But there used to be no mend wounds.

Rather it was a series of cures. Cure Light, Serious, Critical, and Heal like you can still do at healers.

These were replaced by mend wounds that now does all of them depending on your level. You used to get cure light at lvl 1, and Serious at like 9. So now if I am not mistaken at lvl9 the healing of mend wounds gets bumped up to the level of the old cure serious, and on up the line. Up until Heal which is still a separate skill.

I think this is how the imms explained it back in the day when the changes went in.