"How we met" is a pretty common theme for love songs. This one, by song-a-day musician Jonathan Mann, just takes the concept back a bit further in time—staring in amazement and wonder at the bacteria, early mammals and proto-humans that all led up to the birth of that special person. I'll admit it, I teared up a little. (Even if the lyrics are a bit heavily weighted toward humans, contrary to the actual amount of time we've been around. /pedant moment)
Side note: Jonathan Mann is looking for animators to help bring "Baby, It All Led to You" to life. If you've got skillz/ideas, email him.