesis to make food?

Answer 1

Not sure with the geologic time scale but archean rocks registered disapperance of banded iron formation signaling that organisms there used the oxygen.

Answer 2

I believe this had to do with free oxygen becoming available in the atmosphere. Prior to this time life had to exist under the sea, as a protection from the harmful UV radiation from the sun. It is possible that oxygen poisoning might have led to the destruction of plant life, so animal life evolved to take advantage of the oxidizing potential (called Eh, in Chemistry). It took some time before life could venture out onto the land, about another 2 billion years (I think).