Every living creature on the earth are communicating to each other but in a different evolved speech. The question is exactly about “word” so it should be a word and not expression. The answer would be late Homo Habilis and early Homo Erectus were the species who invented words.
Archaeologists thinks that first words were used about 2 million years ago. Homo Habilis were our ancestors which had already developed Broca’s area in the brains which neurologically affect vocal tract musculature and let them pronounce words. Their brain size was much bigger then Australopithecus Afarensis which also says that our ancestors of modern human could develop first words.
Who invented words