In my opinion, Homo Ergaster was the most important evolution stage. This specific species had many advantages, like a larger/wider nose to keep cool. Not only that, but Homo Ergaster also had a hairless body that made it resemble modern humans, and helped in the process of cooling down. With larger brains, Homo Ergaster was able to track animals down by just using their foot prints. But the most important use of the large brain was to help understand other members of their species. They not only resembled modern humans, but started acting like us as well.