This is the fourth edition of the acclaimed work first published nearly twenty years ago. In this significantly expanded edition, the authors summarize the progress to date and set the stage for future research by incorporating recent discoveries in natural history like the Toba explosion 75,000 years ago. This allows them to show Vedic ideas and language to be late Pleistocene and early Holocene evolutionary phenomena that made their way to Europe some 10,000 years ago leaving their imprint on the languages and cultures of Europe. This provides a scientific explanation for the 200 years-old puzzles of why Indian and European languages are related. All these make the present edition the definitive work covering the discoveries of the last three decades and the starting point for all future work in the area following the collapse of the Aryan invasion myth.