"We're finding that when you rub a natural biological surface that has an orientation - that has a grain, if you will - and you run that grain perpendicularly to the long axis of the ridges, we find a dramatic increase in friction, " Dr Dominy told BBC News.

BBC: Artificial finger tests evolutionary origin of prints

"We want to get the lead inside, so instead of chopping the pencil up perpendicularly and exposing just the ends, we strip the wood casing off the outside and expose the length of the lead, " says Litzen.

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