When I was much younger, before I really understood anything, I used to think about a lot the first human -- that fully conscious, fully human baby born to monkeys -- and wonder how he or she managed to get through life. It must have been extremely lonely, I thought.

It probably would have been. Except, of course, that there really wasn't ever any "first human," or first mammal, or first anything. We tend to think of evolution in these cartoonish terms -- holy crap, that fish just turned into a bird! -- but that isn't how it works at all.

I'm reading River Out of Eden by Richard Dawkins now, and he handles this particular concept wonderfully, explaining how these sorts of big changes really happen in just about the easiest-to-understand manner I've ever seen:

When we think of the divide leading to all the mammals -- as opposed to, say, the stream leading to the gray squirrel -- it is tempting to imagine something on a grand, Mississippi/Missouri scale. The mammal branch is, after all, destined to branch and branch and branch again, until it produces all the mammals -- from pigmy shrew to elephant, from moles underground to monkeys atop the canopy. The mammal branch of the river is destined to feed so many thousands of important trunk waterways, how could it be other than a massive, rolling torrent? But this image is deeply wrong. When the ancestors of all the modern mammals broke away from those that are not mammals, the event was no more momentous than any other speciation. It would have gone unremarked by any naturalist who happened to be around at the time. The new branch of the river of genes would have been a trickle, inhabiting a species of little nocturnal creature no more different from its nonmammalian cousins than a red squirrel is from a gray. It is only with hindsight that we see the ancestral mammal as a mammal at all. In those days, it would have been just another species of mammal-like reptile, not markedly different from perhaps a dozen other small, snouty, insectivorous morsels of dinosaur food.

The same lack of drama would have attended the earlier splits between the ancestors of all the great groups of animals: the vetebrates, the mollusks, the crustaceans, the insects, the segmented worms, the flatworms, the jellyfish, and so on. When the river that was to lead to the mollusks (and others) parted from the river that was to lead to the vertebrates (and others), the two populations of (probably wormlike) creatures would have been so alike that they could have mated with one another. The only reason they didn't is that they had become accidentally separated by some geographical barrier, perhaps dry land separating previously divided waters. Nobody could have guessed that one population was destined to spawn the mollusks and the other the vertebrates. The two rivers of DNA were streamlets barely parted, and the two groups of animals were all but indistinguishable.

Kind of makes you wonder where the first Cockroach sapiens is living right now.