ABSTRACT
Life on earth is estimated to have begun some 3.5 to 4 billion years ago. From the earliest formation on earth from a cosmic cloud, it took hundreds of millions of years for conditions to stabilize enough for organic molecules to accumulate. Conditions in the earliest days on earth were unsuitable for life itself. The atmosphere was devoid of oxygen and was made up of mostly hydrogen, ammonia, methane, and water vapor. This combination offered no shield against the sun’s radiation. The „rst cells might have been formed when hollow spheres of self-sealing fatty membrane coalesced around groups of self-replicating molecules. Because it allowed selective control of the environment, it was the „rst critical step in the evolution of complex cells. For some 2 billion years, simple unicellular microorganisms were the only form of life on earth. Some of these early cells developed the ability to photosynthesize, giving off oxygen as a waste product and in time producing an oxygen rich atmosphere. Around 1.5 billion years ago, more highly developed cells, eukaryotic cells, appeared. From these cells, which have a nucleus and complex internal structure, evolved the single cell protozoa and algae and all multicellular life.
