The questions that the Hindu sages were asking around 1000 B.C. were quite advanced—this is apparent from a reading of the eighteen Puranas, especially the Markandeya Purana, which is the oldest, and has Sage Markandeya as its central character. Here’s an excerpt from the Markandeya Purana in which enquires are being made about cosmology, genealogy, evolution, and geography:
“How did this universe, both moveable and immoveable come into existence? And how will it fall into dissolution at the proper time, most excellent priests? And how came the families that sprang from the gods, the rishis, the pitris, created things, etc.? And how did the Manvantras occur? And what was the history of the families of old? and whatever creations and whatever dissolutions of the universe have occurred; how the ages have been divided; and what the duration of the Manvantaras has been; and how the earth remains stable; and what is the size of the world; and what are the oceans, mountains and rivers and forests according to their situation; what is the number of the worlds, the bhur-loka, svar-loka, etc., including the lower regions; and what is the course of the sun, moon and other planets, of the stars and heavenly bodies also. I wish to hear of all this which is destined to subversion; and what will be the end when this universe is dissolved.” ~ (The Markandeya Purana, translated by F. E. Pargiter, 1904, Canto 45.9-14)
The word “Manvantras” in the above passage means a cycle of the universe—every Manvantaras repeats 71 Chatur Yugas (world ages), lasting for 306,720,000 years. According to tradition, Vayasa, the legendary writer of the epic Mahabharata, is the compiler of all the Puranas. It's impossible to have the exact date of the Markandeya Purana, but most modern scholars place this text between 550 B.C. and 1000 B.C. The eighteen Puranas consist of around 400,000 verses—the Markandeya Purana is believed to have 9000 verses, but most surviving manuscripts show only 6900 verses.