quaternary

adjective

• 1fourth in order or rank; belonging to the fourth order.

• 2 (Quaternary) Geology of, relating to, or denoting the most recent period in the Cenozoic era, following the Tertiary period and comprising the Pleistocene and Holocene epochs (and thus including the present).

• 3 Chemistry denoting an ammonium compound containing a cation of the form NR₄⁺, where R represents organic groups or atoms other than hydrogen.
• (of a carbon atom) bonded to four other carbon atoms.

noun

• the Quaternary period or the system of deposits laid down during it.

  The Quaternary began about 1,640,000 years ago. Humans and other mammals evolved into their present forms and were strongly affected by the ice ages of the Pleistocene