The scientific concept of the origin of life on earth begins with the premise that life first appeared billions of years ago with the formation of microscopic organisms out of inanimate matter. In the billions of years which followed, small organisms evolved into higher and more complex forms of life through random mutations, and one species evolved into another.
Over the years, a process referred to as "natural selection" weeded out those mutations and organisms less fit to survive than others. Thus, it was mostly the more "fit" that passed on their genetic character traits to subsequent generations. And that's how we and all other life forms got here.
On the surface, this sounds great. However, a deeper analysis of the underlying mechanism and the fossil record, leaves little doubt that mutations of a random nature could not possible have been the driving force behind the development of life on earth.
When it comes to a random process, there is always the question of whether it can produce organization. An analogy might be the old monkey on a typewriter: given enough time, can a monkey on a typewriter produce the works of Shakespeare purely by random keystrokes? Let's assume for the purpose of this discussion that this is possible — and that random mutations, given enough time, can also eventually produce the most complex life forms.
Let's begin by rolling a die (one "dice"). To get a "3," for example, you'd have to roll the die an average of six times (there are six numbers, so to get any one of them would take an average of six rolls). Of course, you could get lucky and roll a 3 the first time. But as you keep rolling the die, you'll find that the 3 will come up on average once every six rolls.
The same holds true for any random process. You'll get a "Royal Flush" (the five highest cards, in the same suit) in a 5-card poker game on average roughly once every 650,000 hands. In other words, for every 650,00 hands of mostly meaningless arrangements of cards (and perhaps a few other poker hands), you'll get only one Royal Flush.
Multi-million dollar lotteries are also based on this concept. If the odds against winning a big jackpot are millions to one, what will usually happen is that for every game where one person wins the big jackpot with the right combination of numbers, millions of people will not win the big jackpot because they picked millions of combinations of meaningless numbers. To my knowledge, there hasn't been a multi-million dollar lottery yet where millions of people won the top prize and only a few won little or nothing. It's always the other way around. And sometimes there isn't even one big winner.
How does this relate to evolution?
Let's take this well-understood concept about randomness and apply it the old story of a monkey on a typewriter. As mentioned earlier, for the purpose of this discussion, we'll assume that if you allow a monkey to randomly hit keys on a typewriter long enough he could eventually turn out the works of Shakespeare. Of course, it would take a very long time, and he'd produce mountains and mountains of pages of meaningless garbage in the process, but eventually (we'll assume) he could turn out the works of Shakespeare.
Now, let's say, after putting a monkey in front of a typewriter to type out Shakespeare, you decide you also want a copy of the Encyclopedia of Britannica. So you put another monkey in front of another typewriter. Then, you put a third monkey in front of third typewriter, because you also want a copy of "War And Peace." Now you shout, "Monkeys, type," and they all start banging away on their typewriters.