cai2 originally was a hieroglyph. The horizontal stroke is the ground, the vertical one being the stem and the left sliding stroke, the root. The whole thing describes 'the prospect for growth'.
cai2 neng2 means 'ability' and literally translates as 'talent'-'capable'.
neng2 originated from the hieroglyph which denoted a pitch black muscular animal very much like the bear. The sense of 'capability' and 'power' was derived from the animal's features.
tian1 cai2 means 'genius' and literally translates as 'heaven'-'talent'.
tian1 is derived from . The first character is ren 'man' (a stick figure of a walking person). With arms stretched out, man looks overwhelming in size and so the middle character da means 'big'. But what is superior to all? The answer is 'heaven' and therefore da with a top horizontal stroke (indicating above man) becomes tian meaning 'heaven'.