All living things have evolved into three groups, or domains, of closely related organisms: Archaea, Bacteria, and Eukaryota. Archaea and Bacteria are small, simple cells surrounded by a membrane and a cell wall, with a circular strand of DNA containing their genes. They do not contain a nucleus or other internal structures that higher cells may have. These are called prokaryotes. Basically all the life you see today, including plants and animals, belongs to the third domain, Eukaryota. Eukaryotic cells are more complex than prokaryotes; the major difference between the eukaryotic and prokaryotic cells is the nucleus. The nucleus in a eukaryotic cell is where genetic material is stored. These cells also contain their own source of energy, produced by an organelle called the mitochondria. An organelle is basically a cell within a cell that performs a specialized function. These organelles produce chemical energy, but also are a major factor in understanding the evolution of eukaryotic cells. Although there are many differences between eukaryotic cells and prokaryotic cells in size, complexity, and internal components; there is a similarity between prokaryotic cells and the organelles of eukaryotic cells. The complex eukaryotic cell began a whole new chapter for life on Earth, because they were evolving into multicellular organisms. Evidence supports the idea that eukaryotic cells are actually the descendents of separate prokaryotic cells that joined together in what is called a symbiotic relationship. The first living things would have been prokaryotes. The endosymbiotic hypothesis suggests that free living cells were absorbed by another cell, where the engulfed cell would perform mutually benefiting functions. This strategy would greatly expand the number of environments in which the cells could survive. Mitochondria or the “power house of the cell” is thought to be descends of bacteria that were absorbed through endocytosis. A process where cells absorb things...
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