How are you related to a duck-billed platypus? Do you know?
At a quick glance, you might suspect that mammalian evolution is like a flight of stairs. At the bottom, there’s reptiles. Take a step up, and you find the weirder egg-laying mammals, including our friend, the platypus. Another step up, and you’ve reached the marsupials, which are much less reptilian, but still don’t quite have the same reproductive system as we do. Finally, at the top, you find the majority of the mammal kingdom—creatures like us, neither egg-laying nor pouch-wearing.
This progression seems so obvious. It’s neat and tidy, and orderly. It’s also wrong.
The process of evolution is actually much, much messier. In its wake, it leaves multiple dead ends, whole orders or classes gone extinct, or whittled down to a sole surviving species—the last bastion of a once-proud lineage. And evolution does not work towards making humans. That’s entirely the wrong way to think about it. The creatures alive today are not our ancestors. Instead of a stair-step, with humans at the top, we stand alongside the kangaroo and the platypus, each of us at the end of its own narrow road. If we look back, into the past, we can see those paths turn and branch and cul-de-sac. Go far enough, and our paths meet at a crossroads. But in between that common ancestor and ourselves, the road is littered with cousins that didn’t quite make it.
It’s not an easy road map to follow. Not for us laypeople, certainly. That’s clear anytime you read a news story about a new fossil discovery, or watch yet another school board debate whether evolution should be taught at all. But the path of evolution wasn’t obvious to the people who first traced it, either. In fact, many of the mistakes and misconceptions common with the general public today were first made by natural historians and paleontologists, themselves. Evolution is messy, and the evolution of evolution was doubly so.
It’s that story—how the theory of evolution arose and how the details shifted to fit new evidence—that science blogger Brian Switek tells in his book Written in Stone, a handy primer for anyone who wants to better understand evolutionary theory, or the way that science, in general, works.