Question

Which discovery vastly improved the quality of life for Homo erectus?

Answer 1

It's fire. 
The other inventions came very, very later. 

Answer 2

  The discovery that it´s contemporary with the Homo Erectus is the Fire.

  It was a turning point in the cultural aspect of the human evolution. It provided warmth and protection during the night because during those times the human kind was not at the peak of the food chain. Also allowed early humans to cooked their food, this advance alarge the life expectancy by not eating raw food.

  This cultural advance allowed the human geographic dispersal.

  I hope this answer help you. Regards.