File Name: origin of cells and unicellular evolution .zip
Living things fall into three large groups: Archaea, Bacteria, and Eukarya. The first two have prokaryotic cells, and the third contains all eukaryotes.
A relatively sparse fossil record is available to help discern what the first members of each of these lineages looked like, so it is possible that all the events that led to the last common ancestor of extant eukaryotes will remain unknown.
However, comparative biology of extant organisms and the limited fossil record provide some insight into the history of Eukarya. The earliest fossils found appear to be Bacteria, most likely cyanobacteria.
They are about 3. Structures this size, which might be fossils, appear in the geological record about 2. Data from these fossils have led comparative biologists to the conclusion that living eukaryotes are all descendants of a single common ancestor.
Mapping the characteristics found in all major groups of eukaryotes reveals that the following characteristics must have been present in the last common ancestor, because these characteristics are present in at least some of the members of each major lineage. This major theme in the origin of eukaryotes is known as endosymbiosis, one cell engulfing another such that the engulfed cell survives and both cells benefit.
Over many generations, a symbiotic relationship can result in two organisms that depend on each other so completely that neither could survive on its own. Before explaining this further, it is necessary to consider metabolism in prokaryotes. Many important metabolic processes arose in prokaryotes, and some of these, such as nitrogen fixation, are never found in eukaryotes.
The process of aerobic respiration is found in all major lineages of eukaryotes, and it is localized in the mitochondria. Aerobic respiration is also found in many lineages of prokaryotes, but it is not present in all of them, and many forms of evidence suggest that such anaerobic prokaryotes never carried out aerobic respiration nor did their ancestors. Without oxygen, aerobic respiration would not be expected, and living things would have relied on fermentation instead.
At some point before, about 3. That is, they evolved the ability to photosynthesize. Hydrogen, derived from various sources, was captured using light-powered reactions to reduce fixed carbon dioxide in the Calvin cycle.
The group of Gram-negative bacteria that gave rise to cyanobacteria used water as the hydrogen source and released O 2 as a waste product. Eventually, the amount of photosynthetic oxygen built up in some environments to levels that posed a risk to living organisms, since it can damage many organic compounds.
Various metabolic processes evolved that protected organisms from oxygen, one of which, aerobic respiration, also generated high levels of ATP. It became widely present among prokaryotes, including in a group we now call alpha-proteobacteria. Organisms that did not acquire aerobic respiration had to remain in oxygen-free environments. Originally, oxygen-rich environments were likely localized around places where cyanobacteria were active, but by about 2 billion years ago, geological evidence shows that oxygen was building up to higher concentrations in the atmosphere.
Recall that the first fossils that we believe to be eukaryotes date to about 2 billion years old, so they appeared as oxygen levels were increasing. Also, recall that all extant eukaryotes descended from an ancestor with mitochondria. These organelles were first observed by light microscopists in the late s, where they appeared to be somewhat worm-shaped structures that seemed to be moving around in the cell.
Some early observers suggested that they might be bacteria living inside host cells, but these hypotheses remained unknown or rejected in most scientific communities. As cell biology developed in the twentieth century, it became clear that mitochondria were the organelles responsible for producing ATP using aerobic respiration.
In the s, American biologist Lynn Margulis developed endosymbiotic theory, which states that eukaryotes may have been a product of one cell engulfing another, one living within another, and evolving over time until the separate cells were no longer recognizable as such. In , Margulis introduced new work on the theory and substantiated her findings through microbiological evidence.
Much still remains to be discovered about the origins of the cells that now make up the cells in all living eukaryotes.
Broadly, it has become clear that many of our nuclear genes and the molecular machinery responsible for replication and expression appear closely related to those in Archaea. On the other hand, the metabolic organelles and genes responsible for many energy-harvesting processes had their origins in bacteria.
Much remains to be clarified about how this relationship occurred; this continues to be an exciting field of discovery in biology. For instance, it is not known whether the endosymbiotic event that led to mitochondria occurred before or after the host cell had a nucleus.
Such organisms would be among the extinct precursors of the last common ancestor of eukaryotes. Figure 1. In this transmission electron micrograph of mitochondria in a mammalian lung cell, the cristae, infoldings of the mitochondrial inner membrane, can be seen in cross-section.
One of the major features distinguishing prokaryotes from eukaryotes is the presence of mitochondria. Each mitochondrion measures 1 to 10 or greater micrometers in length and exists in the cell as an organelle that can be ovoid to worm-shaped to intricately branched Figure 1. Mitochondria arise from the division of existing mitochondria; they may fuse together; and they may be moved around inside the cell by interactions with the cytoskeleton.
However, mitochondria cannot survive outside the cell. As the atmosphere was oxygenated by photosynthesis, and as successful aerobic prokaryotes evolved, evidence suggests that an ancestral cell with some membrane compartmentalization engulfed a free-living aerobic prokaryote, specifically an alpha-proteobacterium, thereby giving the host cell the ability to use oxygen to release energy stored in nutrients.
