"The transition from the classic prokaryotic model to the deluxe eukaryotic one is arguably the most important event in the history of life on Earth. And in more than 3 billion years of existence, it happened exactly once...
It is not for lack of opportunity. The world is swarming with countless prokaryotes that evolve at breathtaking rates. Even so, they were not quick about inventing eukaryotic cells. Fossils tell us that the oldest bacteria arose between 3 and 3.5 billion years ago, but there are no eukaryotes from before 2.1 billion years ago. Why did the prokaryotes remain as simple cells for so damn long?...
There are many possible explanations, but one of these has recently gained a lot of ground. It tells of a prokaryote that somehow found its way inside another, and formed a lasting partnership with its host. This inner cell—a bacterium—abandoned its free-living existence and eventually transformed into the mitochondria. These internal power plants provided the host cell with a bonanza of energy, allowing it to evolve in new directions that other prokaryotes could never reach.
If this story is true, and there are still those who doubt it, then all eukaryotes—every flower and fungus, spider and sparrow, man and woman—descended from a sudden and breathtakingly improbable merger between two microbes...
According to the sudden-origin ideas, mitochondria were not just one of many innovations for the early eukaryotes. “The acquisition of mitochondria was the origin of eukaryotes,” says Lane. “They were one and the same event.” If that is right, the rise of the eukaryotes was a fundamentally different sort of evolutionary transition than the gradual changes that led to the eye, or photosynthesis, or the move from sea to land. It was a fluke event of incredible improbability—one that, as far as we know, only happened after a billion years of life on Earth and has not been repeated in the 2 billion years since. “It’s a fun and thrilling possibility,” says Lane. “It may not be true, but it’s beautiful.”...
In 2004, James Lake changed the rules of engagement. Rather than looking at any single gene, he and his colleague Maria Rivera compared the entire genomes of two eukaryotes, three bacteria, and three archaea. Their analysis supported the merger-first ideas: They concluded that the common ancestor of all life diverged into bacteria and archaea, which evolved independently until two of their members suddenly merged. This created the first eukaryotes and closed what now appeared to be a “ring of life.” Before that fateful encounter, life had just two major domains. Afterward, it had three."
Evolution isn't a straight line from basic to complex, inadequate to ideal. It's random changes that are under varying degrees of selective pressures.
FB: "Prokaryotes are stuck in an energetic canyon that keeps them simple and small. They have no way of climbing out. If anything, evolution drives them in the opposite direction, mercilessly pruning their genomes into a ring of densely packed and overlapping genes. Only once did a prokaryote escape from the canyon, through a singular and improbable trick—it acquired mitochondria."
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