Prokaryotes are the smallest
organisms in size, but theyre grand in almost every other way.
Their forms are ancient, represented in fossils going back to the
first life on Earth. If we count individuals, there are far more
prokaryotes today than all other organisms combined.
Prokaryotes can be found in
every imaginable habitat, including some so extreme that no other
forms of life could survive there.
In this Lesson, well survey
the diversity of prokaryotic organisms. Their external shapes may be
simple, but their lifestyles and biochemistry are very complex.
Although some prokaryotes are
well-known causes of disease, many others are beneficial, and
neither humans nor any other species could survive without them.