Definition of life

The origin of life can be approached through four interlinked questions:
1	When did life first appear on Earth?
2	What were conditions like when life first appeared?
3	How did life appear?
4	Where did life first appear?
It is useful to review what is meant by ‘life’ and the probable nature of the first life, as well as to consider the essential requirements for life, which we will do in Sections 2.1.1 and 2.1.2 respectively. Sections 2.2–2.5 then consider in turn each of questions 1 to 4.