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 →
Evolutionary biologists are like puzzle-solvers. That’s true for any of the sciences, of course, but there’s one kind of puzzle that evolutionary biologists particularly like to solve: the order of assembly puzzle. Here’s how this puzzle works. Take a complex system that works very well. Now, break it down into its component parts and figure out how they work together (sometimes, this step is done by the physiologist in the next lab over). Finally, figure out the order in which the parts were originally assembled. But there’s a catch: every time you add a part, the system has to form a working whole. It doesn’t have to have the same function as the finished system, but it does have to be a working system. You can’t break an old system until you have a new system in place. Continue reading →
When I’m teaching, I frequently find myself caught between a fascinating digression and the need to keep things simple. For example, if I’m teaching my Human Anatomy class about the lower jaw, I always have to fight an urge to launch into the complexities of lower jaw evolution.
“See? This is the mandible,” I say, “and it’s just one piece of bone.” But all the while, I am clamping my teeth down firmly on my tongue because the exciting truth is that the human mandible isn’t just one bone: it’s really composed of two symmetrical bones, which are called the “dentaries”and are fused together at the midline, but most other mammals have unfused dentaries, and in fact all other vertebrate classes (and the ancestors of all mammals!) have a mandible that is composed of several bones, which include the dentary, the angular, the surrangular and…
You see how it is? The paired dentary thing might be a bit interesting to my pre-nursing students, because human fetuses have that condition. But they could care less about the number of bones in the mouth of a carp and which of those elements are retained by toads and snakes.