It took 200,000 years for humans to reach 3 billion in number. It took 40 years for the next 3 billion.
11
Early American humans hunted car-size armadillos and used their shells for houses.
12
An average heart beats 100,000 times a day, pumping some 2,000 gallons of blood through its chambers. Over a 70-year life span, that adds up to more than 2.5 billion heartbeats.
73
A person can live without food for about a month, but only about a week without water.
156
Pet dogs and elephants are the only animals that seem to instinctively understand human gesture.
14
On average, there's 100 billion neurons in the human brain.
83