Everyone remembers the "Simpsons" episode in which beleaguered Homer - not yet the walking punchline he was to become in later seasons - works himself nearly to death, taking second shifts at the Kwik-E-Mart with Apu, in order to afford a pony for young Lisa.

The evolutionary history of the horse is one of the most-covered subjects in modern biology. And no wonder - of all modern animals, the horse has behind it the most intact and visible family tree.

Trying to pin down the moment at which horse and human history were first entwined is like trying to name the inventor of the wheel. The domesticated horse looms so large in human culture, and casts its shadow from so far back in time, that we may never know much about its origins.