The theory I like at the moment is that the gradual changes theory is wrong.
Instead evolution has long stable periods punctuated by die back followed by expand and fill.
It isn't mutations, but instead it is the effect of either a new environment causing a die back or expansion into a new territory making a reduced population so that natural variations can be favored and their expression reinforced.
At the beginning of homo erectus we had a serious die back, possible to just a few family groups and changed from savanna to forest, coastal and river regions. It seems to have worked because we then rebounded and spread through out eur-asia and back into africa. These groups then evolved further to neanderthal in europe and sapiens in africa.