Figure 1.1 This NASA image is a composite of several satellite-based views of Earth. To make this whole-Earth image, NASA scientists combine observations of different parts of the planet. (credit: NASA/GSFC/NOAA/USGS)

Viewed from space, Earth offers no clues about the diversity of life it harbors. The first forms of life on Earth are thought to be microorganisms that existed for billions of years in the ocean before plants and animals appeared. The mammals, birds, and flowers that we see in modern times are mostly recent species, originating 130 to 200 million years ago. In fact, only in the last 200,000 years have humans started looking like we do today.

Organisms evolve in response to each other. One of the best examples is disease-causing organisms, which have to adapt to overcome the defenses of the organisms they infect. One such organism that has evolved to specialize in infection in humans is Plasmodium, the organism that causes malaria. Biologists use the process of science to learn about the world and the organisms living in it. For example, people have suspected for quite some time that people with blood type O are less likely to die from severe malaria. Now, a team of scientists have been able to explain why. By examining data from several experiments, and by using both inductive and deductive reasoning, the scientists concluded that A and B type blood reacts with a protein excreted by Plasmodium. This reaction causes severe illness. However, type O blood does not react with the protein. You can read more about the response of type A and B blood groups to infection by Plasmodium.