Amidst all the marvelous diversity of life on Earth—between 10 and 30 million distinct species are thought to populate the planet—it's not surprising that some living things have similar body parts and structures, while others have nothing in common. To help us better understand the natural relationships that exist between species, scientists called taxonomists attempt to place organisms in distinct groups through careful examination of their characteristics.

For hundreds of years, philosophers and naturalists sought ways to classify the various living things that populate the planet. Until recently, they focused solely on physical characteristics, such as body part size and bone structure. Charles Darwin determined that the closest species are not necessarily those that look the most alike—the guiding principle for Carl Linnaeus's traditional classification scheme—but rather those that share the most recent common ancestry. In the course of evolution, Darwin reasoned, a new trait will emerge in an individual organism that will be passed on to its descendants. Therefore, two organisms that share this trait are more closely related than two organisms that lack this trait.

Today, taxonomists use genetics to further assist them in classifying organisms. They compare DNA sequences to determine evolutionary relationships between specimens they study. The millions of chemical letters of DNA code stored in the cells of all living things provide a distinctive genetic profile of a species. The more similar sequences there are between species, the closer their evolutionary relationship.

Understanding the evolutionary relationships between organisms helps us understand how the diversity of life on Earth came to be. By combining information from DNA analysis of fossils, researchers can draw evolutionary trees that depict where and approximately when different species branched off from common ancestors. These diagrams are hypotheses, and, as such, represent our best understanding of the true evolutionary relationships based on existing evidence.

Continued advances in taxonomic research benefits both scientists and the organisms they study. The ability to place plants or animals in groups allows scientists to apply new information about one living thing to other, related ones. Furthermore, we can use existing information about a plant or animal to improve its care, monitor its health, and, in the case of threatened or endangered species, help in establishing recovery programs.

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