Geoscience Reference
In-Depth Information
This is why tetrapods are considered to represent a subgroup within
vertebrates. In this example, the fossil record appears to confirm
this idea, because the first known vertebrate lived about 500 million
years ago, whereas the first known tetrapod appeared about 350 mil-
lion years ago.
However, the fossil record does not always confirm the evolution-
ary sequence of characteristics suggested by looking at features shared
by larger and smaller groups. This is not too surprising because only
a small fraction of the organisms that once inhabited the earth are
preserved as fossils, and many of those are yet to be discovered. How
else could we explain why paleontologists discover new species of
extinct dinosaurs and other organisms every year? Because the fossil
record is so incomplete, most contemporary paleontologists prefer
not to rely on the age of the fossils when reconstructing the family
trees of ancient organisms. This task is more accurately achieved by
comparing only the physical characteristics, such as backbones and
limbs, preserved in the fossils.
Reconstructing the genealogy of plants and animals is often fur-
ther complicated since the enormous array of characteristics found
in these organisms—from physical features to molecular genes—did
not evolve only once in a linear fashion. Similar characteristics have
evolved in animals that are apparently not closely related. Among
living animals, for example, wings evolved in insects, bats, and birds.
A warm-blooded metabolism is found in mammals, birds, and tuna.
Humans, birds, and kangaroos are all bipedal, meaning that they
walk exclusively on their two hind legs. Other examples of features
shared by distantly related groups abound.
An important component of cladistics involves sorting out whether
these similar characteristics evolved from the same ancestor or dif-
ferent ancestors. To make this decision scientists rely on the principle
of parsimony, often referred to as Occam's razor. Given alternative
genealogical interpretations, the principle chooses the simplest expla-
nation that accounts for the arrangement of groups on the family tree.
In other words, it identifies the evolutionary sequence of events that
is supported by the most pieces of evidence and contradicted by the
fewest pieces of evidence. For example, there are two alternative
interpretations involving the genealogy of bats: Are bats more closely
related to birds or to mammals? Although bats and birds both have
