Ballistic movements require a surprising amount of planning. Slow movements leave time for improvisation: when raising a cup to your lips, if the cup is lighter than you remembered, you can correct its trajectory before it hits your nose. Thus, a complete plan is not needed. You start in the right general direction and then correct your path, just as a moon rocket does.
For sudden limb movements lasting less than one eighth of a second, feedback corrections are largely ineffective because reaction times are too long. The brain has to determine every detail of the movement in advance, as though it were silently punching a roll of music for a player piano.
Hammering requires scheduling the exact sequence of activation for dozens of muscles. The problem of throwing is further compounded by the launch window--the range of times in which a projectile can be released and still hit the target. When the distance to a target doubles, the launch window becomes eight times narrower; statistical arguments indicate that programming a reliable throw would then require recruiting 64 times as many neurons to sing like a choir.
If mouth movements rely on the same core facility for sequencing that ballistic hand movements do, then enhancements in language skills might improve dexterity, and vice versa. Accurate throwing abilities open up the possibility of eating meat regularly, of being able to survive winter in a temperate zone. Language ability would initially be an incidental benefit--a free lunch, as it were, because of the linkage. Only later would language pay its own way.
Is there actually a sequencer common to movement and language? Much of the brains coordination of movement occurs at a subcortical level in the basal ganglia or the cerebellum, but novel combinations of movements tend to depend on the premotor and prefrontal cortex. Two major lines of evidence point to cortical specialization for sequencing, and both suggest that the lateral language area has a lot to do with it.
Doreen Kimura of the University of Western Ontario has found that stroke patients with language problems (aphasia) resulting from damage to left lateral brain areas also have considerable difficulty executing unfamiliar sequences of hand and arm movements (apraxia). By electrically stimulating the brains of patients being operated on for epilepsy, George A. Ojemann of the University of Washington has also shown that at the center of the left lateral areas specialized for language lies a region involved in listening to sound sequences. This perisylvian region seems equally involved in producing oral-facial movement sequences--even nonlanguage ones.
These discoveries reveal that parts of the language cortex, as people sometimes think of it, are more multipurpose than had been suspected. The language cortex is concerned with novel sequences of various kinds: both sensations and movements, for both the hands and the mouth.
The big problem with inventing sequences and original behaviors is safety. Even simple reversals in order can be dangerous, as in Look after you leap. But once we get good enough, we can simulate future courses of action and weed out the nonsense off-line; as philosopher Karl Popper said, this permits our hypotheses to die in our stead. Creativity--indeed, the whole high end of intelligence and consciousness--involves playing mental games that shape up quality before acting. What kind of mental machinery might it take to do something like that?
By 1874, just 15 years after Darwin published On the Origin of Species, American psychologist William James was talking about mental processes operating in a Darwinian manner. In effect, he suggested, ideas might somehow compete with one another in the brain, leaving only the best or fittest. Just as Darwinian evolution shaped a better brain in two million years, a similar Darwinian process operating within the brain might shape intelligent solutions to problems on the timescale of thought and action.