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    "segs": [ {
      "utf8": "You may take them for granted, \nbut your teeth are a marvel."
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    "segs": [ {
      "utf8": "They break up all your food \nover the course of your life,"
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  }, {
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    "segs": [ {
      "utf8": "while being strong enough\nto withstand breakage themselves."
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    "segs": [ {
      "utf8": "And they’re formed using \nonly the raw materials"
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  }, {
    "tStartMs": 21183,
    "dDurationMs": 3220,
    "segs": [ {
      "utf8": "from the food they grind down \nin the first place."
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    "segs": [ {
      "utf8": "What’s behind their impressive strength?"
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    "segs": [ {
      "utf8": "Teeth rely on an ingenious structure \nthat makes them both hard and tough."
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    "segs": [ {
      "utf8": "Hardness can be thought of as the ability \nto resist a crack from starting,"
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    "segs": [ {
      "utf8": "while toughness is what stops \nthe crack from spreading"
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  }, {
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    "segs": [ {
      "utf8": "Very few materials have both properties."
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    "segs": [ {
      "utf8": "For instance, glass is hard but not tough,"
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  }, {
    "tStartMs": 46214,
    "dDurationMs": 2749,
    "segs": [ {
      "utf8": "while leather is tough but not hard."
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  }, {
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    "segs": [ {
      "utf8": "Teeth manage both by having two layers:"
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    "segs": [ {
      "utf8": "a hard external cap of enamel, made up\nalmost entirely of a calcium phosphate,"
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    "segs": [ {
      "utf8": "and beneath it, \na tougher layer of dentin,"
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  }, {
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    "segs": [ {
      "utf8": "partly formed from organic fibers \nthat make it flexible."
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  }, {
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    "segs": [ {
      "utf8": "This amazing structure is created \nby two types of cells:"
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  }, {
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    "segs": [ {
      "utf8": "ameloblasts that secrete enamel"
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  }, {
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    "segs": [ {
      "utf8": "and odontoblasts that secrete dentin."
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    "segs": [ {
      "utf8": "As they form teeth, \nodontoblasts move inward,"
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  }, {
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    "segs": [ {
      "utf8": "while ameloblasts move out \nand slough off when they hit the surface."
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    "segs": [ {
      "utf8": "For enamel, this process produces long, \nthin strands,"
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  }, {
    "tStartMs": 85567,
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    "segs": [ {
      "utf8": "each about 60 nanometers in diameter."
    } ]
  }, {
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    "segs": [ {
      "utf8": "That’s one one-thousandth \nthe width of a human hair."
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  }, {
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    "segs": [ {
      "utf8": "Those are bundled into rods, \npacked together,"
    } ]
  }, {
    "tStartMs": 95695,
    "dDurationMs": 2698,
    "segs": [ {
      "utf8": "tens of thousands per square millimeter,"
    } ]
  }, {
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    "segs": [ {
      "utf8": "to form the shield-like enamel layer."
    } ]
  }, {
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    "segs": [ {
      "utf8": "Once this process is finished, \nyour enamel can’t repair itself again"
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  }, {
    "tStartMs": 105264,
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    "segs": [ {
      "utf8": "because all the cells \nthat make it are lost,"
    } ]
  }, {
    "tStartMs": 108745,
    "dDurationMs": 3479,
    "segs": [ {
      "utf8": "so we’re lucky that enamel \ncan’t be easily destroyed."
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    "segs": [ {
      "utf8": "Odontoblasts use a more complex process,\nbut unlike ameloblasts, they stick around,"
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  }, {
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    "segs": [ {
      "utf8": "continuing to secrete dentin \nthroughout your life."
    } ]
  }, {
    "tStartMs": 121845,
    "dDurationMs": 3331,
    "segs": [ {
      "utf8": "Despite the differences in teeth \nacross the mammalian order,"
    } ]
  }, {
    "tStartMs": 125176,
    "dDurationMs": 4828,
    "segs": [ {
      "utf8": "the underlying process of tooth growth \nis the same whether it’s for lions,"
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  }, {
    "tStartMs": 130004,
    "dDurationMs": 1031,
    "segs": [ {
      "utf8": "kangaroos,"
    } ]
  }, {
    "tStartMs": 131035,
    "dDurationMs": 929,
    "segs": [ {
      "utf8": "elephants,"
    } ]
  }, {
    "tStartMs": 131964,
    "dDurationMs": 1602,
    "segs": [ {
      "utf8": "or us."
    } ]
  }, {
    "tStartMs": 133566,
    "dDurationMs": 3508,
    "segs": [ {
      "utf8": "What changes is how nature sculpts \nthe shape of the tooth,"
    } ]
  }, {
    "tStartMs": 137074,
    "dDurationMs": 2315,
    "segs": [ {
      "utf8": "altering the folding and growth patterns"
    } ]
  }, {
    "tStartMs": 139389,
    "dDurationMs": 3845,
    "segs": [ {
      "utf8": "to suit the distinct diets \nof different species."
