The chance that higher life forms might have emerged in this way is comparable to the chance that a tornado sweeping through a junkyard might assemble a Boeing 747 from the materials therein.The crux of the argument is that the complexity of life is too high to be without a designer (commonly proposed to be the proponent's god or gods). Many of have written about why this is a fallacious argument, but I'd like to cover what I see as the origin of this invalid example, and propose an example that, I think, accurately illustrates evolution in action.
This example is used by people who have one or both misunderstandings about evolution:
- Evolution is purely random.
- If something is without an intelligent designer, it is implicitly random.
Evolution, at its core, is defined as small changes occurring over time within a population. This can be as simple as the emergence of a new hair color, described as microevolution, to something as complex as a new species (known as speciation), which can be a product of macroevolution.
These changes that manifest are often random - a faulty duplication of genes, be it by a faulty reproductive process or some external agent such as radiation. However, this duplication - which is, for all intents and purpose, random - is not the only component of evolution. A counterpart to the creation of new genes is selection - natural selection, genetic drift, and other methods - which eliminate the "useless" genes. Although there is no (yet discernible) intelligence driving these methods (excepting forms of artificial selection), these are the counter-balances to the randomness of these random gene mutations.
Take, for example, the recent discovery of humans who are immune to HIV. We know this immunity has a genetic cause - how and when the mutation occurred, we don't know yet (and may never know). Now, let's take the scenario where HIV becomes a true world-wide epidemic; anyone not immune will be killed by it. Once everyone without the immunity is dead, the only people left behind will be those with an immunity to the virus.
In this example, the spread of the HIV virus is an example of natural selection - a change occurred in the environment, and it was only those with the special genes that grant the immunity left behind. Those genes will continue to reproduce, and the genetic composition without those genes will have been wiped out. On a side note, this opens the door for a fantastic discussion of the genetic origins of charity, empathy, hate, and other traits in humanity.
The take-away from this is that, while there is no design or purpose evident in natural selection, it is not random; if it were random, we'd have organs or growths that have absolutely no functional purpose, either now or in our history (even the once-considered-useless appendix has been proposed to have a use after all). Additionally, it's worth noting that we've worked through this entire example without any indication of an intelligent designer - while that does not disprove the existence of an intelligent designer, it does render "tornado in a junkyard" argument obsolete.
In conclusion, it's fair to note that what I've written here isn't anything new, but that I'm merely standing on the shoulders of giants. Richard Dawkins really awakened my interest in science, and I'd recommend his books, such as The Selfish Gene, for more information on genes and how they've shaped the development of our world.