The Emergence of Primates
Primates have four different kinds of teeth—incisors (front biting teeth), canines (pointed teeth behind and next to the incisors), premolars (medium-sized grinding teeth behind the canines), and molars (larger grinding teeth in the back of the mouth). Knowing the number and kinds of teeth that a certain primate had enables scientists to determine the shape of the animal's mouth. For example, some of the earliest primates had four premolars, while lemurs have only three, and humans have two. The size of a single tooth can tell paleontologists how large a certain primate was—a tiny tooth implies a tiny primate. In addition, the pattern of bumps and crannies on each tooth can tell scientists what the animal ate. For example, insect-eating animals generally had molars with sharp cusps (bumps) that allowed them to puncture and slice their food, while animals that ate fruits generally had molars with low cusps that enabled them to crush the fruits.
Because mammals inherit the shapes of their teeth from their parents, paleontologists can trace the primate family tree by analyzing the patterns they discover in each tooth. For example, if an early species had sharp cusps on a particular molar, and several later species had progressively lower cusps on corresponding molars, paleontologists might infer that the sharp-cusped species was an ancestor of the successively lower-cusped species. Paleontologists piece together such clues to determine how different animals are related to one another. Sometimes, however, the information seems contradictory. For example, patterns on molars from a group of species might indicate a clear line of evolutionary descent, but evidence from incisors of the same group might seem conflicting and inconclusive.
To resolve such conflicts, scientists have devised rules that allow them to pay more attention to some characteristics than to others. For example, one rule states that the only characteristics that can be used to trace branching patterns in a family tree are those that are clearly shared by a group of animals and that are derived (originated and evolved) in that group. Scientists must be able to trace a progression of small changes in characteristics from an ancestral species through its various descendants. Thus, if two animals differ greatly in a certain characteristic, the two animals may nevertheless be related if evidence from the characteristics that they do share points to their kinship.
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