Single-celled or simple multicellular eukaryotic organisms that generally do not fit in any other kingdom are called protists. Most pro-tists are microscopic, but a few protists, such as some algae, are several meters in length. Protists are defined by exclusion—most protists are eukaryotic organisms that cannot be classified as fungi, plants, or animals. As a result, protists are the most diverse group of eukaryotes. Protists have varying body plans, types of movement, and means of obtaining food. Protists are made up of eukaryotic cells, each containing a nucleus and other organelles. Most living protists contain mitochondria. Some protists, such as Euglena shown in Figure 25-1, also contain chloroplasts.
Protists emerged early in the history of domain Eukarya. Some of the oldest eukaryotic cells were protists. Protists emerged as much as 2 billion years ago.
By analyzing genetic information in the nucleus, mitochondria, and chloroplasts of protists, scientists have found strong evidence that the first protists arose from prokaryotic cells. Nuclear genes in protists and in other eukaryotes resemble archaeal and bacterial genes. Additionally, the genetic information found in the mitochondria and chloroplasts of protists and other eukaryotes is similar to genetic information found in bacteria and cyanobacteria. Because of these genetic similarities, scientists hypothesize that protists and other eukaryotes arose from ancient prokaryotes that lived in endosymbiosis. In endosymbiosis, an organism lives inside a larger organism. Scientists think that mitochondria and chloro-plasts arose from ancient prokaryotes that lived inside larger prokaryotes. Over time, the endosymbiotic prokaryotes became organelles within eukaryotic protists.
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