The prokaryotes as the bacteria and archaea are collectively known, constitute two of the three domains of life. The third domain is the eukaryotes. Eukaryotes include most forms of life that we're familiar with, including all of the complex organisms like plants, fungi, and animals. These organisms are familiar to us in part because many of them are large enough for us to see, because they're multicellular, meaning that they consist of many cells that work together as a single individual. But multicellularity isn't what distinguishes the eukaryotes from the prokaryotes. In fact, a great many eukaryotes, most of them actually are single-celled organisms, not much larger than a bacterium, and some are actually smaller than bacteria. What distinguishes the eukaryotes from the prokaryotes is the structure of their cells. Eukaryotes have a nucleus which houses their genome. The DNA that makes up the genome is also packaged differently. The DNA of prokaryotes is formed into a circular chromosome, while eukaryotes have one or more linear chromosomes. Outside of the nucleus of eukaryotic cells is the cytoplasm, a gelatinous liquid that contains various other structures called organelles. The organelles have their own membranes, similar to the membranes that surround the entire cell. Examples of organelles include the mitochondria, which generates chemical energy for the cell to use. In the case of plants, the chloroplast which contains the pigments used in photosynthesis. Eukaryotic cells also have a cytoskeleton, a network of the structures that provide support to the cell, that also helps move organelles around within the cell and makes it possible for them to take on different shapes. Eukaryotic cells evolved from prokaryotic ancestors, specifically archaea. The complexity of eukaryotic cells is the result of a series of evolutionary events. Some of these included a process that was once considered a wild and crazy idea. The person most closely associated with this wild and crazy idea was biologists Lynn Margulis. As a graduate student, she became intrigued by the observation that DNA is sometimes found in the cytoplasm of eukaryotic cells. This seemed odd because the DNA genome of eukaryotes is housed in the nucleus. She began studying single-celled eukaryotes like euglena, which live in ponds and engage in photosynthesis. She found that the chloroplasts, the organelles that contain chlorophyll inside the euglena had their own DNA. Margulis saw this as a clue. She noted, as a few others had before, that organelles like chloroplasts and mitochondria resemble bacteria. Both are surrounded by membranes, both have DNA in the form of a circular chromosome, both have structures such as ribosomes for translating their DNA into proteins, and both replicate themselves through a process of binary fission. Margulis believed that these similarities were not coincidences. Rather, she argued that organelles like chloroplasts and mitochondria were once free living bacteria. According to this theory, the ancestors of eukaryotes acquired their organelles when one cell and archaea engulfed another, a bacterium. Rather than consuming it, the two joint forces through a process called endosymbiosis. Margulis wasn't the first to come up with the idea of endosymbiosis. Several others had suggested it before her, but she was the first to accumulate a substantial amount of evidence to support it. But her theory was met with more than just a little skepticism at first. The paper in which she presented the idea along with the evidence she had amassed, was rejected some 15 times before finally being accepted for publication in 1967. The idea was considered wild and crazy in part because until then, evolution was mostly thought to be about what Darwin called the struggle for existence. The emphasis was on competition, not cooperation. Margulis saw the history of life differently. Organisms could come together peacefully and their union could create something new. Lynn Margulis was also a woman working in a scientific field dominated by men. For much of her career, she was regarded as an outsider with ideas on the fringes of the scientific mainstream. When asked if she ever grew tired of being the subject of controversy, she replied, "I don't consider my ideas controversial, I consider them right". Her determination and perseverance would eventually pay off. One of the definitive tests of the theory of endosymbiosis involved a collaboration between Lynn Margulis and Carl Woese. The biologist who's worked with ribosomal RNA, revealed the existence of the archaea. They compared the genetic material within the mitochondria to the genetic material within the nucleus of the same cell, as well as that of free-living bacteria. The result was clear and profound. The mitochondria, as well as the chloroplasts inside eukaryotic cells are more closely related to bacteria than they are to the nuclei inside the very same cell they both reside in. Lynn Margulis is wild and crazy idea that eukaryotic cells came about through endosymbiosis, finally became widely accepted.