Based on cell type, there are two basic types of organisms on Earth: Prokaryotic and Eukaryotic.
Prokaryotic cells are divided into the domains Bacteria and Archaea: these families include close relatives to the earliest forms of life on our planet. Eukaryotic cells on the other hand make up the more familiar domain Eukarya: these cells are the building blocks of life for complex organisms such as bumblebees, frogs, cats and humans.
“The dream of every cell is to become two cells.” – François Jacob
Prokaryotes are the smallest forms of life known to science that can live independently. Unlike eukaryotes, prokaryotes do not have a specialized subunits within itself, known as organelles (nucleus, nucleolus, et cetera).
The first life on earth consisted of prokaryotic cells; most of them we observe today are tiny single cells, but some can form larger, multi-celled structures.
The most familiar prokaryotes are bacteria. Bacteria are a very diverse group that have several shapes, depending on the species. The archaea are also quite diverse, but resemble the bacteria in general appearance.
Prokaryotes, mainly in domain Archaea, are famous for thriving in extreme environments: extreme thermophiles live in hot places such as volcanic springs and hydrothermal vents; extreme halophiles live in water of high saline content and acidophiles thrive at very low pH; methanogens are poisoned by oxygen and live in places like swamps and the gut of animals.
In fact, many biologists think that the life that exists elsewhere in our solar system resembles thermophiles and methanogens we have on Earth.
“If it’s true that our species is alone in the universe, then I’d have to say that the universe aimed rather low and settled for very little.” – George Carlin