Our author says that this book is an account of history. I take him at his word. Now let us see what this history is about.
Homo Sapiens are wanderers by nature. Instead of staying in Africa where they evolved, they decided to explore other lands in the world. As they wandered and settled in different regions of the world, they changed due to different environments and genetic mutations. This book tells us that Time, with capital T, is the cause of these changes. This is the Time that Kṛṣṇa identifies with in the Bhagavdgītā.
Some of these wanderers came to India in multiple waves and following different routes. The early wanderers are said to have come to India over 50,000 years ago. This was not a monolithic group, but consisted of many different tribes and cultures. We will meet some of them in this book. One of these groups eventually created the Indus Valley Civilization.
More waves of these wanderers came to India following much longer routes, having first reached and settled in Europe. They shaped the landscape in Europe and created many cultures there over thousands of years. Wander lust caused some of them to leave Europe, in multiple waves again, and move to other lands in Asia, including India. There they encountered others who had settled in India about 50,000 years earlier. What happened as a result of these encounters is the stuff of history that is described in this book