Geologically

Formation of contents, subcontinents, rivers, etc have been investigated by various well known geologists and accepted by historians and the present day intelligentsia.
Geologists have systematically studied various periods in which land formation consolidated on the Earth and the Earth broke up in to various continents in millions of years. The singular land mass broke up into northern and southern plates hundreds of million years ago and then the southern plate, which was called “Gondwana” broke up about 170 millions ago, into western and eastern sub plates. The East Gondwana consisted of the Antarctica, Indian sub continent and Madagascar broke up about 120 millions ago. While the Indian part of Gondwana began moving north, Antarctica began moving south, opening up the Antarctic Ocean. The Indian plate moved north and collided with the largest land mass in the North called Asia about 45 million years ago. Himalayan mountain ranges are part of that collision. So geologically, the Indian subcontinent is separate from the Asian continent.

Probably Pakistan and Sri Lanka may be geologically considered part of the Indian sub continent, but Nepal, Tibet and Afghanistan are the high altitude regions in between the larger Asian land mass and the smaller Indian sub continent. That is the logic of the geographical of division of these countries. Myanmar was definitely not part of Indian subcontinent. The border between India, Tibet, Nepal, Myanmar and Afghanistan should at the Peaks of those mountain ranges, the other side belonging to them and the Indian side belonging to India. The border between India and Pakistan was a political creation at the time of both these countries became free of British colonial rule.

Human Migration and DNA evidence

This is more complicated to understand. The earliest of human beings is said to have originated in the Tanzanian valley of Africa about 70,000 years ago and completed about 40,000 years ago. DNA of the inhabitants of these countries mentioned is not identical. They have branched off several steps down. I am bored. I will write in more detail about the DNA of the Indian people later