We normally spend most of our time trying to figure out what’s true. But we can often learn just as much about the world by considering what isn’t true. Which of these is the “correct” silhouette of George Washington?
It therefore behooves us to consider what is NOT. Let’s take some examples from biology. We’ll start with one of the most important phenonema in the history of biology: the beak of the finch:
Now, we could ask, “Why do birds have beaks?” But we could just as usefully ask, “Why don’t birds have teeth?” After all, teeth are made of immensely hard enamel that can withstand almost everything. Indeed, teeth are far and away the most common fossils because they just don’t rot, erode, or decompose. Beaks, on the other hand, are made of keratin, the same protein that makes our fingernails. Keratin is one of the most common organic molecules in the world; almost every animal with any kind of hard body parts uses keratin. But keratin isn’t as hard as dental enamel; it wears out in use and it rots. Therefore, every animal with keratin hard parts must continually add more keratin to make up for wear. That’s why we must continually trim our fingernails and toenails: in the modern world, we don’t wear them down as fast as they grow, so they keep overgrowing. Cats must sharpen their claws by breaking off the outer layers that have dulled with use. Bird beaks are constantly growing, but their tips are constantly being worn away by use.
As it happens, the reason for beaks becomes obvious when you compare the use of beaks with the use of teeth:
Beaks are really great for reaching food that’s inside a hole or buried inside things. Teeth, on the other hand, work only with food that’s already sticking out where you can reach it easily. If you’re a herbivore, that’s no problem because plants have to put their leafy parts out in the open to catch sunlight. That’s why there are so few bird herbivores; beaks aren’t very good for eating grass or leaves.
The point of this tale is that the reason why birds have beaks only becomes clear when you ask why they don’t have teeth.