According to the recently published results of a study by biologist David Carrier, the human hand, when formed into a fist, was an evolutionary advantage for early humans:

For this latest study, he and co-author Michael Morgan, a medical student, conducted three experiments. First, they analyzed what happened when men, aged from 22 to 50, hit a punching bag as hard as they could. The peak stress delivered to the bag -- the force per area -- was 1.7 to 3 times greater with a fist strike compared with a slap.

"Because you have higher pressure when hitting with a fist, you are more likely to cause injury to tissue, bones, teeth, eyes and the jaw," Carrier said. [...]

In terms of the size and shape of hand anatomy, the scientists point out that humans could have evolved manual dexterity with longer thumbs, but without the fingers and palms getting shorter.

Gorilla hands are closer in proportion to human hands than are other apes' hands, but they and no other ape -- aside from us -- hits with a clenched fist.

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If i remember it right Gorillas use their hands for Knuckel walking... I als remember that fists are not suitable for hitting a head of a person, ... this was the reason why boxing gloves have been invented... The higher pressure when hitting with a fist also leads to higher stress values for the tinier bones of your hand... That is hitting something with a fist also has a higer rist of self-injury...

All in all i am not convinced....

I rather thing of other possible reasons for a shorter and thicker Thumb such as a shorter lever for compensating the force of 4 fingers...