Author’s note: There were errors in the original version of this article. Please see the Post Scripts for more details.
In 1901, Hans Spemann revolutionized biology by doing something very strange. He had been watching various embryos grow, and he got bored. That’s the short version of his motivation.
Basically, the embryologists of the day had already spent oodles of time carefully documenting the normal development of various animals from egg to embryo to hatchling. They had established that most animals – vertebrates included – go through a succession of embryological stages called the morula, blastula and gastrula. The morula is just a dense ball of cells (In Latin, morula means “mullberry”), the blastula is a hollow ball of cells, and the gastrula is like a blastula with an indentation somewhere. The indentation keeps growing inward until it meets the other side and becomes a tube that runs through the embryo’s whole body. This tube becomes the digestive tract, which, if you think about it, is just a tube running through an animal’s whole body. In some species, called the protostomes, the original indentation becomes the mouth, but in other animals (deuterostomes) it becomes the anus. We are deuterostomes. In fact, it’s all very interesting, because the same program seems to occur in wildly different organisms, from worms to molluscs to starfish and humans.
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 →