Natural evolution up to a human is just as far fetched as saying that a god created man in 7 days (or whatever). 4 billion years may be too little for evolution to naturally evolve up to us. But if evolved in steps, on other planets, then maybe its total time is 10 billion years. Then again the age of the universe may be largely greater than 20 billion years (I always had a feeling that this age is too short), or maybe there are infinite universes and the age is eternity. Also it is possible to have the sum greater than the parts. There is a pseudoscientific theory that claims that our technology is evolving at an ever increasing rate towards a "singularity" upon which computers will surpass human intelligence. After that, the computers would "design" other smarter computers and we as humans would no longer even know what happens. Now if some process, in different terms and with different structures occured on other planets with alien structures "designing" structures that "surpass" there own (just as humans may end up designing computers smarter than us) , then you can see how evolution may be quite feasable through this.