The ability to survive is crucial to a a civilization's continuity. Evolution basically means to adapt or perish. Only the strong (in every possible aspect) adapt, and with those strengths they become more resilient. Resilience is an advantage when dealing with adversities such as unforseen disasters. Once one experiences them, it becomes part of their resilience - at least for the survivors.
Considering that slavery originated sometime in the 1620s in North America, and that it lasted until 1865, the correct answer to how much it lasted should be around 245 years.
