The earliest evidence for stone tipped spears is from Africa around 1.5 million years ago and they were used by the pre-human called Homo Erectus. The stone points suggest that the spears were thrusting hand spears not throwing spears.
As far as figuring out where the vitals are and considering that they had to get right next to the animal they were hunting. Then they probably had figured it out pretty quick. Otherwise they have a wounded and pissed off wildabeast right on top of them. Gazelle rib bones at the vitals area with impact cut marks that date to that time period have been found in Ethiopia.
So to answer your question, at least 1.5 million years ago.


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