Proto-Indo-European language
There is no direct evidence of PIE because it was never written. All PIE sounds and words are reconstructed from later Indo-European languages. The asterisk symbol is used to mark reconstructed PIE words, for example: * 'water', * 'dog', or * 'three (masculine)'. Many words in modern Indo-European languages seem to have come from such "proto-words" by regular sound changes, such as Grimm's law.