Alpha-proteobacteria are a large group of bacteria that includes species symbiotic with plants, disease organisms that can infect humans via ticks, and many free-living species that use light for energy. Several lines of evidence support that mitochondria are derived from this endosymbiotic event. Most mitochondria are shaped like alpha-proteobacteria and are surrounded by two membranes, which would result when one membrane-bound organism was engulfed into a vacuole by another membrane-bound organism.
The mitochondrial inner membrane is extensive and involves substantial infoldings called cristae that resemble the textured, outer surface of alpha-proteobacteria. The matrix and inner membrane are rich with the enzymes necessary for aerobic respiration. Mitochondria divide independently by a process that resembles binary fission in prokaryotes. Specifically, mitochondria are not formed from scratch de novo by the eukaryotic cell; they reproduce within it and are distributed with the cytoplasm when a cell divides or two cells fuse.
Therefore, although these organelles are highly integrated into the eukaryotic cell, they still reproduce as if they are independent organisms within the cell. However, their reproduction is synchronized with the activity and division of the cell. Mitochondria have their own usually circular DNA chromosome that is stabilized by attachments to the inner membrane and carries genes similar to genes expressed by alpha-proteobacteria.
Mitochondria also have special ribosomes and transfer RNAs that resemble these components in prokaryotes. These features all support that mitochondria were once free-living prokaryotes. Mitochondria that carry out aerobic respiration have their own genomes, with genes similar to those in alpha-proteobacteria. However, many of the genes for respiratory proteins are located in the nucleus. When these genes are compared to those of other organisms, they appear to be of alpha-proteobacterial origin.
Additionally, in some eukaryotic groups, such genes are found in the mitochondria, whereas in other groups, they are found in the nucleus. This has been interpreted as evidence that genes have been transferred from the endosymbiont chromosome to the host genome. This loss of genes by the endosymbiont is probably one explanation why mitochondria cannot live without a host. Some living eukaryotes are anaerobic and cannot survive in the presence of too much oxygen. Some appear to lack organelles that could be recognized as mitochondria.
In the s to the early s, many biologists suggested that some of these eukaryotes were descended from ancestors whose lineages had diverged from the lineage of mitochondrion-containing eukaryotes before endosymbiosis occurred. However, later findings suggest that reduced organelles are found in most, if not all, anaerobic eukaryotes, and that all eukaryotes appear to carry some genes in their nuclei that are of mitochondrial origin. In addition to the aerobic generation of ATP, mitochondria have several other metabolic functions.
One of these functions is to generate clusters of iron and sulfur that are important cofactors of many enzymes. Such functions are often associated with the reduced mitochondrion-derived organelles of anaerobic eukaryotes. Therefore, most biologists accept that the last common ancestor of eukaryotes had mitochondria.
Some groups of eukaryotes are photosynthetic. Their cells contain, in addition to the standard eukaryotic organelles, another kind of organelle called a plastid. When such cells are carrying out photosynthesis, their plastids are rich in the pigment chlorophyll a and a range of other pigments, called accessory pigments, which are involved in harvesting energy from light.
Photosynthetic plastids are called chloroplasts Figure 2. Figure 2. Stacks of thylakoid membranes compartmentalize photosynthetic enzymes and provide scaffolding for chloroplast DNA. Like mitochondria, plastids appear to have an endosymbiotic origin. This hypothesis was also championed by Lynn Margulis. Plastids are derived from cyanobacteria that lived inside the cells of an ancestral, aerobic, heterotrophic eukaryote. This is called primary endosymbiosis, and plastids of primary origin are surrounded by two membranes.
The best evidence is that this has happened twice in the history of eukaryotes. Almost all photosynthetic eukaryotes are descended from the first event, and only a couple of species are derived from the other.
Cyanobacteria are a group of Gram-negative bacteria with all the conventional structures of the group. However, unlike most prokaryotes, they have extensive, internal membrane-bound sacs called thylakoids. Chlorophyll is a component of these membranes, as are many of the proteins of the light reactions of photosynthesis. Cyanobacteria also have the peptidoglycan wall and lipopolysaccharide layer associated with Gram-negative bacteria. Chloroplasts of primary origin have thylakoids, a circular DNA chromosome, and ribosomes similar to those of cyanobacteria.
Each chloroplast is surrounded by two membranes. In the group of Archaeplastida called the glaucophytes and in Paulinella , a thin peptidoglycan layer is present between the outer and inner plastid membranes.
All other plastids lack this relictual cyanobacterial wall. The outer membrane surrounding the plastid is thought to be derived from the vacuole in the host, and the inner membrane is thought to be derived from the plasma membrane of the symbiont. There is also, as with the case of mitochondria, strong evidence that many of the genes of the endosymbiont were transferred to the nucleus.
