What's interesting to me is the large array of proto-homo species that have been discovered. Focusing on identifying which were the direct ancestors to our line is understandable, but I find the 'failed' cousins of our ancestors almost as interesting.

Apparently these creatures were exploiting rich ecological niches, since a broadly similar group of adaptations have been found at multiple sites at multiple dates. Some combination of climate change and other environmental pressures in conjunction with the ruthless efficiency of our ancestors is probably why there aren't today mountain humans and lowland humans, as there are mountain gorillas and lowland gorillas. Our ancestors had the adaptability necessary to out-compete and out-survive all of their cousins.

It brings to mind the fairly recent reproductive bottleneck, wherein all modern humans are descended from just a few hundred breeding individuals. Similar bottlenecks must have happened to all the cousin species of our ancestors: the difference is that none of the cousin species survived their final brush with extinction.

I wonder how many of those cousin species extinctions were specifically attributable to our ancestors, whether by being out-competed or by being directly hunted and killed, and how many were attributable to more generalized environmental pressures? I suppose in most cases it was due to a combination of factors, but I can't help but speculate about a world where multiple species of genus homo had survived.