I found this clip within the "NY Times" and it seems that sanskrit would be the oldest derivative towards "Mankind." This is a partial clipping, not the full article...
Mr. Barzun has written "that the word 'man' has its roots in the Sanskrit 'manu,' which means human being," you note, continuing: "Its counterpart in Latin is 'homo.' Thus, he argues, the word 'man' has always had a double meaning -- both male human being and human being in general. . . .
It is true that the word "man" -- which appears to derive from the same Indo-European root as "mind" and thus originally meant thinking creature -- has long had the two meanings mentioned, both of which are attested for Sanskrit "manu."
Mankind must be stopped before they pollute the universe with their form of friendship...WAR