I believe the answer is: increase in food supply and disease immunity.
Prior to 2000 years ago, society experience massive shortage in food supply which cause a lot of people died due to hunger which always keep the population low.
The fact that there is no vaccine also contribute to the low population because it cause a lot of death for infants and children before they grow up.
Explanation: Karma is when you die it will determine if you are going to go to be reborn or not, Dharma is a persons duty, reincarnated is going one life to the next and caste is how they dived there people starting off with Brahmins which are the priest, teachers(grus) then the Kshatriya which were the warriors ,rulers then the Vaishyas which were the merchants and landowners. Then the Sudras which were just servants then at the VERY bottom was the untouchable's which had the WORST jobs that the uppercases people didn't want such as cleaning the streets etc.
I hope this helps I just used my notes from last year lol.
One question is this a test
King,
We should start a colony for all those pesky prisoners and all those men that owe debt. We could finally get rid of them. I shall call it Georgia.
Best Regards,
James Oglethorpe