Evolution What the Fossils Say and Why it Matters

138 Evolution and the Fossil Record

lack tails) and our grasping hands and stereovision from our earliest primate ancestors
(nearly all primates have stereovision and grasping hands with opposable thumbs). Like-
wise, we inherited our hair and mammary glands from our distant mammalian ancestors
(all mammals have them) and our four-legged bodies and lungs from our distant tetrapod
ancestors (all tetrapods, including amphibians, reptiles, birds, and mammals, have them).
If we wanted to define what makes us human, it would involve features related to our large
brains and bipedality, our most recently developed evolutionary novelties, not our primitive
features, such as a lack of tail, stereovision, grasping hands, hair, mammary glands, four
legs, or lungs. The older classification schemes often mixed together primitive features and
advanced features in their definitions, but Hennig pointed out that only the shared derived, or
shared advanced, characters are really valid in defining natural groups.
We can define relationships based on these shared derived characters. Humans and
monkeys (fig. 5.3) are more closely related to each other than they are to any other organisms
in this diagram because they have many shared evolutionary novelties not found elsewhere
in the animal kingdom, including opposable thumbs, stereovision, and many other features
that define primates. A group including humans, monkeys, and cows could be defined by
shared derived characters such as hair and mammary glands, unique features that define
the class Mammalia. A group including mammals plus frogs could be defined on the shared
presence of four legs and lungs, features that are found in all tetrapods. Likewise, sharks are
more closely related to frogs and mammals than they are to lampreys because they have the
advanced features of jaws and true vertebrae in their backbones. Notice that we did not use
characters like jaws to define a group such as the mammals because jaws are primitive for
mammals but derived at a much deeper level, at the level of the earliest jawed vertebrates
(the gnathostomes). Characters are primitive or derived relative to the level at which they are
being used, and according to Hennig, we must use them only at the level at which they first
appear as evolutionary novelties.

FIGURE 5.3. Evolutionary relationships of an assortment of vertebrates, showing the shared specializations
(evolutionary novelties) that support each branching point (node) on this cladogram.

LAMPREY SHARK FROG COW MONKEY HUMAN

Opposable thumb, stereovision

Hair, mammary glands

Four limbs, lungs

Jaws, vertebrae