Darwin was right
Darwin's suggestion has received conclusive support from a detailed and convincing study of the human hand by the distinguished anatomist Richard W Young:
It has been proposed that the hominid lineage began when a group of chimpanzee-like apes began to throw rocks and swing clubs at adversaries, and that this behaviour yielded reproductive advantages for millions of years, driving natural selection for improved throwing and clubbing prowess. This assertion leads to the prediction that the human hand should be adapted for throwing and clubbing, a topic that is explored in the following report. It is shown that the two fundamental human handgrips, first identified by J. R. Napier, and named by him the ‘precision grip’ and ‘power grip’, represent a throwing grip and a clubbing grip, thereby providing an evolutionary explanation for the two unique grips, and the extensive anatomical remodelling of the hand that made them possible. These results are supported by palaeoanthropological evidence.
(Evolution of the human hand: the role of throwing and clubbing - see link for full text of paper)
Unlike australopithecine and human hands, chimpanzee hands lack a strong opposable thumb:
The grips of the chimpanzee differ profoundly from those of humans (Napier, 1960). For suspension from horizontal supports, chimpanzees use a ‘hook grip’ of the four flexed fingers (Napier, 1960; Marzke & Wullstein, 1996). With vertical supports, a diagonal hook grip is used (Susman, 1979; Marzke et al. 1992). The thumb may touch the support, but does not squeeze it against the palm. Chimpanzees use this grip when flailing with sticks, but when the arm swings forward the hand tends to lose its grip, possibly due to weakness of the thumb and its inability to overlap the index finger (Marzke et al. 1992; Marzke & Wullstein, 1996). Because the thumb is weak and short, its distal phalanx is relatively immobile and its distal pad cannot be opposed to those of the fingers, it cannot generate a firm pinch or squeeze (Marzke, 1992a, 1997; Marzke & Wullstein, 1996). (Young)
Building on earlier research by Napier and Marszke, Professor Young demonstrates that surviving fossils of australopithecine hand bones show from the beginning a decline in specialisation for tree climbing and a new specialisation for throwing and clubbing, with the throwing and clubbing adaptation improving in the later fossils. These later fossils correspond to those in which the canine teeth become less well adapted to use as weapons.