Because otherwise, the timing doesn’t work:
Since the 1960s, many scientists have argued that the emergence of eukaryotes — cells containing a clearly defined nucleus — happened in response to the oxygenation of Earth’s surface environment. But new research led by Stanford University and University of Exeter scientists suggests eukaryotes in fact emerged in an anoxic environment in the ocean.
“We can now independently date eukaryogenesis and key oxygenation transitions in Earth history,” said lead author Dr. Daniel Mills, a researcher in the Department of Geological Sciences at Stanford University, the Department of Biology at Portland State University, and the Penn State Extraterrestrial Intelligence Center.
“Based on fossil and biological records, the timing of eukaryogenesis does not correlate with these oxygen transitions in the atmosphere (2.22 billion years ago) or the deep ocean (0.5 billion years ago).”News Staff, “Eukaryotic Cells Emerged in Anoxic Environment, New Study Suggests” at SciNews (April 27, 2022)
So, on this view, complex life started as a merger (symbiosis) between the simplest life (Archaea) and bacteria:
“Mitochondria-bearing eukaryotes likely resulted from a merger between archaea and bacteria, and the DNA in modern Asgard archaea is more closely related to the DNA found in eukaryote nuclei today than it is to other archaea.”
“This is additional evidence that the host that took in the bacterium was an archaeon.”News Staff, “Eukaryotic Cells Emerged in Anoxic Environment, New Study Suggests” at SciNews (April 27, 2022)
On the whole, it might be easier to conclude that the timing is somewhat off than that complex life started without oxygen. But symbiosis is an intriguing theory nonetheless.
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You may also wish to read: Claim: Complex cells started without oxygen Yes, the ham sandwich was invented that way too. It started without any ham… (The University of Exeter’s take on the story)