Amanda Heidt writes:
This year, University of Paris-Saclay biologist Purificación López-García embarked with colleagues on a journey into life’s ancient past. The researchers traveled to the altiplanos of the northern Atacama Desert, high-altitude stretches of rocky soil and shrubbery in South America that are among the driest places in the world. Despite their inhospitable reputation, these plateaus may hold clues about the very origins of complex life. Amidst the dunes and barren mountains, there are pockets of life—warm, briny pools crusted over with colorful microbial mats of cyano-bacteria and archaea stacked atop one another like crepes. Long before Earth resembled its current state, López-García says, these microbial mats “were the forests of the past,” adding that scientists now use these clumps of microscopic life “as analogs of past ecosystems that certainly occurred at the time when eukaryotes first appear[ed].”
Each layer of these living mats is composed of different types of microbes that rely upon one another. At the surface, where light and oxygen are plentiful, photosynthesizing cyanobacteria dominate, while just below, heterotrophs that can persist in low-oxygen environments feed on their byproducts. Deeper down, the mats become dark and smelly, the result of the sulfate reducers and methanogens that populate these oxygen-bereft zones. Here, these partnerships become even more essential, with the castoffs of one group serving as fuel for another.
These close metabolic associations between organisms, a type of symbiosis known as syntrophy, may have prefaced the evolution of complex life by creating alliances that turned permanent over time, López-García says. In this way, individuals of different microbial species could have nested within one another to create a host with one or even several symbionts. This is exactly what scientists suspect happened to form a whole new type of cell, the eukaryote, which thrived and subsequently diversified into the macroscopic array of life we see today, including humans. So-called eukaryogenesis is not defined the same way by all researchers, but broadly, the term describes an evolutionary surge toward increasing cellular complexity between 1 and 2 billion years ago.
During this time, some of the defining characteristics of modern eukaryotic cells—the nucleus, mitochondria, cytoskeleton, cell membrane, and chloroplasts, among others—made their debut. These occurred between the first and last common ancestors of all living eukaryotes, known by their acronyms, FECA and LECA, respectively. Most of the details of these evolutionary leaps, however, remain unsettled. Researchers do not uniformly agree on which branch of life eukaryotes sprang from, which microbial players might have contributed to the process, or on the order of specific evolutionary milestones along the way. But the recent identification of the Asgard archaea, thought to be the closest living relatives to modern eukaryotes, has enlivened discussions about eukaryogenesis.
This year, University of Paris-Saclay biologist Purificación López-García embarked with colleagues on a journey into life’s ancient past. The researchers traveled to the altiplanos of the northern Atacama Desert, high-altitude stretches of rocky soil and shrubbery in South America that are among the driest places in the world. Despite their inhospitable reputation, these plateaus may hold clues about the very origins of complex life. Amidst the dunes and barren mountains, there are pockets of life—warm, briny pools crusted over with colorful microbial mats of cyano-bacteria and archaea stacked atop one another like crepes. Long before Earth resembled its current state, López-García says, these microbial mats “were the forests of the past,” adding that scientists now use these clumps of microscopic life “as analogs of past ecosystems that certainly occurred at the time when eukaryotes first appear[ed].
Each layer of these living mats is composed of different types of microbes that rely upon one another. At the surface, where light and oxygen are plentiful, photosynthesizing cyanobacteria dominate, while just below, heterotrophs that can persist in low-oxygen environments feed on their byproducts. Deeper down, the mats become dark and smelly, the result of the sulfate reducers and methanogens that populate these oxygen-bereft zones. Here, these partnerships become even more essential, with the castoffs of one group serving as fuel for another.
These close metabolic associations between organisms, a type of symbiosis known as syntrophy, may have prefaced the evolution of complex life by creating alliances that turned permanent over time, López-García says. In this way, individuals of different microbial species could have nested within one another to create a host with one or even several symbionts. This is exactly what scientists suspect happened to form a whole new type of cell, the eukaryote, which thrived and subsequently diversified into the macroscopic array of life we see today, including humans. So-called eukaryogenesis is not defined the same way by all researchers, but broadly, the term describes an evolutionary surge toward increasing cellular complexity between 1 and 2 billion years ago.
