Biological life's enormous complexity is a serious objection to atheistic evolutionary theory. Many evolutionists recognize the serious scientific problems associated with the origin of the first cell non-living chemicals.
In an effort to avoid the obvious: that an all-knowing God created life, some people think that perhaps aliens from other planets were somehow involved in 'seeding' our planet with the building blocks for life millions of years ago. But in this scenario the problem for the evolutionary story of life still remains. It has just been moved to another planet.
The truth is: there is no intelligent life on other planets, God created life fully-functioning right at the very start.
Alien Intrusion has a lot more information on aliens and the creation evolution issue.