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If there is no "inexorable trend toward more advanced forms of life" why did more advanced forms of life evolve with increasing velocity ever since the appearance of life some 3-3.6 billion years ago.
Firstly, the vast majority of the Earth's biomass continues to be relatively simple organisms. It's just that you don't tend to think about them. (Just stop and ponder, for a moment, how many individual bacteria there are on planet Earth. Whew!)
And think about it this way. Imagine the first living thing. It's literally the simplest organism that could possibly still be considered alive. Actually, imagine a population of them. Now, imagine that one thing changes about a few of them.
If that change was for the simpler, they all die, because they're no longer complex enough to be alive. If the change was for the complex, they live, and have advanced capabilities or something.
When you're at the bottom of the barrel, there's no place to go but up.
To extend crashfrog's pont, once the first organism has evolved, any other
must be more complex. This has the effect of evolution filling in the ecological niches for increasingly complex organisms. As long as all the "simple" niches are filled, the only way evolution can proceed is towards greater complexity,since an existing "simple" organism, would almost certainly be better suited to the ecological niche than a new mutant.
However, if these simple niches do exist, there is nothing to prevent species losing functionality to fill them. The classic examples are cave or underground dwelling animals, who tend to lose eyes and body colour, and parasites, who lose the ability for independent existance.
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