12 The History of Life

MAIN IDEAEukaryotic cells may have evolvedthrough endosymbiosis.The fossil record shows that eukaryotic organisms had evolved by 1.5 billionyears ago. Unlike a prokaryote, a eukaryote has a nucleus and other membrane-boundorganelles. Eukaryotic cells were all aerobic. While the firsteukaryotes were made of only one cell, later eukaryotic organisms becamemulticellular, or made of many cells. All cells in multicellular organisms todayare eukaryotic.One hypothesis of eukaryote evolution, proposed more than a hundredyears ago, did not get much attention until the 1970s. Biologist Lynn Margulisof the University of Massachusetts found evidence to support the theory ofendosymbiosis. Endosymbiosis (EHN-doh-SIHM-bee-OH-sihs) is a relationshipin which one organism lives within the body of another, and both benefitfrom the relationship.The theory of endosymbiosis suggests that early mitochondria and chloroplastswere once simple prokaryotic cells that were taken up by larger prokaryotesaround 1.5 billion years ago. Instead of being digested, some of the smallerprokaryotes may have survived inside the larger ones as illustrated in FIGURE 12.13.This relationship would have had its advantages. If it took in a prokaryote thatacted as a mitochondrion, the larger cell got energy in the form of ATP. If ittook in a prokaryote that acted as a chloroplast, the larger cell could use photosynthesisto make sugars. In exchange, the mitochondria and the chloroplastsfound a stable environment and nutrients.Margulis based her theory on several factors. Unlike other organelles,mitochondria and chloroplasts have their own DNA and ribosomes. Theycan copy themselves within the cell in which they are found. Mitochondriaand chloroplasts are also about the same size as prokaryotes, their DNA formsa circle, and their gene structures are similar to that of prokaryotes.Analyze What evidence supports the theory of endosymbiosis?VOCABULARYEndosymbiosis can be brokendown into endo-, meaning“within,” sym-, meaning“together,” and biosis, meaning“way of life.”FIGURE 12.13 EndosymbiosisThe theory of endosymbiosis proposes that the mitochondria found ineukaryotic cells descended from ancestors of infection-causing bacteria.Likewise, chloroplasts are considered descendants of cyanobacteria.early nuclearenvelopebacteriumcellBacterium enters cell.Over generations,bacteria evolve asmitochondria.HOST CELLBIOLOGYWatch endosymbiosisin action atClassZone.com.Infection-causing bacteria entering ahost cellcolored SEM; magnification 13,000Chapter 12: The History of Life 373