What's obvious is that you don't have a grasp on the time frames involved to get from the first self-replicating molecule to something as relatively complex as a sponge, much less to humans and other modern animals, and "I can't understand how it works" is not the same as "too complex". So here's a visual aid that I'm just going to hope doesn't get unnecessarily folded by Y!Answers. (edit: yeah, it folded it, so I'm going to split it into three lines and just assume there's no breaks. Yeesh, thanks, Y!A.)
Every character in the line above represents about 50 million years, starting with the formation of the Earth. The periods represent prebiotic time. The asterisks represent time periods for which there is indirect and/or unconfirmed evidence of life. The c's represent single celled (and towards the end of the line of c's, simple multicelled) life starting with the earliest non-controversial microfossils, dating back about 3.5 billion years. And the C's represent complex life.
The vast majority of the history of life on Earth is microbial. On this scale, humans just don't appear; mammals weren't even on the scene until the fourth from the last C. The Chicxulub meteor, the impact that ended the age of the dinosaurs, happened just a bit to the left of the final C.
The last pixel on the rightmost edge of the last C encompasses all of human history. Not merely all recorded human history, but *all* human history dating back to the earliest /h. habilis/ and /h. erectus/. We /h. sapiens/ don't even rate a full pixel across on this scale.
As the late author Robert Anton Wilson pointed out, "Most of our ancestors were not perfect ladies and gentlemen. The majority of them weren't even mammals."
I'm going to recommend Aron Ra's ongoing series on The Systematic Classification of Life, which I'll include as the source link, if you're serious about this question. You might be surprised at how obvious it is that life *did* evolve.