    } ]
  }, {
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    "segs": [ {
      "utf8": "Cows have flat molar teeth with parallel \nridges for grinding tough grasses."
    } ]
  }, {
    "tStartMs": 148513,
    "dDurationMs": 5679,
    "segs": [ {
      "utf8": "Cats have sharp crested molars, \nlike blades, for shearing meat and sinew."
    } ]
  }, {
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    "dDurationMs": 4891,
    "segs": [ {
      "utf8": "Pigs have blunt, thick ones, \nuseful for crushing hard roots and seeds."
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  }, {
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    "segs": [ {
      "utf8": "The myriad molars of modern mammals"
    } ]
  }, {
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    "segs": [ {
      "utf8": "can be traced back to a common form\ncalled “tribosphenic,\""
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  }, {
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    "segs": [ {
      "utf8": "which first appeared \nduring the dinosaur age."
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  }, {
    "tStartMs": 167972,
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    "segs": [ {
      "utf8": "In the 19th Century, \npaleontologist Edward Drinker Cope"
    } ]
  }, {
    "tStartMs": 171961,
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    "segs": [ {
      "utf8": "developed the basic model \nfor how this form evolved."
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      "utf8": "He hypothesized that \nit started with a cone-like tooth,"
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  }, {
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    "dDurationMs": 3380,
    "segs": [ {
      "utf8": "as we see in many fishes, \namphibians, and reptiles."
    } ]
  }, {
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    "segs": [ {
      "utf8": "Small cusps were then added, \nso the tooth had three in a row,"
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  }, {
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    "segs": [ {
      "utf8": "aligned front to back,\nand connected by crests."
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    "segs": [ {
      "utf8": "Over time, the cusps were pushed out \nof line to make triangular crowns."
    } ]
  }, {
    "tStartMs": 195524,
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    "segs": [ {
      "utf8": "Adjacent teeth formed a continuous \nzigzag of crests for slicing and dicing."
    } ]
  }, {
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    "dDurationMs": 3921,
    "segs": [ {
      "utf8": "A low shelf then formed \nat the back of each set of teeth,"
    } ]
  }, {
    "tStartMs": 205114,
    "dDurationMs": 2607,
    "segs": [ {
      "utf8": "which became a platform for crushing."
    } ]
  }, {
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    "segs": [ {
      "utf8": "As Cope realized, the tribosphenic molar \nserved as the jumping-off point"
    } ]
  }, {
    "tStartMs": 212545,
    "dDurationMs": 3349,
    "segs": [ {
      "utf8": "for the radiation of specialized\nforms to follow,"
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  }, {
    "tStartMs": 215894,
    "dDurationMs": 2498,
    "segs": [ {
      "utf8": "each shaped by evolutionary needs."
    } ]
  }, {
    "tStartMs": 218392,
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    "segs": [ {
      "utf8": "Straighten the crests \nand remove the shelf,"
    } ]
  }, {
    "tStartMs": 220899,
    "dDurationMs": 3604,
    "segs": [ {
      "utf8": "and you’ve got the conveniently \nbladed teeth of cats and dogs."
    } ]
  }, {
    "tStartMs": 224503,
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    "segs": [ {
      "utf8": "Remove the front cusp, raise the shelf, \nand you’ve got our human molars."
    } ]
  }, {
    "tStartMs": 230335,
    "dDurationMs": 3690,
    "segs": [ {
      "utf8": "A few additional tweaks get you \na horse or cow tooth."
    } ]
  }, {
    "tStartMs": 234025,
    "dDurationMs": 4209,
    "segs": [ {
      "utf8": "Some details in Cope’s intuitive\nhypothesis proved wrong."
    } ]
  }, {
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    "segs": [ {
      "utf8": "But in the fossil record,"
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  }, {
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    "segs": [ {
      "utf8": "there are examples of teeth \nthat look just as he predicted"
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    "segs": [ {
      "utf8": "and we can trace the molars of all living\nmammals back to that primitive form."
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    "segs": [ {
      "utf8": "Today, the ability to consume \ndiverse forms of food"
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  }, {
    "tStartMs": 251743,
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    "segs": [ {
      "utf8": "enables mammals to survive in habitats"
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  }, {
    "tStartMs": 254119,
    "dDurationMs": 2926,
    "segs": [ {
      "utf8": "ranging from mountain peaks \nand ocean depths"
    } ]
  }, {
    "tStartMs": 257045,
    "dDurationMs": 2224,
    "segs": [ {
      "utf8": "to rainforests and deserts."
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  }, {
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    "segs": [ {
      "utf8": "So the success of our biological class \nis due in no small measure"
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    "dDurationMs": 2590,
    "segs": [ {
      "utf8": "to the remarkable strength \nand adaptability"
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  }, {
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    "segs": [ {
      "utf8": "of the humble mammalian molar."
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