If you compare a chicken with a crocodile, you might find it hard to imagine
that they are closely related. And it might strike you as implausible that birds
are even more closely related to dinosaurs. In fact, they are dinosaurs—in the
same major group as Tyrannosaurus (FIGURE 16.1).
How can biologists know that? How confident can we be in these outra-
geous claims? Can we be sure that multicellular organisms evolved indepen-
dently many times, that headless echinoderms evolved from ancestors with
heads, that α- and β-hemoglobins evolved by gene duplication in ancient
vertebrates? Any biology or anthropology textbook will state that humans and
chimpanzees are each other’s closest relatives, and will describe how humans
evolved in Africa and spread from there throughout the world. How do we
learn about the history of life on Earth?
Two sources of information inform us about the past: fossils and living organ-
isms. Paleontology reveals organisms and events we could not have imagined
without fossils, as we will describe in the next two chapters. But the record of
even the best-fossilized groups of organisms is incomplete, and many kinds of
organisms—especially those that lack skeletons or other hard structures—have a
very poor fossil record or none at all. Many features we are interested in—behav-
ior, physiology, life history, genome structure, and more—cannot be ascertained
from fossils. In these situations, we piece together information from living spe-
cies, chiefly through analyzing their phylogenetic trees and the history by which
differences among species have arisen (see Chapter 2). Phylogeny, moreover,Skeletal evidence that birds have evolved from theropod dinosaurs has been
reinforced by the discovery of feathers in many dinosaurs. These tail feathers of a young
mid-Cretaceous coelurosaurian theropod, exquisitely preserved in amber, were found
recently in China [37]. Their structure supports a hypothesis about the steps by which
feathers evolved. (Courtesy of Royal Saskatchewan Museum [RSM/R. C. McKellar].)Phylogeny:
The Unity and
The History of Life
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