DAVID POGUE (New York Times Technology Columnist): When rock star Bruce Bradley's making a tool, he's thinking several steps ahead, using more brain than brawn, a key point for Dietrich Stout.

DIETRICH STOUT (Emory University): There's a structure to the actions. There are certain rules. In order to do this, you have to do that, but all the time they are related to the goal. And it's the same thing in language.

DAVID POGUE: Just as words in their proper places form sentences, each individual strike must be done in the proper order to get the desired result: a flat stone blade.

BRUCE BRADLEY (University of Exeter): So what we're looking at is complex, sequential, abstract thought. Because what is language? It's complex sequential thought, okay?

DAVID POGUE: I see. So in order to have language, you first have to have the same sorts of mental skills that, that we're seeing.

BRUCE BRADLEY: That's what we're trying to figure out.

DAVID POGUE: But for the tool-to-language hypothesis to work, there would have to be similarities between what the brain is doing while making a handaxe, and how it behaves while forming a sentence.

That's something Dr. Cynthia Thompson, at Northwestern University, knows a lot about.

CYNTHIA THOMPSON (Northwestern University): The focus of my research is to look at what parts of our brain are actually activated when we compute very difficult sentences.

DAVID POGUE: Thompson works with people who have trouble speaking because their brains have been damaged by stroke or other injuries.

Kristen Carlstedt, is one of her research subjects.

Following a stroke, four years ago, Kristen became one of a million Americans with a condition called "aphasia".

CYNTHIA THOMPSON: Kristen has classic agrammatic aphasia.

So, she can understand single words, she can produce single words.

ELLYN (Therapist): Okay, what's this?

KRISTEN CARLSTEDT (Research Subject): Um, pitcher…

CINDY THOMPSON: But people with agrammatic aphasia have problems with complex sentence processing.

ELLYN: The cat is chased by the dog.

What would you say for this one?

KRISTEN CARLSTEDT: The dog, no, the cat is…hmm.

DAVID POGUE: Thompson searches for answers in the images of brains of Kristen and other patients like her.

In M.R.I. brain scans, dark areas mean damaged tissue. Thompson found that agrammatic aphasia patients do share a striking characteristic, damage to the left hemisphere of the brain, home to a mysterious region known as Broca's area.

One of the many parts of the brain associated with language, Broca's area's seems especially important when it comes to grammar.

CINDY THOMPSON: Broca's area is, is crucial to sentence processing. When we have patients who have stroke, and they have damage that encroaches on Broca's area, that's when we start to see problems with sentences.

DAVID POGUE: Okay, fine, but does Broca's area have anything to do with stone tool making?

DIETZ STOUT: What we want to know is do, in fact, particular kinds of stone tool making, that we see coming along at particular times in human evolution, draw on or recruit Broca's area.

DAVID POGUE: Stout, himself, recruited a French neurologist, Thierry Chaminade, to set up an experiment that could answer that question.

Since it's impossible to have people actually make stone tools while having their brains scanned, Chaminade set up the next best thing: a special projector that could beam images of stone tool making into the M.R.I. machine.

Studies have shown that watching an activity excites the same areas of the brain as actually performing the activity.

THIERRY CHAMINADE (Aix Marseille University): The same brain area responds when you observe someone else doing the action. So perceiving and performing an action activates the same regions.

DAVID POGUE: So, in order to see what areas of the brain are activated by making stone tools, Chaminade projected images onto a screen, in the MRI, and had subjects watch videos of Bruce Bradley at work.

In one video, the subjects were observing the creation of a simple tool, the Oldowan chopper. In another, they were observing a complex tool, the Acheulean handaxe.

The results were, pardon the pun, striking. Watching the video of simple choppers resulted in mild activity in Broca's area, but observing the complex Acheulean handaxe caused four times more activity.

So, the same area of the brain we use in forming complex sentences is also hard at work when we make complex tools.

THIERRY CHAMINADE: These areas of the brain are necessary both for language and stone tool making. These two things co-evolved.

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