An interesting scenario is the straight-up earth. On the surface, it sounds like a good idea, with the extremes of summer and winter going away. Certainly different type sof plants and animals would have evolved in the temperate areas of the earth, with no "growing season" in summer, or winter to prepare for.

I think that a straight up earth would not have given rise to the variety of life that it did with the tilt. The atmosphere would tend to band, with variance only caused by the water to land differences.

The effect on primitive life would be less variety (thus less context) on any one part of the planet. The life that formed in one part might not have the ability to venture into another part as well as it does/did. On the straight-up earth, an organism would have to travel north or south in order to experience any temperature change. To gain context, it would have to then return to the original spot, having to again apply old survival techniques. On the earth that we have, though, changes in climate come to non-tropical areas on their own. These changes further add context, and facillitated further cognitive development in primitive life.

Another idea that a straight up, banded-atmosphere planet might not be such a great place is Jupiter. What accounts for the red spot, a violent storm that spins the opposite direction from the rest of Jupiter's atmosphere. Is it a function of the banding?
NEED MORE INFO ON DIFFERENCE BETWEEN TROPICAL AND TEMPERATE PLANTS!!!