NASA / Reuters For most of the time that humans have existed, our ranks have grown really, really slowly. There were an estimated 4 million people on Earth in 10,000 BCE, and after the following 10 millennia, the planet-wide population had only reached 190 million. Even in 1800, the total number of humans was still under 1 billion. The climb since then—made possible by advances in medicine, sanitation, and food production—has been astounding. By […]