I heard this thousands of times. But, the fact is: single-cell life went to multi-cell life, then went to more and more sophisticated lives.
No one will believe it is not a trend of something. You can not pin point the force. Don't say it is not there.
Imagine a cliff with lots of ledges in lots of configurations: large, small, flat, tilted, cracked. Then imagine lots of rocks falling from the top, down the side of the mountain--they, too, are in every possible configuration: large, small, flat, round.
What will happen over time?
Small rocks will fall in cracks, flat rocks may land on ledges and stay there. Round rocks will probably bounce and roll to the bottom. After a while, you'll get very clear patterns and distinctions--stacks of flat rocks on certain ledges, while other ledges are distinctly rock-free. In some cracks you'll find small rocks, while in others, only sand; and in any given crack, you'll never find a rock larger than a certain amount in any direction (the amount being the widest spot in the opening of the crack). Round rocks will form unstable piles at the bottom.
There are patterns, trends and seeming progressions. It may even look like the rocks are being deliberately organized. After all, no matter how long you stare at a bunch of rocks, they never just up and stack themselves. And how would the rocks know, anyway, whether they are round, and thus belong at the bottom; or small, and thus belong in a crack; or flat, and thus belong on a ledge?
The point is, when
everything happens, in every possible configuration, the stuff that is stable will stick around and the stuff that is unstable will disappear. Over time, the stable things will build upon themselves. On the mountain, this looks like a round rock continuing to bounce, because it is rarely stable on anything, while a flat rock lands and stays put--perhaps on top of another flat rock, forming a tower.
In life, this looks like organisms which can't survive, dying, and organisms which can survive, surviving. And then those surviving organisms change, and the ones that can't survive anymore, don't, and the ones that can, do.
Evolution only looks so dramatic because we only see the "successes," (the ones that were stable enough to stick around long enough to have a chance of leaving enough fossils that we might one day find some). If all you ever saw of falling rocks were beautifully stacked towers, that would look designed, also. We don't have any fossils from the creatures that utterly failed, and they're not around to be studied.
We do have some that only
sorta failed, though--giant humans, for example. They stuck around long enough to be some kind of success, but we can pinpoint exactly why they would have died out. Being big means you have a low surface area to volume ratio, which means your skin is less efficient at radiating heat, which means you're susceptible to overheating. Probably, people got pushed in the direction of being huge because bigger, stronger people were less vulnerable to predation by other animals. But once that stopped being a major issue, the heat got them, and they died out. Whoops. That rock teetered on the edge for a while, but, like most, it eventually it fell.
In short, evolution amounts to nothing more than the natural occurrence of stability. Fling enough poo at the wall, and eventually, some will stick.