What would be the effects on humanity if the Volcano in what is now Lake Toba which put Human kind on the brink never erupted.
Well, disregarding any other butterflies, the population of India would be radically different. In OTL, India was very badly hit by the ash fall from Toba, and the human population there (unlike pockets in Southeast Asia) was entirely killed off. India was repopulated later by people from the Middle East. So, the population of India would be probably closer to Melanesians and Australian Aboriginals, at least genetically.
Also, generally, humanity would be genetically far more diverse than in OTL.