Being a sloooow reader, I've been reading a book titled " La Pensée antique - des présocratiques à saint Augustin" for at least three weeks, and probably won't be finished with it before next Summer. It is very well laid out, with short chapters devoted to all the philosophical currents that were emerging in ancient Greece before Socrates. I had heard the names Heraclitus and Democritus before, without knowing if they were authors, athletes or philosophers (Greece was busy breeding them like rabbits between 600-400 BC). Now I know !
According to Whitehead, "Modern philosophy is but a footnote on a page from Plato". There are indeed the germs (and stems, leaves and fruits) of just about every philosophical current when one reads about Anaxagore, Parmenides, Empedocles, Thales and Pythagoras.
Nietzsche and Marx, Sartre and Heidegger merely developed what had long been exposed before them (if sometimes imperfectly). The most interesting discovery I made while reading this is that the main difference between now and what had been said 2500 years ago lies more in the evolution of language than of thought. Therefore, as one of the writers(*) comments, philosophy has progressively detached itself from the common sense and capacity of understanding of the mass to evolve in a science, bundle of concepts and language that only fellow philosophers can understand.
This has long been my own contention but I'm happy to see that my intuition is abetted by the Faculty

(*) Sorbonne professor, ancient Greek specialist, Académie française Member and eminent hellenist
Jacqueline de Romilly.