Hi
The answer is : 1
Collective learning.
Because they found skulls in South Africa that was from ages ago{proof}
It greatly changed the view of the Vietnam War back in America. People already didn't want U.S boots down on Vietnam, but the My Lai massacre made the U.S People really change views and turn on the war. This also affected the government, which then affected how the U.S commanders dictated the war.
The Back Death wiped out 3/4 of Europe. This caused Europeans to become less reliant on the Church. One of the cause of The Black death was movement on the silk road by many different people with diseases and infections.
