A couple of ideas ...
1) Our environment has been fairly stable. It may be that more extreme, prolonged changes could elicit higher rates of mutation. It could also be that mututation rate is constant but some of us die off, resulting in differential survival.
2) Margulis' notions about symbiosis are perhaps still underexplored. Her model of kefir bubbling in the vat is intriguing; it may be more than just a model.
3) I see rumors that 95% of our genetic material is inactive. Is it a vestige of our former selves? If so, can the sequences be switched on again? Certainly yes, but through what means?
4) Baldwin ... GG Simpson didn't seem to care for the model and there are comments that Baldwin and somebody defined the famous effect. Then Baldwin appears to have squelched the somebody. If these things are true, we may have an Effect that exists more for the persistence of its inventor. (Darn manics are everywhere!) I'm new to things; if you have a more favorable history, please share it.