An Amazon tribe is the only one known to man that has no concept of numbers or math, with only the words "one," "two," and "many" to describe quantities.

A study appearing today in the journal Science reports that the hunter-gatherers seem to be the only group of humans known to have no concept of numbering and counting. Not only that, but adult Piraha apparently can't learn to count or understand the concept of numbers or numerals, even when they asked anthropologists to teach them and have been given basic math lessons for months at a time ... the Piraha are the only people known to have no distinct words for colours.
They have no written language, and no collective memory going back more than two generations. They don't sleep for more than two hours at a time during the night or day. Even when food is available, they frequently starve themselves and their children, Prof. Everett reports.

They communicate almost as much by singing, whistling and humming as by normal speech.
They frequently change their names, because they believe spirits regularly take them over and intrinsically change who they are.
They have no creation myths, tell no fictional stories and have no art.

That last part in particular intrigues me. It makes them seem culturally to be the equivalent of chimps - they seem to lack most of the essential qualities which most people feel distinguishes humans from other animals.