The Early Earth was not a place to be. Hot, devoid of oxygen and subject to cataclysmic cosmic explosions, our planet was about 4.5 billion years ago unstable to life as we know it. And yet from this strange early stage, thanks to the metabolism of primary single-celled organisms, the Earth’s atmosphere has changed. Oxygen levels first rose in the shallow waters and surface atmospheres, and 2 billion years later in deeper waters.
This Great Oxidation Event, which began about 2.4 billion years ago, set the stage for a turning point in the history of life on Earth that is also among its most enduring mysteries: the evolution of organisms cells, and eukaryotes. Unlike single-celled organisms such as bacteria and archaea, the nucleus is closed by organs, the basic cell structure that would overwhelm every plant, animal, protector and fungus on the planet.
“The puzzle is that living eukaryotes are so different from their closest relatives, the archaea,” said Susannah Porter, UC Santa Barbara paleobiologist. Not only do they have nuclei and other organelles like mitochondria, most eukaryote cells are several thousand times larger than those of bacteria and archaea (collectively known as prokaryotes). The leap from simple small anaerobic cells with free floating genetic information to large oxygen-deprived cells with particularly complex structures is a major gap in early life history on Earth.
Porter and UC Santa Barbara professor Alyson Santoro is poised to help fill that gap. Supported by the Gordon and Betty Moore Foundation and the Simons Foundation, the two will use their knowledge and research teams in efforts to shed light on the origins of the eukaryotic cell.
“Our projects are under the same big screen issue, but they are very different,” said Santoro, a microbial ecologist. “And the nice part of the approach that the Moore-Simons program takes is that they have an open mind about the kinds of projects that they could fund to help answer this question. “
Santoro and Porter lead two of the 20 teams selected from more than 90 candidates for the Moore-Simons project.
Mitochondria, and other smoking guns
The origin of the eukaryotic cell has been the subject of controversy for decades.
“One theory is that an archaeon captured a bacterium, and that a bacterium became a symbiont within the archaeon,” said Santoro, a microbial ecologist, about the concept known as the Endosymbiotic Theory, a version of events called whether it was widely accepted. Instead of one prokaryote destroying the other, the two created a relationship, in which the bacterium produced energy for the host archaeon in exchange for its new conditions. It is an arrangement that has grown over time and many enduring generations, and has led to mitochondrion, the “powerhouse” called eukaryotic cells, which derives its power from aerobic relief. As a result, it allows the eukaryotic host to live in oxygenated environments.
The proliferation of eukaryotes possessed by mitochondria – organelles responsible for generating energy through cellular relaxation – indicates elevated levels of oxygen in the atmosphere, the researchers said. This occurred during a gap of around 2 billion years that begins with anoxic conditions and ends with an oxygenated environment in which “life was trying a lot of things,” said the researchers, with these early entrepreneurs. (single-celled organisms) adds structures and functions to deal with or take advantage of the changing environment, including complex cytoskeletons, sterol synthesis and cyst formation as well as aerobic relief. These series of early eukaryotes died with little evidence of their existence.
“We have a fossil record of activists but it’s hard to say how far it will go back, because part of the problem,” Porter explained, “is that they are older, fewer eukaryotic characters. It’s hard to tell if you’re looking at a fossil of a eukaryote. “
Porter drew a resemblance between extinct dinosaurs and modern birds – the only dinosaurs that survived to the present day (yes, birds are a kind of dinosaur). The gap between birds and their closest relatives, the crocodiles, is huge, she said, and there is no way we can find out how birds evolved — feathers, flying , empty bones, etc. – without a rich fossil record of dinosaurs including intermediate versions that have some but not all bird marks.
“And so our project is to try to use proxies for specific eukaryotic features,” Porter explained. Looking at fossils that could be eukaryotes, their connection to the rocks and the sediment where they were discovered and found, Porter, along with her postdoc Leigh Anne Riedman and her colleagues at McGill University in Canada hope to sense the evolution of these lost sequences.
“We’re going to put all the data together and try to put together a story where early species are consistently found only in anoxic environments, and are associated with hungry environments,” Porter said. “The hope is that we can then start putting restrictions on when we see eukaryotic fossils in environments with oxygen using that as a surrogate to say, ‘okay, there must be mitochondria have evolved before this time because they live in oxygenated environments. ‘”
Reconstruction events
Researchers discovered early eukaryotes in large clues in 2015 thanks to some Viking gods – that is, a recently discovered group of archaea classified under the new superphylum “Asgard,” which has the characteristics crossing the line between Archaea and Eukarya. Members of this group act as anaerobic archaea – simple cells functioning in anoxic environments – although their genes encode eukaryotic signature proteins, placing them as the closest prokaryotic relatives of the eukaryotes.
“The reason these microbes weren’t discovered was because we didn’t have the DNA sequencing technologies to make enough of these rare members of the community,” Santoro said. she said, most of it is impossible to do culture in the lab.
There may be another rare archaea, which, like the Asgards, may have provided valuable hints. To that end, Santoro is developing a method to detect archaeological cells in environmental samples, select for specific traits and arrange them for genomic sequencing, in collaboration with colleagues from Oxford University, the Heimholtz Institute and University of Texas.
“We use an instrument called a flow cytometer, which is the only device used to perform blood cell counts,” she said. By passing the environmental sample through a thin capillary tube, she explained, and striking it with a violet laser, she and her colleagues can identify an interesting archaea with the way they bloom. The ones that light up are sorted and saved for sequencing.
“Many experiments may have failed on the road to becoming eukaryotes” – genetic guidance that may have been missing at one time but was used, Santoro said. “Some of the signs of these failed experiments may be hanging out in today’s archaea genomes. That’s what we’re hoping to find.”
The conclusions drawn from these and from the other studies in the Moore-Simons project could go a long way toward understanding the many mysteries surrounding the origin of the eukaryotic cell. They may offer insights into not only a series of events but also other puzzles, such as how the eukaryotic cell grew so large compared to its simple-cell parents, or why, if the cell is like as a result of an archaeon swallowing a bacterium, which is a more bacterial membrane than an archaeal. They could even break down existing lines of thought, or, according to the Moore-Simons project, pave the way for “the development of new ideas with the potential to reflect new but potentially invisible perspectives on eukaryogenesis . ”
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