He wrote the book "" (2002), which explains how genetic data has been used to trace human migrations over the past 50,000 years, when modern humans first migrated outside of Africa. According to Wells, one group took a southern route and populated southern India and southeast Asia, then Australia. The other group, accounting for 90% of the world's non-African population (some 5.4 billion people as of 2014), took a northern route, eventually peopling most of Eurasia (largely displacing the aboriginals in southern India, Sri Lanka and Southeast Asia in the process), North Africa and the Americas. Wells also wrote and presented the 2003 PBS/National Geographic documentary of the same name. By analyzing DNA from people in all regions of the world, Wells has concluded that all humans alive today are descended from a single man who lived in Africa around 60,000 – 90,000 years ago, a man also known as Y-chromosomal Adam.