There is a congenital defect called oligodactyly which is when an organism is born with fewer fingers and/or toes than normal. If some advantage to this arrangement were to be found by an organism with this condition, and its offspring were more likely to be born with it than not, then you might get speciation. There is some evidence that oligodactyly can be endemic in humans (the Wadoma people of Zimbabwe are known for ectrodactyly, an extreme form of oligodactyly).
Narrower hands would allow access to food in smaller openings, such as bee hives secreted in trunks or animal burrows, without tool-use. There would be disadvantages to grip strength and object manipulation, however.