Hinduism is a very ancient religion, and neither Hinduism nor the caste system has remained static over this long period.

The earliest mention of castes is found in the Vedas, which categorize people into four groups (later called varnas) - priests, warriors, farmers/merchants, and laborers. Some people think of this as the origins of the caste system, but it does not resemble the caste system that followed later, in that:

• there were no proscriptions against inter-caste marriages
• there is no mention of caste being hereditary, meaning there was social mobility between castes
• there are no social or religious strictures against any caste, all castes were entitled to read and recite the Vedas

From what we know of their culture, the Vedic people were not organized by caste or social status, but by tribe and kinship.

The earliest hints of a more rigid caste delineation date to the time of the Manusmriti a collection of Hindu texts that go into detail about dharmashastra, or the principles behind dharma or religious duty. They treat caste much more rigidly, defining a place for everyone in society. People are expected to know their place and live accordingly. It also establishes the inheritance of caste, and proscriptions against inter caste marriage.

The Manusmriti is a collection of texts, and was written over a long period. Most scholars date it to after 500 BC, probably between 200 BC and the turn of the millenium.

Some time after the Manusmriti, its notions about caste became widespread among the population, and the present caste system emerged.

However, there is a lot of argument over exactly how rigid this caste system has been over the last 2000 years since the Manusmriti was compiled. There were (and have always been) various sects of Hinduism that attached no (or little) importance to caste, for example, the Tantrics. But the majority of the population did follow caste strictures.

There is some scholarship that shows that caste distinctions became much more rigid in the last 200 years, starting with British rule, since the British favored certain castes while penalizing others, setting up frictions between them. This is not too different from say the Hutu/Tutsi divide in Rwanda or the Congo, where two ethnically distinct populations had those distinctions markedly emphasized during colonial rule, due to the rulers favoring one group over the other.

The emergence of the caste system in the period of the Manusmriti is also linked to a crisis in Hinduism. This was a period when Hinduism was faltering. About 700 BC, Jainism broke off from Hinduism. Around 500 BC, Buddhism broke off from Hinduism. By 300 BC, Hinduism was under threat. Rulers of vast empires, such as Ashoka, adopted Buddhism as the state religion, and many minor kings did so as well.

This was very important in a period when royal sponsorship of religion was crucial. Huge sources of income for the Hindu priests were now diverted to building Buddhist monasteries instead, or sending Buddhist ambassadors to China and southeast Asia.

It seems likely that the caste system was a response to such threats. The priestly class, suddenly deprived of its revenue, took measures to protect itself. Religious rites and rituals became a province of Brahmins, and Brahmins only. When your kid was born, you needed a Brahmin for the naming ceremony. When someone wed, he needed a Brahmin to perform the ceremony. When someone died, a Brahmin was needed to do the death rites. It was job security, not just for them but for their kids. Caste became hereditary.

There is some evidence for this timeline in genetic studies. Studies such as this one on modern Indian populations show that there was extensive intermixing between populations all across India until about 1900 years ago, when it came to an abrupt halt. After this, endogamy becomes much more pronounced, not strictly according to varna, but according to jati, which is the Indian sub-caste system that determines intermarriages.

So if you translate this to a historic timeline, when Indo-European speaking people trickled into northern India beginning around 4,000 years ago, there were no rigid caste lines. They intermarried with locals extensively, as did all Indian sub-populations. Up until 1900 years ago, which is a few hundred years after the Manusmriti, the time it took for the caste system to establish itself.

As for how much the caste system is part of Hinduism and how much it's a social phenomenon, there is no single answer, because Hinduism isn't monolithic. As I mentioned above, it seems to have social beginnings - an attempt by Brahmins and higher castes to protect their turf at a time when they were losing ground and royal favor to other groups. Various Hindu subgroups throughout the last 2,000 years have both embraced and condemned the caste system. A similar situation exists today, though the trend is very definitely against it. It is, of course, illegal according to law, and has been since Indian independence.