[Eukaryogenesis is] arguably one of the most important events in the history of life, after the origin of life itself.—Daniel Mills, Ludwig-Maximilians-Universität München
During this time, some of the defining characteristics of modern eukaryotic cells—the nucleus, mitochondria, cytoskeleton, cell membrane, and chloroplasts, among others—made their debut. These occurred between the first and last common ancestors of all living eukaryotes, known by their acronyms, FECA and LECA, respectively. Most of the details of these evolutionary leaps, however, remain unsettled. Researchers do not uniformly agree on which branch of life eukaryotes sprang from, which microbial players might have contributed to the process, or on the order of specific evolutionary milestones along the way. But the recent identification of the Asgard archaea, thought to be the closest living relatives to modern eukaryotes, has enlivened discussions about eukaryogenesis.
Today, at the microbial mats in the Atacama Desert and other sites throughout the world, scientists are investigating what the earliest eukaryotic cells may have looked like, the partnerships they may have struck up with other organisms, and how their molecular machinery might have functioned and evolved. Already, the discovery of the Asgards has solidified certain aspects of eukaryogenesis while raising new questions about others. “I think this is the most exciting development in biology right now. So much is being discovered and so many predictions are being met,” says Daniel Mills, a geobiologist and postdoctoral researcher at Ludwig-Maximilians-Universität München who recently coauthored a paper suggesting that eukaryotes likely evolved in the absence of oxygen. Eukaryogenesis, he adds, is “arguably one of the most important events in the history of life, after the origin of life itself.”
The Path to Complexity
Eukaryogenesis is broadly defined as the evolutionary path taken by increasingly complex lifeforms as they diverged from the simpler prokaryotes that dominated the early part of Earth’s biological history. The functional period of eukaryogenesis started just prior to the symbiosis between two prokaryotes and ended when the last common ancestor of modern eukaryotes arose. During this time, many of the most recognizable eukaryotic features appeared, including organelles such as mitochondria, nuclei, and chloroplasts, as well as cellular processes such as phagocytosis. The ordering of these events in time remains unclear.
ORIGINAL HOST UNKNOWN
While the identity of original host in the symbiotic partnership that birthed modern eukaryotic cells remains mysterious, some researchers say the evidence suggests it was an archaeon rather than a bacterium. Scientists call this host, which lived more than a billion years ago, the first eukaryotic common ancestor, or FECA.
ORIGIN OF MITOCHONDRIA
At some point in the past, the prokaryote host formed a partnership with an alphaproteobacterium and permanently engulfed it, creating the mitochondrion. Researchers debate whether phagocytosis was needed to establish this relationship, but mitochondria did help power much of eukaryotes’ subsequent radiation.
APPEARANCE OF UNIQUE FEATURES
Numerous other features and processes associated with modern eukaryotic cells evolved during this time, including the nucleus and cytoskeleton. The order of their appearance is uncertain.
BIRTH OF MODERN LIFE
The last eukaryotic ancestor (LECA) shared by all living eukaryotes today was already a complex cell by the time eukaryotes began to radiate. Over hundreds of millions of years, LECA gave rise to the complex organisms that exist today, including fungi, protists, plants, and animals.
See complete article at The Scientist.
“Mysterious…” It appears that evolutionary scientists are unable to offer any mechanism consistent with the known workings of nature that could explain eukaryogenesis. From my point of view as a physicist, just saying that unique features appeared, or that “ancestral” cells “gave rise to the complex organisms that exist today”, is mere story-telling. If someone tried to publish a physics paper postulating, for example, that superconductivity “arises from a low temperature environment”, and failed to give a mathematical or even a conceptual physics explanation for their postulate, it would justifiably never come print.