The definite article with a whole class

The definite article is used before a noun when it represents a whole class of things:

The computer has made text editing easier.
The polar bear is threatened with extinction.

In less formal styles, the + noun can be replaced by a plural form without an article:

Computers have made text editing easier.
Polar bears are threatened with extinction.

The indefinite article is not possible in this sense as it would refer to one or any member of the class but not the class as a whole:

A polar bear is threatened with extinction.
A computer has made text editing easier.

Man or mankind can be used to represent the human race and has no article:

Man will eventually destroy himself.

However, the more gender-neutral term humankind is preferable to man or mankind:

Humankind will eventually destroy itself.