What are the most numerous organisms on the planet?
The single-celled prokaryotic organism otherwise known as bacteria. And all it takes is one to quickly grow, under just the right conditions, into millions and billions. Luckily, we know how to control bacteria when necessary. But bacteria do serve many important purposes. In fact, we could not survive without them.
Prokaryotes are the smallest living organisms. They were the first organisms to evolve, and lived on this planet all by themselves for two billion years. What did they do during that time? They evolved. These prokaryotic organisms are the most abundant and successful organisms around today, able to live in any and all environments.