The growth of our human population is extraordinary. For more than two million years, humans lived with no permanent home, finding plants to eat and hunting animals for meat. Then, just 10,000 years ago, we invented agriculture. At that time, there were only about five million humans, but this fgure quickly doubled. The population reached a billion in 1805, and since then it has multiplied seven times.

Câu hỏi

The word “it” in paragraph refers to _______.