Chimpanzees display numerous signs of intelligence, from the ability to remember symbols to cooperation, tool use, and perhaps language. They are among species that have passed the mirror test, indicating self-awareness. Therefore, the common ancestor to both humans and chimps is probably when it happened. The chimpanzee–human last common ancestor, or CHLCA, is the last common ancestor shared by the extant Homo (human) and Pan (chimpanzee and bonobo) genera of Hominini. Due to complex hybrid speciation, it is not possible to give a precise estimate on the age of this ancestral population. While "original divergence" between populations may have occurred as early as 13 million years ago (Miocene), hybridization may have been ongoing until as recently as 4 million years ago (Pliocene).