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How did life evolve from simple, single-celled creatures to complex animals capable of manipulating nature? For most of the three billion years of Earth's history, life is limited to single celled bacterium, what changed?
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"Lichens are not, strictly speaking, organisms. They are partnerships. Each one is a sandwich of bread-like fungus with jelly-like algae on the inside. The fungi protect the algae, and the algae feed the fungi. This is what lichens probably did as far back as six hundred million years ago. The earliest life on land was symbiotic." - Stanley A. Rice, "Life of Earth: Portrait of a Beautiful, Middle-aged Stressed-out World"
A eukaryote is an organism with complex cells, or a single cell with a complex structure. In these cells the genetic material is organized into chromosomes in the cell nucleus.
Animals, plants, algae and fungi are all eukaryotes. There are also eukaryotes amongst single-celled protists. In contrast, simpler organisms, such as bacteria and archaea, do not have nuclei and other complex cell structures. Such organisms are called prokaryotes.
Eukaryotes evolved in the Proterozoic eon. The oldest known probable eukaryote is Grypania, a coiled, unbranched filament up to 30mm long. The oldest Grypania fossils come from an iron mine near Negaunee, Michigan. The fossils were originally dated as 2100 million years ago, but later research showed the date as about 1874 million years ago. Grypania lasted into the Mesoproterozoic era.
Another ancient group is the acritarchs, believed to be the cysts or reproductive stages of algal plankton. They are found 1400 million years ago, in the Mesoproterozoic era.
Eukaryotes are often treated as a superkingdom, or domain, and the classification of the Eukaryota is under active discussion, and several taxonomies have been proposed. All modern versions have five kingdoms, but disagree about which groups go into each kingdom.
Animals, plants, algae and fungi are all eukaryotes. There are also eukaryotes amongst single-celled protists. In contrast, simpler organisms, such as bacteria and archaea, do not have nuclei and other complex cell structures. Such organisms are called prokaryotes.
Eukaryotes evolved in the Proterozoic eon. The oldest known probable eukaryote is Grypania, a coiled, unbranched filament up to 30mm long. The oldest Grypania fossils come from an iron mine near Negaunee, Michigan. The fossils were originally dated as 2100 million years ago, but later research showed the date as about 1874 million years ago. Grypania lasted into the Mesoproterozoic era.
Another ancient group is the acritarchs, believed to be the cysts or reproductive stages of algal plankton. They are found 1400 million years ago, in the Mesoproterozoic era.
Eukaryotes are often treated as a superkingdom, or domain, and the classification of the Eukaryota is under active discussion, and several taxonomies have been proposed. All modern versions have five kingdoms, but disagree about which groups go into each kingdom.
Structure
Eukaryotic cells are usually much bigger than prokaryotes. They can be up to 10 times bigger. Eukaryote cells have many different internal membranes and structures, called organelles. They also have a cytoskeleton. The cytoskeleton is made up of microtubules and microfilaments. Those parts are very important in the cell's shape. Eukaryotic DNA is put in bundles called chromosomes, which are separated by a microtubular spindle during cell division. Most eukaryotes have some sort of sexual reproduction through fertilisation, which prokaryotes do not use.
Prokaryotes do not have sexes, but they can pass DNA to other bacteria. Their cell division is asexual. Bacterial conjugation is when bacteria give each each DNA by touching each other or making something like a bridge to go between them. And eukaryotes have sets of linear chromosomes located in the nucleus and the number of chromosomes is usually typical for each species.
Eukaryotic cells are usually much bigger than prokaryotes. They can be up to 10 times bigger. Eukaryote cells have many different internal membranes and structures, called organelles. They also have a cytoskeleton. The cytoskeleton is made up of microtubules and microfilaments. Those parts are very important in the cell's shape. Eukaryotic DNA is put in bundles called chromosomes, which are separated by a microtubular spindle during cell division. Most eukaryotes have some sort of sexual reproduction through fertilisation, which prokaryotes do not use.
Prokaryotes do not have sexes, but they can pass DNA to other bacteria. Their cell division is asexual. Bacterial conjugation is when bacteria give each each DNA by touching each other or making something like a bridge to go between them. And eukaryotes have sets of linear chromosomes located in the nucleus and the number of chromosomes is usually typical for each species.
