Tree of Life
The tree of life, also called a phylogenetic tree, is a graphic tool that biologists use to portray evolutionary relationships among plants, animals and all other forms of life. The tree reveals evolutionary histories: Each "fork in the road," or branching point, indicates a common ancestor splitting into two descendants. And the fewer branching points there are between any two species, the more closely they are related--a feature that has great predictive value. Indeed, a botanist who discovers a useful pharmacological property in one plant species might investigate "sister" species for similar properties.
Beginning with Darwin, scientists focused on shared anatomical features to determine evolutionary relationships. More recently, they have learned that the history of evolution is also recorded in DNA, the set of instructions for building bodies encoded in all living cells. When plants and animals reproduce, they pass copies of their DNA on to their offspring. But over time, the DNA of a species changes, usually as a result of copying errors known as mutations. Scientists can compare DNA to help determine evolutionary relationships: in general, the greater the difference in DNA between two species, the more time must have passed since the two groups were one, since they diverged from a common ancestor.
All in the Family
At a glance, neither a mosquito nor a chicken appears to be closely related to humans. But a chicken's anatomy--its lungs and backbone, for instance--is much more similar to a human's than is a mosquito's. DNA comparisons confirm that chickens and humans are closer relatives: 79% of chicken genes are similar to ones found in humans, while "only" 43% of mosquito genes have human parallels. In fact, all forms of life--including plants, bacteria and humans--possess many genes that are very similar in DNA sequence, testament to the fact that all life is descended from a common ancestor.
One in a Million
This monkey frog is but one example of the millions of species on Earth. The tree of life on Earth portrays the evolutionary relationships among groups of plants, animals and all other forms of life. Any one of those groups encompasses many subgroups, and each subgroup many individual species. Here, another tree "zooms in" on amphibians and depicts the relationships among more than 60 amphibian families and subfamilies. In turn, the third tree focuses on one family of amphibians--tree frogs--and details the relationships of about 200 of its nearly 800 distinct species.