Read Online Body Farm 2 - Flesh And Bone by Jefferson Bass - Free Book Online Page A
Authors: Jefferson Bass
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“Good,” I said. “Most of you still have some teeth. Clearly UT’s admission standards have gone up lately.” I heard a few chuckles, saw a few more teeth. “Okay, now I want you all to stick a finger in your mouth and run it all around your upper and lower jaw to count how many teeth you have. This is an experiment; we’ll gather some data on evolution, or ‘secular change,’ as we usually call it in physical anthropology.” I demonstrated, reaching my index finger back to my upper right molars and tracing a line around my mandible, counting aloud as I went: “Un, oo, ree, or, ive, ix,” ending at “unny-eight.” I went to the chalkboard and wrote “28” in foot-high numerals. I turned back to face them. “By the way,” I added, “if you’ve had your wisdom teeth extracted, or any other teeth, add those to your total. Ready? Count.”
A few students tried counting with their tongues; most used an index finger, as I had done, but a sizable subset of the girls used the long nail of a pinkie so as to be more delicate about the procedure. As the students fished around in their mouths, it looked as if they were trying to dislodge popcorn hulls from their teeth. Then, almost as if choreographed, a hundred fingertips rubbed across pants legs and skirts to wipe off traces of saliva.
“Okay,” I said, “now let’s analyze our data. How many of you had thirty-two teeth, which is what’s considered normal for an adult human?” A sprinkling of hands shot up, representing about a quarter of the class. “How many had twenty-four?” I saw roughly the same number of hands. “And how many had twenty-eight?” Half the students raised their hands.
“See, this is interesting,” I said. “Only a quarter of you have thirty-two, which is considered a full set of teeth—for modern humans. But for our ancestors thirty or forty million years ago, the norm was forty-four—which, by the way, is still the standard for most mammalian teeth. If you’d lived forty million years ago, you’d have had twelve more teeth. Where would you put them? Anybody in here feel like they’ve got room enough for a dozen more molars?” I shook my head dramatically. “And why is that? Because our jaws have gotten smaller. And why is that?” Faces went blank; shoulders shrugged.
I had started slowly, but now I was gathering momentum, like a rhino on the run. “A couple hundred million years ago, our ancestors, the first mammals, began evolving from swamp lizards,” I said. “They were small mammals about the size of squirrels or shrews, called ‘preprimate insectivores’; they lived on the ground and ate bugs. They had long snouts, sort of like anteaters, and their eyes were on the sides of their heads.” I tapped both temples for emphasis. “Well, at the same time, another group of animals was emerging: the dinosaurs. Now, what happens when a tyrannosaurus or a brontosaurus steps on a preprimate insectivore?” I smacked one palm down on top of the other. “Splat,” I said. “So some of the brainier insectivores decided they’d be safer up in the trees, where they wouldn’t get stepped on. Good idea; more of those survived. But not all. If you’re skittering around in the trees, jumping from branch to branch, it’s hard to see which branch to grab if your eyes are on the sides of your head and you’ve got a big snout in the middle of your face. So some of these critters fall out of the trees and get eaten. Or stomped on.” I made the splat again. “So over time—remember, we’re talking millions of years—the survival rate, and the reproductive rate, is higher in the ones with smaller snouts and eyes closer to the front of their heads. But to lose that snout, they have to lose some teeth—if you’ve got forty-four teeth, you’re going to have a mighty big snout. So natural selection favors those with smaller snouts. Fewer teeth. The fossil record documents all these changes in great detail.”
The young
A few students tried counting with their tongues; most used an index finger, as I had done, but a sizable subset of the girls used the long nail of a pinkie so as to be more delicate about the procedure. As the students fished around in their mouths, it looked as if they were trying to dislodge popcorn hulls from their teeth. Then, almost as if choreographed, a hundred fingertips rubbed across pants legs and skirts to wipe off traces of saliva.
“Okay,” I said, “now let’s analyze our data. How many of you had thirty-two teeth, which is what’s considered normal for an adult human?” A sprinkling of hands shot up, representing about a quarter of the class. “How many had twenty-four?” I saw roughly the same number of hands. “And how many had twenty-eight?” Half the students raised their hands.
“See, this is interesting,” I said. “Only a quarter of you have thirty-two, which is considered a full set of teeth—for modern humans. But for our ancestors thirty or forty million years ago, the norm was forty-four—which, by the way, is still the standard for most mammalian teeth. If you’d lived forty million years ago, you’d have had twelve more teeth. Where would you put them? Anybody in here feel like they’ve got room enough for a dozen more molars?” I shook my head dramatically. “And why is that? Because our jaws have gotten smaller. And why is that?” Faces went blank; shoulders shrugged.
I had started slowly, but now I was gathering momentum, like a rhino on the run. “A couple hundred million years ago, our ancestors, the first mammals, began evolving from swamp lizards,” I said. “They were small mammals about the size of squirrels or shrews, called ‘preprimate insectivores’; they lived on the ground and ate bugs. They had long snouts, sort of like anteaters, and their eyes were on the sides of their heads.” I tapped both temples for emphasis. “Well, at the same time, another group of animals was emerging: the dinosaurs. Now, what happens when a tyrannosaurus or a brontosaurus steps on a preprimate insectivore?” I smacked one palm down on top of the other. “Splat,” I said. “So some of the brainier insectivores decided they’d be safer up in the trees, where they wouldn’t get stepped on. Good idea; more of those survived. But not all. If you’re skittering around in the trees, jumping from branch to branch, it’s hard to see which branch to grab if your eyes are on the sides of your head and you’ve got a big snout in the middle of your face. So some of these critters fall out of the trees and get eaten. Or stomped on.” I made the splat again. “So over time—remember, we’re talking millions of years—the survival rate, and the reproductive rate, is higher in the ones with smaller snouts and eyes closer to the front of their heads. But to lose that snout, they have to lose some teeth—if you’ve got forty-four teeth, you’re going to have a mighty big snout. So natural selection favors those with smaller snouts. Fewer teeth. The fossil record documents all these changes in great detail.”
The young
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