How do we learn about extinct things? Can we use evolutionary theory itself to help us? Yes, but first we need to take a heuristic detour into space.
Suppose you’re an alien from another star system, maybe a thousand light-years from ours. Your scientists pick up radio wave transmissions from Earth, but they are garbled. You know there’s a civilization here, and you can figure that the transmissions came from the third planet, but you don’t have much more detail yet. So, you pack up your spaceship and head over our way.
Unfortunately, in the intervening time, us silly humans manage to blow up the Earth. When you arrive in the Sol system and come out of cryosleep, all that’s left here is a shiny new ring of asteroids where our big blue marble used to be. (Sigh …it happens.)
Well, the time has come to talk about dinosaurs. In a blog about evolution, it was inevitable. Dinosaurs are, so to speak, the elephant in the room.
It is widely believed that dinosaurs are big and go “Rawr!’. While it is certainly true that dinosaurs go “Rawr!” (of course, there’s a Santa Claus, boys and girls!), it happens that many were not very large.
Some were, of course, brain-meltingly huge. But others were the size of a chicken. In fact dinosaurs came in such a diversity of sizes and shapes, that talking about dinosaurs is a bit like talking about mammals: there are just too many of them to be able to generalize much. Mammals can be giant like an elephant – or a whale – or they can be tiny like a shrew. And they’ve changed dramatically over evolutionary time. Same thing with dinos: the first dinosaurs were quite different from the ones that got smacked down by that asteroid at the end of their reign.
But there’s one thing that seems to have remained the same over the entire age of the dinosaurs: the carnivorous ones always had a bipedal stance. From Allosaurus to Velociraptor, they all seem to have run on two beautifully engineered legs. Herbivores came in a surprising array of shapes both two-legged and four; but carnivores? Just the two-legged variety.
What makes a mammal a mammal? In grade school, we were taught that all mammals have three distinguishing characteristics: fur, milk and live birth. But there is a problem: not all mammals have all three of these features. Monotremes (the group that includes the platypus and the spiny echidna) have fur and milk, but they do not give birth to live young. They lay eggs!
So, there seems to be a sort of gray area between mammals and non-mammals. Is there something wrong with our definition of mammals, or do we have a deeper problem? Perhaps there are other gray areas that we need to worry about. Continue reading →