The Great Mystery of Biology: The Eukaryote Cell's Origin

"The origin of eukaryotes is one of the hardest and most intriguing problems in the study of the evolution of life, and arguably, in the whole of biology. " (Koonin, 2015)

All living things are composed of either prokaryote or eukaryote cells.  The prokaryote cells are simple, basically a blob of protoplasm encased in a cell membrane whiel the eukaryote cell is larger, possesses a nucleus (a kind of DNA-packed control room safely enclosed in a membrane), as well as a set of specialized organelles ("little organs") that are able to perform necessary tasks like storing molecules or protein manufacture.  Significantly the eukaryote cell has its own power plant in the form of mitochondria.  

For nearly 2 billion years after the appearance of life on Earth, the prokaryote model had Earth to itself.  It was and continues to be highly successful at not only surviving but also thriving.  Prokaryotes can be found in all of Earth's habitats from clouds to the depths of the sea, using a repertoire of ways to survive, from the ability to cause disease, use noxious substances like crude oil for food, power themselves with energy from the sun, and even to swap genes with one another.  (Yong, 2014)

The eukaryote cell with its nucleus and mitochondria doesn't appear until much later in Earth's history, about 1.5 billion years ago. 

While the prokaryotes "have repeatedly nudged along the path to complexity" and while some groups of prokaryotic cells move in colonies that resemble complex life, "none of them have acquired the full suite of features that define eukaryotes:  large size, the nucleus, internal compartments, mitochondria..."  (Yong, 2014)

This is why the appearance of the eukaryote ("eukaryogenesis") is "regarded as one of the major evolutionary innovations in the history of our planet" because the eukaryote cell with its mitochondria, its own power plant provides "the host cell with a bonanza of energy, allowing it to evolve in new directions that other prokaryotes could never reach," and accounts for the reason why all multicellular life is based on the eukaryotic cell.  (Zaremba-Niedzwiedzka et al., 2017 & Yong, 2014)

In an article in Nature published in January 2017, the authors argue that "most recent insights" support a variety of symbiogenesis of eukaryotic evolution.  Evidence is that a still mysterious host cell from the domain Archaea merged with "an alphaprotobacterial (mitochondrial) endosymbiont." (Zaremba-Niedzwiedzka et al., 2017)

That the mitochondria in the eukaryote cell was once a free living bacteria was first proposed by Lynn Margulis in 1967, at the time a graduate student.  

Margulis argued that one driver of evolution was symbiosis, with evidence based on the fact that the mitochondria in eukaryotic cells look remarkably like bacteria.  Another example for this endosymbiosis are chloroplasts which also look like bacteria.  With the coming of new genetic tools, analysis of the chloroplast genome by the University of Illinois' Carl Woese showed that the chloroplast genes were not at all like the genes in the host cells, but turned out to be the DNA of cyanobacteria.  It was also found that the mitochondrial DNA resembles that which is found in the group of bacteria that causes typhus.  

New technologies have expanded the tools that are available to track the relationships between organisms, adding new data to the quest to solve the mystery surrounding eukaryogenesis. While the bacteria that contributed the mitochondria to the eukaryote was from the group known as alphaproteobacteria, a group well-known to take up life within the cells of plants and animals as both mutualists and pathogens.  (Williams, Sobral, & Dickerman, 2007)

But less is known about the organism from the domain Archaea that was the presumed host in the merger. 

In 2015, a team from Sweden's Uppsala University collected and analyzed sediments from an ocean floor field of hydrothermal vents lying between Norway and Greenland called Loki's  Castle.  The DNA found in the sample show that these Lokiarchaeota are the "best approximations that we have for that ancestral archaeon that gave rise to us all."  (Yong, 2017)

More searches in places like North Carolina, Yellowstone National Park, and New Zealand, have revealed many more varieties from this group of archaea, which the group has named Asgard (a name from Norse mythology).
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The DNA from these organisms have turned up genes that until now that were thought to be unique to eukaryotes. There are genes in the asgard archaea that serve in eukaryotes for building internal skeletons, although the archaea do not have internal skeletons. Other genes are associated with the pinching off of the outer membrane of cells to create little pockets that are used to move molecules around, another eukaryotic capability not found in archaea.

It would be wrong to say that these discoveries have solved the mystery of eukaryogenesis. A lead researcher in this field describes these cells, not as eukaryotes but “primed to become eukaryotes.”(Yong, 2017)

The discovery of the Agard archaea gets us closer to the link that connects the most ancient life to our own.
 
Resources:
Koonin, E. V. (2015). Origin of eukaryotes from within archaea, archaeal eukaryome and bursts of gene gain: eukaryogenesis just made easier? Philosophical Transactions of the Royal Society B, 370(1678). Retrieved from http://rstb.royalsocietypublishing.org/content/370/1678/20140333

Williams, K. P., Sobral, B. W., & Dickerman, A. W. (2007). A Robust Species Tree for the Alphaproteobacteria. Journal of Bacteriology, 189(13). Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1913456/

Yong, E. (2014). The Unique Merger That Made You (and Ewe, and Yew). Retrieved from http://nautil.us/issue/10/mergers--acquisitions/the-unique-merger-that-made-you-and-ewe-and-yew

Yong, E. (2017). A Break in the Search for the Origin of Complex Life. The Atlantic. Retrieved from https://www.theatlantic.com/science/archive/2017/01/our-origins-in-asgard/512645/
Zaremba-Niedzwiedzka, Caceres, E. F., Saw, J. H., Bäckström, D., Juzokaite, L., & Vancaester, E. (2017). Asgard archaea illuminate the origin of eukaryotic cellular complexity. Nature Immunology, 541(7637), 353-358. Retrieved from http://www.nature.com/nature/journal/v541/n7637/full/nature21031.html

See the South Carolina Academic Standards and Performance Indicators for Science 2014: Biology I, Cells as a System, H.B.2.
Endosymbiotic Theory