I assume we're talking about apes, not monkeys.
I'd say it's definately our development of language and writing. With that invention, it became possible to store the knowledge of mankind in an external, durable, corporeal form, which allowed the ideas of past generations to be studied by the young and built upon. In this way, humanity's knowledge was allowed to accumulate and advance at ever-accelerating rates, and human culture became the strength of the species, rather than our mere selves.
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If songs were lines in a conversation, the situation would be fine.