India�s population

Seriously fertile river valleys and Deltas.
Historicly, the Ganges/Indus/Brahmaputra river valleys/Deltas region is where the larger portion of the population of the subcontinent (still is really). The envirmomental conditions therein also facilitated the development of urbanised civilisations, much like everywhere else.

It should be noted that population growth rates in India are most probably higher now than they have ever been, so there wasn't so much of an explosion (other than the initial urban revolution, but thats the same everywhere else in the world), but a more gradual increase over time.
But basicly its the same as what happened in parts of China, Mesopotania, and just about every other river valley civ, the rise of urban civilisation resulted insome degree of political centralisation and the development of more intergrated regional economies etc.
Technology is argueably the least important ingredient, and serios development there off tends to come after the initial urban revoution (smaller percentage of people working on fields, more diverse economy, established learning instituions of some sort etc.).

India's population= too many people.

Many sources claimed ancient India had 2nd largest population after China. There were probably over 50 miillions people in India in Medieval time (my Indian friend told me this).  So, what was the cause of this population explosion?

Second largest when? Round about 0 AD Rome and China had the largest populations, both around 60 million. India at that time wasn't a single entity, but its combined population would have been a bit lower from what i know. Earlier when united under the Mauryans, it would easily have been in the top 3 (along with China and one of Alexander's successor empires most probably, no idea in what order).
I think 50 million might be a little low for the whole of the sub-continent during the medieval peroid, but again, when, 1000 AD? 1400 AD? Makes a difference.

What population explosion and again when? Explosion implies peroid of rapid population growth, and, before the 1800s, India's population had been gradualy growing at a fairly stable rate since ancient times.
Towards the end of the 1800s, the whole of the sub-continent (Pakistan, India & Bangladesh) had a population of around 400 million, and now the 3 combined have a population of  1.35 billion, thats a population explosion.

Thats for the whole pla, but it does show fairly even growth untill the late 1700s when it starts to grow more exponentialy.

Originally posted by Bosnjo India needs like other 3rd World Countries a 1 Baby Policy.

The 1 baby policy is a good idea, but it wouldn't work because so many people will consider it un democratic. However I do not agree that India is a 3rd world country, that is a short sighted argument, based on the poorer countryside areas compared to the cities, which are booming and the economy is growing due to the Indians emphasis on education in urban areas. The boom is caused by illiterate people who have plenty of food (there has never been a famine in India) who believe that just by having loads of kids that they can somehow work their way out of poverty.

The term is obsolete anyways, the cold war ended over a decade ago.