Learning Objectives
Mitochondria and the origin of eukaryotesEukaryotes evolved during the Proterozoic eon approximately 1.6 BYA. Prior to the origin of eukaryotes, all life on Earth was prokaryotic (lacking nucleus or other membrane-bound organelles). The leading hypothesis, called the endosymbiotic theory, is that eukaryotes arose as a result of a fusion of Archaean cells with bacteria, where an ancient Archaean engulfed (but did not eat) an ancient, aerobic bacterial cell. The engulfed (endosymbiosed) bacterial cell remained within the archaean cell in what may have been a mutualistic relationship: the engulfed bacterium allowed the host archean cell to use oxygen to release energy stored in nutrients, and the host cell protected the bacterial cell from predators. Over many generations, a symbiotic relationship developed between the two organisms so completely that neither could survive on its own. Microfossil evidence suggests that eukaryotes arose sometime between 1.6 and 2.2 billion years ago. The dependents of this ancient engulfed cell are present in all eukaryotic cells today as mitochondria. Show
The first eukaryote may have originated from an ancestral prokaryote that had undergone membrane proliferation, compartmentalization of cellular function (into a nucleus, lysosomes, and an endoplasmic reticulum), and the establishment of endosymbiotic relationships with an aerobic prokaryote which led to mitochondria. Some early eukaryotes later engulfed a photosynthetic bacterium similar to cynanobacteria, which led to chloroplasts in modern-day photosynthetic eukaryotes. By Phil Schatz – http://philschatz.com/biology-concepts-book/contents/m45513.html, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=68096564 The video below gives an overview of the endosymbiotic theory for the origin of eukaryotes: Chloroplasts and photosynthetic eukaryotesThe information below was adapted from OpenStax Biology 23.1 Some groups of eukaryotes are photosynthetic. Their cells contain, in addition to the “standard” eukaryotic organelles, photosynthetic organelles called chloroplasts. Like mitochondria, chloroplasts appear to have an endosymbiotic origin. Chloroplasts are derived from cyanobacteria that lived inside the cells of an ancestral, aerobic, heterotrophic eukaryote. Evidence suggests that engulfment of a cyanobacteria-like organism has happened twice in the history of eukaryotes: in one case, the common ancestor of the major lineage/supergroup Archaeplastida took on a cyanobacterial endosymbiont; in the other, the ancestor of the small amoeboid rhizarian taxon, Paulinella, took on a different cyanobacterial endosymbiont. Almost all photosynthetic eukaryotes are descended from the first event, and only a couple of species are derived from the other. Evidence for the endosymbiotic theoryThere are multiple, independent lines of evidence to support the hypothesis that eukaryotes evolved from an endosymbiotic event between an ancient archaean cell and an ancient aerobic bacterium:
How does this evidence map to the tree of life? Since all eukaryotes have mitochondria, but only photosynthetic eukaryotes have chloroplasts, the principle of parsimony (the idea that the explanation requiring the fewest steps is most likely correct) argues that first, an ancestral eukaryote engulfed the bacteria (which led to mitochondria). Secondly, only in the lineage that lead to plants and algae, a later descendant of this ancestral eukaryote then engulfed a cyanobacteria-like species that led to chloroplasts. This hypothesis is represented in the phylogenetic tree below: The origin and evolutionary tree of life that is based on small-subunit RNA. The branches that perform oxygenic photosynthesis are labeled with ‘O2’. The black arrows indicates the endosymbiotic events that resulted in the origin of eukaryotes from proteobacteria-like organisms (ultimately leading to mitochondria), and later eukaryotic photosynthesis from cyanobacteria-like organisms, which ultimately became chloroplasts in algae and later in plants. Image adapted from work by Govindjee, Dmitriy Shevela – doi:10.3389/fpls.2011.00028, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=59645694 Unique features of eukaryotesThe information below was adapted from OpenStax Biology 23.1 There are many unique characteristics of eukaryotes that allow us to distinguish them as a monophyletic group on the phylogenetic tree of life. The following characteristics must have been present in the last common ancestor (LCA) from which all eukaryotic life emerged:
The video below describes the origin and advantages of sexual vs asexual reproduction in eukaryotes: There are eukaryotic species with exceptions to the list above: for example, some species of rotifers (microscopic, aquatic animals) reproduce asexually without meiosis. But in all cases of exceptions, evidence indicates that a particular trait was lost in that lineage rather than the lineage independently evolving all other traits of eukaryotes. General steps of sexual life cyclesSexual reproduction with meiosis is a defining feature of eukaryotes. Unlike in asexual reproduction, offspring that result from sexual reproduction are not genetically identical to their parents, but instead get half of their genetic information from
each parent. How does this form of reproduction work? The short answer is that a sexual life cycle always involves two changes in ploidy (the number of copies of each chromosome). The first change in the parent’s ploidy: specifically, meiosis, a cell division that reduces ploidy by 1/2, from “2n” (diploid ) to “1n” (haploid), where “n” is the number of copies of each chromosome. This reduction produces haploid cells from
diploid cells. The next change is a doubling from 1n back to 2n (return to original ploidy) by fertilization, or the joining of gametes (typically called egg and sperm). We humans tend to think of fertilization as happening immediately after meiosis (production of gametes) because that’s how it works in animals. But in different eukaryotic lineages, fertilization does not always happen immediately following gamete production; in most fungi, the products of meiosis
undergo mitosis In regard to timing of the changes of ploidy, there are 3 types of sexual life cycles:
The importance of single-celled eukaryotesWhen we think of the term “eukaryote,” we tend to think of large, multicellular things like plants and animals. But there are many (many!) single-celled eukaryotes which significantly impact human health and general ecosystem services. Many species of single-celled eukaryotes are often grouped together as “protists,” a term which is descriptive but evolutionarily problematic because it does not refer to a monophyletic group (much like the term “prokaryote.”) The information below was adapted from OpenStax Biology 23.4 Protists function in various ecological niches. Whereas some protist species are essential components of the food chain and generators of biomass, others function in the decomposition of organic materials. Still other protists are dangerous human pathogens or causative agents of devastating plant diseases.
How did eukaryotic cells evolve from endosymbiosis?The endosymbiotic theory explains how eukaryotic cells evolved. The large and small cells formed a symbiotic relationship in which both cells benefited. Some of the small cells were able to break down the large cell's waste s for energy. They supplied energy not only to themselves but also to the large cell.
What is the best explanation about how eukaryotic cells evolved?The hypothesis that eukaryotic cells evolved from a symbiotic association of prokaryotes—endosymbiosis—is particularly well supported by studies of mitochondria and chloroplasts, which are thought to have evolved from bacteria living in large cells.
What is endosymbiosis the best explanation?A symbiotic relationship where one organism lives inside the other is known as endosymbiosis.
What does endosymbiosis tell us about the evolutionary history of the eukaryotic cell?The endosymbiotic theory is the accepted mechanism for how eukaryotic cells evolved from prokaryotic cells. It involves a cooperative relationship between two cells which allow both to survive—and eventually led to the development of all life on Earth.
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