For almost a thousand million years after its formation, there was no known life on earth. The first simple sea-dwelling organic structures appeared about 3,400 years ago; they may have formed when certain chemical molecules joined together. 
Prokaryotes, single-celled micro-organisms such as blue-green algae were able to photosynthesise and thus produce oxygen. A thousand million years ago sufficient oxygen had built up in the earth’s atmosphere to allow multi-cellular organisms to proliferate in the Precambrian seas (before 570 million years ago). About 363 million years ago, the first amphibians crawled to the land. Around two million years ago the first humans appeared on earth, known as Homo sapiens (Modern humans). This is the early sign of life.
Amra Ramlan
Grade 8
Middlesex College
Negombo