Researchers investigating the genesis of complex life on Earth seem to have unlocked a long-standing enigma surrounding the evolution of animals, flora, and fungi. The findings of this study have been featured in the journal Nature.
To grasp this riddle, one must first be aware that two primary cellular types exist: eukaryotes, which possess numerous internal components like a nucleus and mitochondria, and prokaryotes, which lack all such features.
All that we pertain to as complex life—essentially everything visible to the naked eye, and substantially more besides—is composed of eukaryotic cells. Prokaryotes, conversely, encompass bacteria and another cohort of single-celled organisms termed archaea.
The most widely accepted hypothesis regarding the emergence of complex life posits that an archaeal cell, specifically a type known as an Asgard archaeon, engulfed a bacterium. However, instead of being digested, this bacterium persisted within the archaeon and ultimately gave rise to what we now recognize as mitochondria.
This theory is compelling for several justifications, including the fact that the DNA within mitochondria bears resemblance to the DNA of contemporary alphaproteobacteria, and that eukaryotic cell membranes exhibit greater similarity to archaeal membranes than to bacterial cell membranes.
Nevertheless, a significant point of contention exists: investigations into Asgard archaea, only discovered in 2015 in sediments from a hydrothermal vent site dubbed “Loki’s Castle,” indicate a preference for low-oxygen environments. Yet, it is presumed that the bacteria they supposedly consumed thrived where oxygen levels were abundant—given that the current function of mitochondria is to utilize oxygen to convert sustenance into energy via a process called respiration.
As a consequence of this extensive sequencing effort, the roster of genomes belonging to the closest known archaeal relatives of the host organism that gave rise to eukaryotes has nearly doubled.
However, a new investigation originating from the University of Texas at Austin suggests this issue is largely moot, as numerous Asgards currently exist that do utilize oxygen or are, at minimum, tolerant of it.
“One of the major questions in the biology and evolution of life on this planet is what occurrences led to the formation of complex life (plants and animals),” stated Brett Baker, the study’s lead. “This research offers fresh insights into the lifestyle of our microbial forebears, and we believe they might have been breathing oxygen just like us!”
By compiling data from several marine expeditions, the team processed approximately 15 terabytes of environmental DNA, from which they extracted hundreds of novel Asgard genomes.
Subsequently, employing a machine learning tool adept at analyzing genetic data to predict the proteins they could synthesize, the researchers demonstrated that the genes of one group of Asgard archaea, categorized as Heimdallarchaea, encode proteins resembling components of the electron transport chain—a structure integral to oxygen metabolism.
“This massive sequencing operation nearly doubled the number of genomes belonging to the nearest known archaeal kin of the organism that served as the ancestor to eukaryotes, affording a more complete picture of their ecology and metabolism,” noted study co-author Katherine Eppeler.
Furthermore, some of the specimens they examined were sourced from shallow sediment where aerobic alphaproteobacteria are presently found, suggesting that interactions believed to have initiated eukaryotic life are observable even today.
“The majority of extant Asgards have been located in oxygen-free settings,” Baker elaborated. “But it turns out that those genealogically closest to eukaryotes inhabit oxygenated locales, such as shallow coastal sediments and floating in the water column, and they possess many metabolic pathways that involve oxygen. This strongly indicates that our eukaryotic ancestor likely featured these same processes.”
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