Speciesism refers to the tendency to ascribe rights to other animals based on how closely they resemble human beings. Initially it was easy to decide who got rights and who didn't—humans had souls, and animals didn't so we could kill them and eat them with impunity.
With the realization that souls don't exist it became necessary to find other ways to differentiate humans from other animals so as to keep humans on top. Various measures have been suggested, including tool use and language.
However studies are constantly wearing away at the boundaries between mankind and the "animals" which leads some to suggest that there is really nothing inherently different between mankind and animals and that we are simply part of a continuum.
The idea that since for any criterion that differentiates humans as a whole from other animals (e.g. intelligence, language), there will be individual human beings (e.g. infants) which don't possess it, or to a lesser degree than some individuals of other species, is sometimes called the argument from marginal cases.
 See also
- Peter Singer, one of the major opponents.
- Anthropomorphism — roughly the converse