Before life on Earth, the atmosphere contained chemicals such as methane, hydrogen, ammonia and water. Chemical reations took place in the sea, sugar molecules joined together to form starch and cellulose; amino acids formed proteins. When sugars and nitrogen compounds combined they formed DNA, an oganic chemical capable of reproducing itself.

Mycoplasms are the smallest known free-living cells. They are related to bacteria but lack a cell wall, which means they can change their shape and can pass through fine filters that would retain bacteria. Some are saprophytic (they live on dead or decaying organic material), others are parasitic, living on mammals and causng pneumonia-like diseases. 

The Great wall of China, which can be seen from space, is over 3,400 km long - three times the length of Great Britain. It was built over 2,000 years ago.