For thousands of years, natural factors like rainfall and temperature helped determine the fate of economies and societies. For thousands of years, humans also engaged in human trafficking and kept one another as enslaved people. But as human prosperity increased exponentially beginning in the 19^th century, it may have seemed that such concerns were relics of the past.

Years ago, Mohamed’s family had enough to eat, despite being poor. His daughter owned a vegetable stall at a bustling market in northeastern Nigeria. The family had options: during the dry season, when Lake Chad was shallow, Mohamed could farm; and during the wet season, he could fish or graze his cattle.