Lub-dub... Lub-dub...

Lub-dub is the phonetic representation of a 2 chamber heart beat, with all valves/equipment/etc. functioning properly, as taught to medical students.

I'm sitting here stretching before I begin my morning treadmill & email routine, and I noticed that I have a habit of mentally counting my stretching in groups of eight.

Why is that?

Two thoughts come to mind immediately:
1) I've heard that humans can comfortably remember about 7 digits (see: The Tipping Point by Malcolm Gladwell), which is why we have phone numbers in groups of 7.
2) It just *feels* better to end on an even number. Why should that be? Ideas:
a) The TWO CHAMBER HEART indoctrinates us with a familiarity to the two-step, lub-dub, even before birth
b) The human body's bilateral symmetry gives us two of lots of stuff.

So,
i. what would our culture, music, and lives in general be like if we had a THREE chambered heart, and
ii. What's with all the symmetry. I understand the benefits of redundancy/fault tolerance, but how did it happen in nature? I wonder if a math wiz could prove that it's the simplest solution.

Or is it the simplest solution, rather than dumb chance?

I'd like to hear Ray Kurzweil's thoughts on this, and also Robert Wright's, Malcolm Gladwell's... any of the authors from my reading list.