I'm studying ancient philosophy now. It's interesting that in the 5th-4th centuries BCE, ideas that would later appear in world religions and political systems had already been articulated.
For example, Plato's idea of the Demiurge as a creator is quite close to the biblical narrative - in both a man was first created after a women. He also had his own version of a socialist system, according to which the ruling class (philosophers and warriors) should own no property, money, or families - neither men nor women. The lower classes, however, may have all these things.
It's also interesting that Pythagoras believed in reincarnation and was a vegetarian for that reason. The first recorded idea of saṃsāra and karma appeared in Upanishads in India earlier, in the 8th century BCE.
It's also worth noting that the Christian ethic of humility was preceded by the ideas of the Stoics, who taught that an ascetic life elevates the spirit.
In short, the Buddha Gautama said that the Dhamma (the teaching) has always existed, and the role of past and future Buddhas is to rediscover the eternal teaching for people who have forgotten it.
