It’s a painful question for people who accept evolution to hear. Not because there’s no answer – in fact, the bulk of this article will be devoted to answering that question. No, it’s painful because of the startling ignorance it displays. If you can ask this question, and especially if you can ask it with that condescending confidence that it somehow disproves evolution, then you need to accept one simple fact: you do not understand the Theory of Evolution well enough to have a position on its plausibility.
Let’s see if we can correct that, at least on this particular part of the puzzle.
First of all, evolution (and here I’m going to use “evolution” as a shorthand for the Theory of Evolution), does not say that our ancestors were any currently existing species of monkey. It says that at some distant remove in the past, we shared a common ancestor with modern monkeys. Now if you looked at that ancestor you’d probably call it a monkey, but it would not be a chimpanzee or a macaque or a capuchin, or an orangutan or any other species we see currently running around the planet. Remember, after all, that “monkey” is a broad category covering many different species.
So what happened to this ancient monkey that we and all those other monkey species can be descended from it? Well, let me give you the broad strokes.
Picture, if you will, that ancient ancestor (we’ll call her Mona the Monkey) as part of a band of monkeys happily living in their trees millions of years ago. Maybe that band gets too big for the local environment and part of it splits off to find food elsewhere. Or there’s some sort of local disaster (a flood, fire, earthquake, whatever) that splits the band. Whatever it is, some of that monkey’s children end up living and reproducing separately from the others.
So now we have Band A, the original, and Band B, the group that split off. Band A is still living in the same place doing the same things, so it doesn’t really change much. Band B, however, ends up in a place where the trees are more sparse, making it harder to move between them and forcing the monkeys to spend more time on the ground. That means that in Band B, the monkeys who are able to move better on the ground are more likely to survive and pass the traits that let them do so on to their offspring.
Before too many generations pass, the monkeys in Band B have noticeably longer legs than those in Band A simply due to the fact that the longer-legged monkeys in Band B are more likely to survive in the environment where they live.
Now, say a member of Band B is born with a mutation that increases the amount of muscle mass it puts on as it grows. This makes it better at protecting itself from predators in a physical confrontation, which enables it to spend even more time on the ground. And here’s the important thing: only those members of Band B who are descended from the money with that mutation will carry it themselves. That means that so long as Band B and Band A remain separate, no member of Band A will ever have that mutation.
You following me so far? We now have two bands of monkeys. Band A still strongly resembles Mona the Monkey and are a group of small, agile tree-dwellers. Band B, though, is composed largely of monkeys who, because of a combination of their environment selecting for taller monkeys with longer legs, and a beneficial mutation (yes, they happen) that increases their strength, are now larger and more comfortable on the ground. Both groups are descended from Mona the Monkey. But you notice what didn’t happen to Band A? It didn’t die out. It also didn’t change in the same way Band B did.
In other words, Band B is descended from Band A, yet Band A still exists.
The only difference between the story of the monkey bands I just told and the relationship between us and monkeys is one of scale. The band of monkeys that would eventually become us split off millions of years ago from the band that would go on to become modern monkeys, and both bands have split and/or mutated hundreds if not thousands of times in the intervening millennia. Some of those bands died, some of them survived, some even recombined at a later date. The details vary, but it’s always the same basic story repeated over and over and over. That’s what makes it so elegant – that by these simple steps repeated often enough over enough time you can arrive at such a diverse explosion of living forms.
That is why we can be descended from monkeys, and yet there are still monkeys.
Or, to steal a saying I’ve heard and found amusingly appropriate: the reason that we can be descended from monkeys, and yet there are still monkeys, is exactly the same reason most American can be descended from Europeans, and yet there are still Europeans.
This is, of course, only the most basic sort of layman’s explanation. I’m not a biologist, after all, nor are the people I’m targeting by writing this. I just hope this shows people that there is an answer to this question, and that it’s not so far-fetched as you might have been led to believe. And if this little glimpse has at all piqued your interest in what the Theory of Evolution actually has to say, I hope that you will pursue further reading on the subject from material written by actual evolutionary biologists.