The first non-cellular life forms must have been originated $3$ billion years ago in the form of giant molecules like $RNA$, Protein and polysaccharides inside a self replicating metabolic capsule.

These capsules must have later reproduced their molecules.

The first cellular form of life originated about $2000$ million years ago.

These might have been single cells formed in aquatic environment only.

This theory of abiogenesis i.e. the formation of first form of life arose slowly through the evolutionary forces acting on non-living molecules was accepted by a great majority.

Therefore, once formed, these single cells must have been evolved into the diverse complex biodiversity of today.