Hi Dhruba, All this happenned not in a single day but thousands of years. The continents shifted not due to earth going soft or heating up of atmosphere, it is primarily due to continental drift and this is happenning even now to all the continents in the world. India is moving in the northest direction at a velocity of ~5 cm per year thrusting Himalayas and thatswhy Himalays is continuously rising. Coming to the connections of continents, africa, India and Australia slowly moved northwards splitting from Antartica and the subsequently got splitted themselves. But even today the boundary between Indian and Australian plates is not well defined and hence speculated to be on one plate namely Indo-Australian plate. The land mass between India and Australia would have got submerged gradually. But few hundreds of thousands years back, there was a huge landmass between these continents and gradully got sunken in the ocesn. Even the previouslye existed sea was Tethys sea and not Indian Ocean. With this I want to point out that the changes in continents are happenning due to continental drift(non stop process-a secret of earth's life unlike other planets) and not due to overheating of earth or atmosphere.
