The Evolution of Eukaryotic Cells

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The Evolution of Eukaryotic Cells

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Eukaryotic cells. Getty/Stocktrek Images

As life on Earth started to undergo evolution and become more complex, the simpler type of cell called a prokaryote underwent several changes over a long period of time to become eukaryotic cells. Eukaryotes are more complex and have many more parts than prokaryotes. It took several mutations and surviving natural selection for eukaryotes to evolve and become prevalent.

Scientists believe the journey from prokaryotes to eukaryotes was a result of small changes in structure and function over very long periods of time. There is a logical progression of change for these cells to become more complex. Once eukaryotic cells had come into existence, they then could start forming colonies and eventually multicellular organisms with specialized cells.

So just how did these more complex eukaryotic cells appear in nature?

 

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Flexible Outer Boundaries

Cell membrane lipid bilayer, artwork. Getty/PASIEKA

Most single celled organisms have a cell wall around their plasma membranes in order to protect them from environmental dangers. Many prokaryotes, like certain types of bacteria, are also encapsulated by another protective layer that also allows them to stick to surfaces. Most prokaryotic fossils from the Precambrian time span are bacilli, or rod shaped, with a very tough cell wall surrounding the prokaryote.

While some eukaryotic cells, like plant cells, still have cell walls, many do not. This means that some time during the evolutionary history of the prokaryote, the cell walls needed to disappear or at least become more flexible. A flexible outer boundary on a cell allows it to expand more. Eukaryotes are much larger than the more primitive prokaryotic cells.

Flexible cell boundaries can also bend and fold to create more surface area. A cell with a greater surface area is more efficient at exchanging nutrients and waste with its environment. It is also a benefit to bringing in or removing particularly large particles using endocytosis or exocytosis.

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Appearance of the Cytoskeleton

Cytoskeleton, confocal light micrograph. Getty/Thomas Deernick

Structural proteins within a eukaryotic cell come together to create a system known as the cytoskeleton. While the term "skeleton" generally brings to mind something that creates the form of an object, the cytoskeleton has many other important functions within a eukaryotic cell. Not only do the microfilaments, microtubules, and intermediate fibers help keep the shape of the cell, they are used extensively in eukaryotic mitosis, movement of nutrients and proteins, and anchoring organelles in place.

During mitosis, microtubules form the spindle that pulls the chromosomes apart and distributes them equally to the two daughter cells that result after the cell splits. This part of the cytoskeleton attaches to the sister chromatids at the centromere and separates them evenly so each resulting cell is an exact copy and contains all of the genes it needs to survive.

Microfilaments also aid the microtubules in moving nutrients and wastes, as well as newly made proteins, around to different parts of the cell. The intermediate fibers keep organelles and other cell parts in place by anchoring them where they need to be. The cytoskeleton also can form flagella to move the cell around.

Even though eukaryotes are the only types of cells that have cytoskeletons, prokaryotic cells have proteins that are very close in structure to those used to create the cytoskeleton. It is believed these more primitive forms of the proteins underwent a few mutations that made them group together and form the different pieces of the cytoskeleton.

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Evolution of the Nucleus

Cutaway Drawing of a Nucleus. Getty/Encyclopaedia Britannica/UIG

The most widely used identification of a eukaryotic cell is the presence of a nucleus. The main job of the nucleus is to house the DNA, or genetic information, of the cell. In a prokaryote, the DNA is just found in the cytoplasm, usually in a single ring shape. Eukaryotes have DNA inside of a nuclear envelope that is organized into several chromosomes.

Once the cell had evolved a flexible outer boundary that could bend and fold, it is believed that the DNA ring of the prokaryote was found near that boundary. As it bent and folded, it surrounded the DNA and pinched off to become a nuclear envelope surrounding the nucleus where the DNA was now protected.

Over time, the single ring shaped DNA evolved into a tightly wound structure we now call the chromosome. It was a favorable adaptation so DNA is not tangled or unevenly split during mitosis or meiosis. Chromosomes can unwind or wind up depending on which stage of the cell cycle it is in.

Now that the nucleus had appeared, other internal membrane systems like the endoplasmic reticulum and the Golgi apparatus evolved. Ribosomes, which had only been of the free-floating variety in the prokaryotes, now anchored themselves to parts of the endoplasmic reticulum to aid in the assembly and movement of proteins.

 

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Waste Digestion

Conceptual image of lysosome. Lysosomes are cellular organelles that contain acid hydrolase enzymes that break down waste materials and cellular debris. Getty/Stocktrek Images

With a bigger cell comes the need for more nutrients and the production of more proteins through transcription and translation. Of course, along with these positive changes comes the problem of more waste within the cell. Keeping up with the demand of getting rid of waste was the next step in the evolution of the modern eukaryotic cell.

The flexible cell boundary had now created all sorts of folds and could pinch off as needed to create vacuoles to bring particles in and out of the cell. It also had made something like a holding cell for products and wastes the cell was making. Over time, some of these vacuoles were able to hold a digestive enzyme that could destroy old or injured ribosomes, incorrect proteins, or other types of waste.

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Endosymbiosis

Plant Cell SEM. Getty/DR DAVID FURNESS, KEELE UNIVERSITY

Most of the parts of the eukaryotic cell were made within a single prokaryotic cell and did not require interaction of other single cells. However, eukaryotes do have a couple of very specialized organelles that were thought to once be their own prokaryotic cells. Primitive eukaryotic cells had the ability to engulf things through endocytosis, and some of the things they may have engulfed seems to be smaller prokaryotes.

Known as the Endosymbiotic TheoryLynn Margulis proposed that the mitochondria, or the part of the cell that makes usable energy, was once a prokaryote that was engulfed, but not digested, by the primitive eukaryote. In addition to making energy, the first mitochondria probably helped the cell survive the newer form of the atmosphere that now included oxygen.

Some eukaryotes can undergo photosynthesis. These eukaryotes have a special organelle called a chloroplast. There is evidence that the chloroplast was a prokaryote that was similar to a blue-green algae that was engulfed much like the mitochondria. Once it was a part of the eukaryote, the eukaryote could now produce its own food using sunlight.

 

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Scoville, Heather. "The Evolution of Eukaryotic Cells." ThoughtCo, Aug. 29, 2015, thoughtco.com/the-evolution-of-eukaryotic-cells-1224557. Scoville, Heather. (2015, August 29). The Evolution of Eukaryotic Cells. Retrieved from https://www.thoughtco.com/the-evolution-of-eukaryotic-cells-1224557 Scoville, Heather. "The Evolution of Eukaryotic Cells." ThoughtCo. https://www.thoughtco.com/the-evolution-of-eukaryotic-cells-1224557 (accessed January 22, 2018).