Xenophobia is not a "20th century concept". Xenophobia has existed for as long as man has existed. Cavemen were most likely xenophobic towards other cavemen. Hell, for all you know, some of the Sentinelese might even be xenophobic to other Sentinelese that live on a different part of their island.

Animals of all types also exhibit xenophobia, even the ones that have "no hint of modern civilization".