BTW, if natural selection was so capable of making these incredible IC systems, then why couldn't it produce anything more complex than a jelly fish for about 3.5 billion years, up until the Cambrian Explosion?
This is actually a pretty darn good question.
It is slightly off since complex things arose sometime before the Cambrian and we are only just finding a few traces. That pushes the time back to somewhere like 600 myr ago. In addtion life seems to have arisin somewhere just over 3.5 gyr ago so the correct time is about 3 billion not 3.5 billion but for this discussion the half billion years isn't really germane.
It also is slightly wrong (but perhaps a major misunderstanding) when it says "natural selection". That should be mutations; they give rise to novelty and NS keeps some of the new forms.
Without selective pressures to keep novelty that arises it doesn't get a foothold. I've read suggestions that the snowball earth time of 600 million years ago was the selctive conditions that pushed more complex life.
It may also be that you (and perhaps all of us) are still stuck with our view of life. Most life (very nearly ALL on a count or mass basis) is still "simple". Almost everything on the planet is a bacteirum or virus. The apparent "big deal" of muticellurlar forms rising may be just one little bump that didn't really need to arise for life to do very well.
The quesion also suggests that the step to very simple but somewhat differentiated forms (like worms) isn't a big step while from worms to us is. That may not be really right. It is clear from the physical form and, more, from the genetics that the step from a mouse-like thing to us is not a big step at all. Perhaps (but I don't know enough to really say) that the step from a simple worm to a mouse-like things is a small step too. But the step from green algae to that simplest form of pre cambrian life IS a big deal after all so it took a long time.
Remember, our genetic make up is set up to form novelty with every single individual this may allow evolution to happen faster.
I don't think there is an answer to the real question though. But, as noted, don't go inserting a God in the gap. We keep closing them and it forces you to have a smaller and smaller god as the gaps shrink.