Some of the stuff about trajectory computation, release timing,
and so on, was possibly well developed long before Lucy and her kin.
They likely stemmed from arboreal predecessors, who had occasion to
leap around in the treetops, in the manner of modern monkeys: Missing
the branch you jump for has low survival value. A variety of
present-day arboreal animals with much smaller brains than Lucy handle
projectile-hurling over distances of several meters tolerably well --
they themselves being the projectiles. The common local example is
(*ahem*) squirrels.
There is a subtle point on using weaponry to drive off predators
that may not have come up yet. To defend against a predator does not
require that one kill or injure the predator, or even that one have a
particularly high probability of doing so. It merely requires that
one make the risk to the predator, of pressing home an attack,
unacceptable.
Consider an hypothetical predator that takes one large animal-prey
per week, that starts doing so at age six months, and that attains
sexual maturity some time between two and three years of age. That's
100 prey encounters before the animal gets a chance to pass on its
genes. Even an encounter with prey whose only immediate effect is to
injure the predator, may result in its subsequent death, either from
starvation because of reduced ability to hunt, or from being preyed
upon. Thus if the average risk of even moderate injury, per prey
encounter, is as high as one percent, it is more than likely that the
predator will be out of the gene pool before reproducing.
Hence, as long as there are alternate prey animals, it doesn't
take a highly lethal defense to deter predators. We have the
situation, paradoxical at first sight, that even a prey species that
would be very likely to lose in a fight to the death with a predator,
may be able to deter that predator by virtue of the small chance of a
different outcome. After all, the predators aren't out for fights to
the death as such, all they want is lunch.
-- Jay Freeman, First Extropian Squirrel