Zhang: Molars have to be really perfectly aligned to chew, right? I had a dental crown once that was slightly too high and it caused me no shortage of problems.
Ungar: Nearly all mammals have two sets of teeth. Other animals have a lot more. And that all comes down to chewing. Because if you’re going to chew, you have to have precise contact between upper and lower teeth. If you keep popping in and out new teeth, it’s really hard to get the relationships between uppers and lowers to be good enough for chewing.
Zhang: You write that it’s pretty obvious even to children that the sharp teeth of lions are used to rip meat the big flat molars of, say, giraffes, to chew vegetation. But when it gets to primates and early humans, it gets a lot more complicated to figure out diets based on tooth shape.
Ungar: One of the takeaways of the book is that what you’re adapted to is not necessarily what you do on a daily basis. Understanding ecology—that is the relationship of an organism to its environment—is not the same thing as studying adaptation—that is what your ancestors evolved to do. The mangabeys, for example, monkeys from Kibale in Uganda, these are animals that very, very rarely need to eat hard foods to survive. But they better have teeth that are for consuming hard foods if they need them even rarely, so that they can get through the rough times.
I like to use this analogy: If you eat Jell-O 360 days out of the year but if you’ve got to eat rocks the other five days, your teeth have better be designed to eat rocks.
Zhang: Since the overall tooth shape can’t tell you what our ancestors actually ate, you end up studying what you call foodprints. What are foodprints?
Ungar: Foodprints are like footprints in the sand, traces of actual activities of an organism that was alive. There are several different types of foodprints that are used today. One major foodprint type is the chemistry of the teeth because your teeth are built essentially from the foods you eat. Different types of foods have different chemical signatures. For example, different proportions of isotopes of elements like carbon. Those are going to be reflected in the teeth that are being formed from the foods that are eaten.
There are others. The tartar sometimes on the teeth are preserved, and you can scrape off the tartar, look on the inside, and see if there are little bits and pieces of plant material—whether it’s DNA or whether it’s phytoliths, which are little bits of silica that form in plant parts. These are sometimes identifiable as well. The DNA and stuff inside the tartar, that really has been not explored fully because people have only started to do it and DNA doesn’t last all that long. But it’s useful for Neanderthals and early modern humans.
The foodprint I usually study is microscopic wear or “microwear” on teeth—the scratches and pits that form as a result of use. Different patterns of scratching and pitting are reflecting different diets.