On 24 July, 1999 (estimates vary), the human population of Earth reached 6 billion people. This level has been achieved largely by a process known as 'nookie', but there are other significant contributing factors:
Farming, an activity which involves massively inflating the populations of certain co-existing lifeforms for human food purposes, while eliminating all animals and plants that are not tasty enough.
Disease control, which is a never-ending struggle for control over other organisms in the champion's league of life.
Medicine, which keeps humans healthy and fertile for longer and longer.
The fact that Earth has not been hit by an asteroid or comet in the recent few million years.
The Earth's human population doubles every 35 years or so, meaning that in a mere 100 years, the population of our planet will be somewhere in the region of 40 billion souls - about 8 times the current population. This translates into a population of about 520 million for the UK, and roughly 2 billion for the USA. China, the world's most populous country, will have a population of over 11 billion people. And you thought it was crowded now!