Creationists often say that 3.8 billion years just isn't enough for evolution to occur through random mutation and natural selection. I posted this on another thread and I think it has some merits here.
Let's count the zeros.
So, we believe that the first life on Earth came about say 3.8 billion years ago.
That's 3,800,000,000. It is also important to note that the humans have the longest lifespans among the mammals. So, say that each generation is 50 years.
3,800,000,000 / 50 = 76,000,000
That's 76 million generations, and I'm being conservative, too. Up until 200 years ago, a human generation is only about 30 years. That would make 3.8 billion years to equal 126,666,666. That's about 130 million generations.
But we know for a fact that the vast majority of organisms both in the past and present have much shorter lifespans, thus much more generations per unit time.
Certain insects only have about 10 days per generation. So, for them 3.8 billion years mean 138,700,000,000 generations. That's 138.7 billion generations.
But wait, that's not all. So far, I've only put into consideration a single family line. Especially insects and bacteria, there are literally trillions and trillions of organisms reproducing and mutating. Take 138.7 billion generations and multiply that by a a few trillions or so and you get to see the number for beneficial mutations to occur.
The question is how often do you see this kind of number show up in your everday life? Another reason why you don't want to use common sense for something like this.
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