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A matrix of phenomic, genomic, and fossil evidence supports the scientific account of how the animals exploded in diversity 65 million years ago.
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 modem 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...