In a hunter-gatherer society you'd have far higher chances of survival in a team/tribe. I don't find it surprising at all that feeling lonely (a desire to be part of a tribe) would have such a strong impact on mental (and therefore physical) health. It would act as a strong drive for the lonely person to change their situation, potentially improving their chances of survival and reproduction.
From an evolutionary perspective it's little different to starvation. In fact, you could define loneliness as "social starvation".
From an evolutionary perspective it's little different to starvation. In fact, you could define loneliness as "social starvation".