There are no prereqs for the abilities, except ...;

You know that you need to put increasingly more points into abilities to increase them ?

Getting from 0 to 1 requires 1 point, from 1 to 2 requires 2 points, and so on.

So, to maximize an ability to 5, you need
- 0 --> 1: 1 ability point
- 1 --> 2: 2 ability points
- 2 --> 3: 3 ability points
- 3 --> 4: 4 ability points
- 4 --> 5: 5 ability points
Altogether, you'd need 15 points to maximize an ability.

There are some abilities that can be increased with item bonuses or traits.
For example, there can be +1 bonus to Crafting and/or Blacksmithing on bracers and belts, there is the Scientist talent which gives +1 to Crafting and Blacksmithing, and the Pragmatic trait increases Crafting by +1.

This way, it's possible to bring Crafting to 5, with +2 from gear, +1 from the Scientist talent and +1 from the pragmatic trait, requiring only one point being put into the ability directly.

All spell school abilities (Man-at-Arms, Scoundrel, Pyrokinetic, etc.) do not get any bonus from items, nor talents, nor traits, so you will have to invest the full 15 points to maximize one of them to level 5.