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Internal Membranes
In eukaryotic cells, there are many things with membranes around them. All of them together are called the endomembrane system. Simple bags, called vesicles or vacuoles, are sometimes made by budding off other membranes, just like how children make bubbles with their toys. Many cells take in food and other things using something called endocytosis. In endocytosis, the membrane closest to the outside bends inwards and then pinches off to make a vesicle. Many other organelles that have membranes probably started off as vesicles. The nucleus is surrounded by two membranes membrane that has holes in it so things can go in and out. The nuclear envelope has things sticking out of it that look like tubes and sheets. These are called the endoplasmic reticulum which is often shortened to ER. The ER is works with moving proteins around and allowing them to mature. The ER has two parts, the rough ER and the smooth ER. The rough ER has ribosomes attached to it. The proteins made by the ribosomes attached to the rough ER go to the inside the rough ER, called the lumen. After that, they usually go into vesicles, which grow and pinch off from the smooth ER. In most eukaryotes, the vesicles with proteins inside fuse with piles of flattened vesicles called the Golgi bodies, where the proteins inside are changed again. |
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Vesicles are sometimes changed so they can do one thing very well. This is called specialization, or differentiation. For example, lysosomes have enzymes inside them that break down the food the comes from food vacuoles, and peroxisomes have enzymes that break down peroxide, a poison, so it is not poisonous anymore.
Many protozoa have contractile vacuoles, which are vacuoles that can fuse or pinch off from the outer membrane. Contractile vesicles are often used to get and get rid of unneeded water. Extrusomes shoot out stuff that make predators go away or catch food. In multicellular organisms, hormones are often made in vesicles. In the complicated plants, most of the inside of a plant cell is taken up by a central vacuole. That central vacuole is the main thing that keeps osmotic pressure so the cell can hold its shape.
Many protozoa have contractile vacuoles, which are vacuoles that can fuse or pinch off from the outer membrane. Contractile vesicles are often used to get and get rid of unneeded water. Extrusomes shoot out stuff that make predators go away or catch food. In multicellular organisms, hormones are often made in vesicles. In the complicated plants, most of the inside of a plant cell is taken up by a central vacuole. That central vacuole is the main thing that keeps osmotic pressure so the cell can hold its shape.
The Evolution of Eukaryotes
Because the cell organelles of eukaryotes have different (polyphyletic) origins, the question arises as to whether the group is a unified clade or not. It is certain that the protists are not. Cell organelles are specialised units which carry out well-defined functions, like mitochondria and plastids. It is fairly clear now that all or most of these organelles have their origin in once-independent prokaryotes (bacteria or archaea), and that the eukaryote cell is a 'community of micro-organisms' working together in 'a marriage of convenience'. The first such events took place between ancient bacteria to produce the double-membrane class known as gram-negative bacteria. Since the gram-negative bacteria include the cyanobacteria, this was the first of several such events in the history of the eukaryotes.
Living things have evolved into three large clusters of closely related organisms, called "domains": Archaea, Bacteria, and Eukaryota. Archaea and Bacteria, being prokaryotes, are small, relatively simple cells surrounded by a membrane and a cell wall, with a circular strand of DNA containing their genes.
Because the cell organelles of eukaryotes have different (polyphyletic) origins, the question arises as to whether the group is a unified clade or not. It is certain that the protists are not. Cell organelles are specialised units which carry out well-defined functions, like mitochondria and plastids. It is fairly clear now that all or most of these organelles have their origin in once-independent prokaryotes (bacteria or archaea), and that the eukaryote cell is a 'community of micro-organisms' working together in 'a marriage of convenience'. The first such events took place between ancient bacteria to produce the double-membrane class known as gram-negative bacteria. Since the gram-negative bacteria include the cyanobacteria, this was the first of several such events in the history of the eukaryotes.
Living things have evolved into three large clusters of closely related organisms, called "domains": Archaea, Bacteria, and Eukaryota. Archaea and Bacteria, being prokaryotes, are small, relatively simple cells surrounded by a membrane and a cell wall, with a circular strand of DNA containing their genes.
Evidence supports the idea that eukaryotic cells are actually the descendants of separate prokaryotic cells that joined together in a symbiotic union. In fact, the mitochondrion itself seems to be the "great-great-great-great-great-great-great-great-great granddaughter" of a free-living bacterium that was engulfed by another cell, perhaps as a meal, and ended up staying as a sort of permanent houseguest. The host cell profited from the chemical energy the mitochondrion produced, and the mitochondrion benefited from the protected, nutrient-rich environment surrounding it. This kind of "internal" symbiosis — one organism taking up permanent residence inside another and eventually evolving into a single lineage — is called endosymbiosis. The theory has been propounded by many people since Darwin, popularized by Lynn Margulis, and is now supported by detailed experimental testing.
Ponder this
If our most basic building blocks are no more than a sum of symbiotic organisms, then what makes us 'us'? How would that put the flow of evolution into a different perspective? Did 'we' evolve or our individual cells?
Discuss
Darwin's theory of evolution through natural selection provides the framework on the emergence of new species, but how did endosymbiosis enter into this framework? Do endosymbiosis still play a part in human evolution?
Further readings
Domain Eukaryota, an illustration of the Eukaryotic Domain and its subgroups.
Endosymbiosis, the prevalent theory on the origins of Eukaryotic cells.
Lynn Margulis, a prominent evolutionary biologist known for her contributions in the endosymbiotic theory. Also was Carl Sagan's wife.
Mitochondrial DNA, the DNA that's located in a cell's mitochondria, which is thought to have originated through endosymbiosis.
