Some believe Venus came into our solar system within the last 4 to 5000 years. Here's the arguement:

Venus spins in the opposite direction that it orbits the Sun, which no other planet does. This may be because it came flying past our Sun and was caught by the gravitational field.

If this was the case, it could have happened anytime. However, it's spinning tremendously fast, and also it could well have been the thing which caused the strange happenings on our own planet as it flew by, recorded in the Bible: fired-up things in the sky (chariots of fire), weird tides in the oceans (parting of the Red Sea), unbalanced biology (the plagues of frogs and stuff).

Also, someone has noded the fact that Venus' pattern of craters is utterly dodgy.......

The Babylonians were the first to chart Venus, and looking at their records Venus appears very suddenly about 4-5000 years ago.

Just a theory.