The Indian population is a result of four major migrations, including that of the Aryans , into the country in prehistoric times, says a new book Early Indians : The Story of Our Ancestors and Where We Came From" looks at how and when modern humans first arrived in India; what evidence they left behind; who their descendants are today; who else followed them as migrants to this land; how and when farming started and the world's largest civilisation of its time was built; when and why this civilisation declined; and what happened next.Based on recent genetic studies using ancient DNA as well as findings from disciplines such as archaeology and linguistics, journalist Tony Joseph writes that these migrations, including that of the Aryans from central Asia, were part of global population movements that affected not just India but many regions of Asia and Europe.The first modern humans arrived in India around 65,000 years ago as part of an Out of Africa migration that populated the entire world ultimately. The genetic lineage of these first migrants that the book calls ‘ First Indians ' still dominate the Indian population and accounts for 50-65 per cent of the Indian ancestry today.The second major migration happened 9000 to 5000 years ago, when agriculturists from the Zagros region of Iran moved into India's northwestern part and mixed with the First Indians and helped speed up the farming experiments that were already beginning in the subcontinent.As a result, farming - especially of barley and wheat - spread like wildfire across the northwestern region, thus laying the foundation for the Harappan Civilisation that in its mature phase lasted from 2600 BCE to 1900 BCE.The Harappans, therefore, were a mixture of Zagros agriculturists and First Indians, the book claims.The third major migration happened from southeast Asia around 2000 BCE, when farming-related migrations originally starting from the Chinese heartland overran south-east Asia and then reached India, bringing the Austroasiatic family of languages, such as Mundari and Khasi spoken in the eastern and central parts of the country.The last major migration between 2000 and 1000 BCE brought central Asian pastoralists, who spoke Indo-European languages and called themselves Aryans, to India.The scientific evidence for all of these migrations - for long the subject of vigorous debates among both scientists and lay persons - is now coming from analysis of ancient DNA, or DNA collected from the skeletons of individuals who lived thousands of years ago.By looking at DNA from the same location at different periods, or from different locations at the same period, geneticists can work out which populations moved where and when.India's current population mix also includes many minor migrations that happened during the historical period, says the book, published by Juggernaut, though these did not leave a significant genetic impact.'Early Indians' uses the metaphor of a pizza to explain the Indian population structure today. The First Indians form the base of the pizza, it says, since their ancestry is present in all population groups, no matter what region they live in, what caste they belong to, or what language they speak.Then comes the sauce that is spread all over the base of the pizza - the Harappans, who moved all over the country in search of new, fertile land when their civilization declined around 1900 BC.The rest of the migrants are the cheese, the capsicum, the tomato and other vegetables spread on top of the pizza, though not in a uniform manner - some pieces have a larger share of one and less of the other.Apart from the formation of the Indian population and the development of the Harappan Civilisation, the book also looks into when and how the caste system began.Genetic evidence suggests that the caste system did not begin with the arrival of the ‘Aryans', Joseph says, adding that it began nearly two thousand years later, around 100 CE, probably due to a shift in political ideology at that time.