The Greeks were the first half-bloods and had famous heroes like Hercules. This is when the Olympians were first worshipped, so that is where they had power. Later on, the greeks were conquered and eventually, the Romans ruled the area. This led to the beginning of there being roman half-bloods. They both share the same gods, just in two slightly different forms. They both go on quests, live dangerous lives hunted by monsters, and eventually started to form small half-blood settlements. We know this because, in the books, Chiron explains it to those at camp-half blood figure it out. Jason is from Camp Jupiter and New Rome and as he regains his memory, he also confirms its existence. We finally know for sure when we follow Percy on his journey to the camp.