Plastids, like mitochondria, cannot live independently outside the host. In addition, like mitochondria, plastids are derived from the division of other plastids and never built from scratch. Researchers have suggested that the endosymbiotic event that led to Archaeplastida occurred 1 to 1. Not all plastids in eukaryotes are derived directly from primary endosymbiosis. Some of the major groups of algae became photosynthetic by secondary endosymbiosis, that is, by taking in either green algae or red algae both from Archaeplastida as endosymbionts Figure 3 ab.
Numerous microscopic and genetic studies have supported this conclusion.
The gene regulation mechanisms necessary for the development of complex multicellular animals have been found in sponges. The evolution of multicellular organisms from simple, single-cell organisms was a pivotal turning point in the history of life. Many different lineages of organisms, including animals, independently evolved multicellularity. One advantage of multicellularity is that cells can be programmed to perform particular biological roles. However, in order to assign cells to a specific role or fate, genes within certain tissues need to be activated at precise times during development. This process, which is known as spatiotemporal regulation, is orchestrated by various regulatory genes, including transcription factors and cell-signaling molecules. A startling revelation in the field of evolutionary developmental biology was the discovery that the suite of regulatory genes that controls development in animals is ancient Degnan et al.
Prokaryotic cells (bacteria) lack a nuclear envelope; eukaryotic cells have a nucleus Because cells originated in a sea of organic molecules, they were able to Multicellular organisms evolved from unicellular eukaryotes at least billion.
The appearance of the first cells marked the origin of life on Earth. However, before cells could form, the organic molecules must have united with one another to form more complex molecules called polymers. Examples of polymers are polysaccharides and proteins. In the s, Sidney Fox placed amino acids in primitive Earth conditions and showed that amino acids would unite to form polymers called proteinoids.
Why can this fish live in these tentacles, but other fish cannot? Anemones and Clown Fish have a well-known symbiotic relationship. In the ocean, the Clown Fish are protected from predator fish by the stinging tentacles of the anemone, and the anemone receives protection from polyp-eating fish, which the Clown Fish chases away.
Evolution of cells refers to the evolutionary origin and subsequent evolutionary development of cells.
David A. Cells are the smallest known pieces of biology that are capable of independent reproduction and are therefore the simplest units that can evolve by conventional natural selection. This is problematic because even the simplest self-replicating cell-like entities seem to be too complicated to arise without the guiding hand of selection. To solve this conundrum, I argue that selection began before there were bounded entities in the form of neighborhood selection, an analog of group selection without bounded groups. This, I suggest, acted on chemical consortia bound to mineral surfaces to enhance their autocatalytic abilities.
Sexual reproduction is a nearly universal feature of eukaryotic organisms. Given its ubiquity and shared core features, sex is thought to have arisen once in the last common ancestor to all eukaryotes. Using the perspectives of molecular genetics and cell biology, we consider documented and hypothetical scenarios for the instantiation and evolution of meiosis, fertilization, sex determination, uniparental inheritance of organelle genomes, and speciation.
Ancient animals still alive today, such as the sponge and hydroids shown above, hold important clues about how multicellular animal life evolved. From one came many. Some million years ago, a single cell gave rise to the first animal, a multicellular organism that would eventually spawn the incredible complexity and diversity seen in animals today. New research is now offering scientists a fresh perspective on what that cell looked like, and how multicellularity could have emerged from it — a transition that marks one of the most pivotal events in the history of life on Earth. For well over a century, it has been widely assumed that the ancestors from which the first animal evolved were simple blobs of identical cells. But now, painstaking genomic analyses and comparisons between the most ancient animals alive today and their closest non-animal relatives are starting to overturn that theory. The recent work paints a picture of ancestral single-celled organisms that were already amazingly complex.
The question of how animals evolved from a unicellular ancestor has challenged evolutionary biologists for decades. Because cell adhesion and signaling are required for multicellularity, understanding how these cellular processes evolved will provide key insights into the origin of animals. A critical finding is that choanoflagellates, the closest living unicellular relatives of animals, express members of the cadherin superfamily. Cadherins are pivotal for animal cell adhesion and signaling and were previously thought to be unique to animals, making them crucial to understanding the evolutionary origin and transition to multicellularity. Importantly, the presence of cadherins in choanoflagellates allows a consideration of their ancestral function in the unicellular progenitor of animals. To gain insight into the ancestral structure and function of cadherins, I reconstructed the domain content of cadherins from the last common ancestor of choanoflagellates and metazoans. Conservation of diverse protein domains in the choanoflagellate Monosiga brevicollis and metazoan cadherins suggests that ancestral cadherins served both signaling and adhesive functions.
NCBI Bookshelf. Cooper GM. The Cell: A Molecular Approach.
Living things fall into three large groups: Archaea, Bacteria, and Eukarya.Ceferina M. 09.05.2021 at 04:13
PDF | Cell types are the basic building blocks of multicellular organisms and are sion profiling, fuelled by the rapid progress in single cell.