In view of the nearly countless, wildly varying forms of life that surround us, it is easy to forget that all the species living on Earth today represent only the tip of the iceberg of evolutionary history By most scientific estimates, 99 percent of the species that have ever inhabited our planet are now extinct.

Charles Darwin and Alfred Russel Wallace gave us a theory of evolution that species descended from a common ancestor. In elegant simplicity it explained both the diversity and the similarity of all species on Earth. What these visionaries did not do--what was not possible in their time--was trace all the species' lines of descent in the form of a phylogenetic tree, or Tree of Life.

Figuring out the Tree of Life for the hordes of living species (estimates run anywhere from 4 million to 100 million) and the tens of thousands of known fossil species has been the job of several succeeding generations of scientists. Within just our own class of animals, Mammalia, researchers have identified more than 5,000 living species and as many as 10,000 extinct ones. Paleontologically speaking, we are well off: Mammals boast a particularly good fossil record, including important early transitional species.

Now we are beginning to tackle this great challenge--for life as a whole, and for mammals in particular--with new tools for studying the anatomy of living and fossil species, with modern algorithmic methods, and with extensive genomic data. The goal is to build a phylogenetic tree, using all these living and extinct species, that will provide solid support for our reconstructions of the past.

Why Study the Tree of Life?

The availability of new software that allows scientists from many different labs to work together online as a research team has opened new opportunities for organizing phenomic data (that is, all of an organism's nongenomic traits) on a large scale. At the forefront of such transformations has been the mammal Tree of Life project, a multiyear, multi-investigator initiative funded by the National Science Foundation.

Making a data-rich outline of the lineages of placental mammals, both living and extinct, allows us to interpret how traits evolved over time. With this tree we can tackle intriguing questions, such as: Did whales change their diet before or after they lost their limbs and evolved away from land to an obligatory aquatic life? Or, closer to the issue of human origins, did rodents exist as early as primates?

Delineating the mammal Tree of Life was an undertaking that required the efforts of large and varied groups of specialists. The mammal phenomic team, in which I participated, included 22 other scientists and doctoral students. The scale of our task was unprecedented: Most previous studies had itemized an order of magnitude fewer traits and had left no online images or data references for scientists and the public. We addressed the project with a new web application called MorphoBank that gives scientists the means to organize, edit, and illustrate published information on traits observed in mammals....