The animals in the halls of vertebrate evolution are arranged according to their evolutionary relationships. Each animal is grouped with its closest relatives. The sequence of these groups forms a giant evolutionary diagram, or cladogram. You can follow the main path of the cladogram by following the main path through the halls. Evolutionary branching points on the main path highlight advanced features that the groups of animals farther down the path have in common.

Branching off the main path are alcoves focusing on major groups of closely related animals. In each alcove, an information station offers an interactive overview, in which curators discuss the animals and the features they share. The stations also show videos of the animals' living relatives, or, if the group is extinct, animations of how they might have looked and moved.

Essential to our understanding of the world and our place in it is the understanding that all living things have evolved as a result of inherited changes and diversification over vast periods of time. The fossil halls exhibit animals, both living and extinct, that represent the evolutionary history of vertebrates. As people trace their family history by compiling a family tree, so scientists can reconstruct evolutionary history by compiling evolutionary "family trees." Therefore the Museum, where evolution is a major area of research, decided to organize these halls as an evolutionary tree of vertebrates.

What is the Best Way to Reconstruct Evolutionary History?
Scientists build evolutionary trees using a method called cladistics, in which organisms are grouped according to shared features. The distribution of features forms a set of nested groups, in which smaller groups are contained within larger ones. For example, the group "tetrapods" (animals with 4 limbs) is contained within the larger group "vertebrates" because tetrapods, like other vertebrates, have a backbone and a braincase, which are the defining features of the group called vertebrates. Each group, or clade, is recognized by a set of such advanced features inherited from a common ancestor. A clade contains all descendants of the common ancestor.

What is a cladogram?
A cladogram is a visual reconstruction of the evolutionary history of a group of animals, based on the distribution of newly evolved ("advanced") features. Cladograms are drawn as branching diagrams, with the advanced features noted at the appropriate branching points. Compare the cladograms below, one of which shows the "evolution" of advanced modes of transportation, the other the evolution of vertebrates.

What is an advanced feature?
As animals evolve, they develop new features, or characteristics. The descendants often diversify and form other groups, but they all inherit the advanced features. An advanced feature can be any attribute of an animal, from the shape of its bones and muscles to its genetic chemistry and DNA. The term "advanced" is relative -- it does not necessarily mean that the feature is better or more efficient than the primitive feature that it evolved from.

Why use cladistics?
Although cladistics provides us with the best current method of determining evolutionary relationships, it is not perfect. Contradictions among advanced features often suggest alternative evolutionary trees. In such cases, the cladogram consistent with the most features is chosen for the time being. True evolutionary relationships can never be definitively established, either by examining fossils or studying DNA. But we can get closer and closer to the actual sequence of evolution by testing hypotheses about relationships with as many features as possible.