What set mammals on the road to greatness? Forget about brains and warm blood--it may have been a small change in their molars.
When the dinosaurs disappeared 65 million years ago, our mammalian ancestors had a tremendous opportunity. The world was theirs for the taking. There was just one problem: those would-be world rulers had teeth that were far from world-class. The early mammals were small, humble bug eaters, and so they would have remained if their teeth hadn’t changed. John Hunter, a paleontologist at the State University of New York at Stony Brook, has been studying the evolution of mammalian teeth. He and University of Helsinki evolutionary biologist Jukka Jernvall believe that an inconspicuous dental feature that first appeared 65 million years ago-- called the hypocone--was the key to the success of mammals, enabling them to exploit a wide range of tough, fibrous plant foods.
Unlike dinosaurs, who grew new teeth throughout their lives, mammals get only two sets. Early mammals had triangular upper molars with three cusps, and lower molars shaped a bit like an L lying on its side, so that the short arm of the L stuck up and fitted into gaps between the upper teeth. This design was fine for chomping insects but not so appropriate for chewing and grinding fibrous plants, which require a lot of surface contact between upper and lower teeth.
As mammals gradually diversified and filled the dinosaur-free world, however, some of them began eating plants. And some of them developed a fourth cusp--the hypocone--that squared off the surface of their formerly triangular upper molars. At the same time, the upward- sticking part of the lower molars was shortened by the loss of a cusp. The result: The gaps between the upper molars were filled, and their cusps fitted between the cusps of the lower molars. That increased the overall contact between upper and lower teeth, says Hunter, and it doubled the area devoted to grinding food.
Although the fourth cusp and its connection to herbivory have been known to paleontologists for some time, Hunter and Jernvall have now found evidence that the appearance of this small dental feature was directly related to an explosion in mammal species in the mid-Eocene some 40 million years ago. The researchers analyzed the fossil record to find which feature, among all the adaptations for plant eating, most herbivorous mammals shared. One trait stood out: the hypocone. It seemed to account for why some orders of mammals are more successful than others--assuming the number of species in an order is a measure of its success. Hunter and Jernvall found that over the last 65 million years, 14 of the 26 mammalian orders with hypocones had 10 or more species; only 7 of the 28 mammalian orders without hypocones had that many. Several orders with hypocones spawned more than 30 species on some continents, a diversity unmatched in orders lacking hypocones.
Perhaps most telling, Hunter and Jernvall estimate that hypocones arose independently at least 20 separate times in the course of mammalian evolution. I find it interesting that there are so many groups that have independently evolved that feature, says Richard Stucky, chief curator and mammal expert at the Denver Museum of Natural History. The origin of the hypocone in any one group is just a random event, but the continuation and great frequency of it must be because it gave those organisms that had it an advantage over others.
What happened in the mid-Eocene to make hypocones so useful? Earth got cooler, says Hunter. The early Eocene was a warm period, a time when the Great Plains, for instance, was a forested subtropical paradise. The climate was ideal for hypoconally challenged insect and fruit eaters. (Fruits are easy to digest and don’t need to be chewed as thoroughly as cellulose-rich plant fibers.) But from the mid-Eocene, the climate became progressively cooler, with greater seasonal extremes in temperature and rainfall. Forests became scrub. For most mammals, including our own ancestors, well-developed hypocones seem to have been de rigueur.