Jun 19, 2019 | Updated: 09:31 AM EDT

From Single Cells to Us—The Tale Of Evolution As It's Told At Deep-Sea Vents

May 07, 2015 12:56 PM EDT

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In the search for the origins of life, scientists have found a striking new link between lone cells and complex creatures like humans. The connection comes from observing life under the harsh conditions that exist near volcanoes more than a mile below the surface of the Atlantic Ocean.

Scientists have discovered a new type of microbe near deep-sea vents between Greenland and Norway. This undersea area, called Loki's Castle, has lent its Norse mythology to the new organism found there. The microbe, named Lokiarchaeota, is single-celled, yet it resembles multicellular organisms in previously unseen ways. The Loki of Norse legend is a shape shifter, and this transitional life form seems to reflect this kind of trickster behavior as it bridges the gap between bacteria and us.

The findings were published in the journal Nature this week, describing in some detail how simple microbes evolved into more complex kinds of organisms. The team of researchers was led by scientists from Sweden's Uppsala University along with Bergen University of Norway and Vienna University of Austria.

Life As We Know It

Perhaps the most significant step in the history of life on Earth was the evolution of eukaryotes: creatures who are made up of complex cells. The cells of eukaryotes are run by nuclei and powered by the energy created by mitochondria. Even the humble amoeba is a eukaryote, joined by all plants, fungi, and animals including humans. This kind of life first occurred about 2 billion years ago.

In the 1970s, University of Illinois microbiologist Carl Woese and his team provided a major piece of the puzzle: the three-branched tree of life. The team created this detailed description of evolution on Earth by comparing the genetic material from different species.

The first branch, prokaryotes, included bacteria like E. coli. The second branch, archaea, included species of microbes that live in extreme environments. These archaea, poorly understood, were primarily distinguished by their ability to survive in places like hot springs and the bottoms of swamps. The third branch, eukaryotes, included organisms with complex cell structures which are more closely related to archaea than bacteria.

In more recent decades the details of tree of life have become clearer, as scientists have been capable of comparing the DNA of newly discovered species of microbes. Some of this more recent research have found that eukaryotes are perhaps the progeny of archaea, and are not actually a unique third branch.

However, scientists have long been baffled by the specifics of this evolutionary step. A link in the chain appeared to be missing; some sort of transitional life form was needed to explain the connection between prokaryotes like bacteria and the more complex world of organisms.

The Missing Piece

"First we had to convince ourselves it was true," lead author of  the study from Uppsala University, Thijs Ettema says. "And once we were certain, we did further analysis of the genes. And it turns out that they're quite special."

Lokiarchaeota is that transitional form, painting an all-new picture of the progenitors of eukaryotes and the tree of life as a whole.

"Loki formed a well-supported group with the eukaryotes in our analyses," coauthor Lionel Guy says.

"In addition, we found that Loki shares many genes uniquely with eukaryotes, suggesting that cellular complexity emerged in an early stage in the evolution of eukaryotes," coauthor Anja Spang says.

"The puzzle of the origin of the eukaryotic (complex) cell is extremely complicated, as many pieces are still missing" Ettema says. "We hoped that Loki would reveal a few more pieces of the puzzle, but when we obtained the first results, we couldn't believe our eyes. The data simply looked spectacular."

The DNA analysis of Loki showed that it is not only more closely related to eukaryotes than is any other known type of archaea, but, amazingly, that Loki has genes for many traits previously thought to be exclusive to eukaryotes. For example, Dr. Ettema's team found that Loki has many genes that encode the proteins required to build the skeleton.

The team also found that although Loki was far less complex as any true eukaryote, it was much more complex than other archaea and bacteria. And although the study explains that Loki lacks mitochondria and a nucleus, the placement of the microbe in the tree of life and their genomic analysis clearly shows how the first full-blown eukaryotes may have evolved from archaea like Loki.

"By studying its genome, we found that Loki represents an intermediate form in-between the simple cells of microbes, and the complex cell types of eukaryotes," Ettema says.

The study was reviewed by several preeminent experts who were not connected to the research.

"These findings clinch the case for the origin of eukaryotes from within the archaeal diversity and point to a specific part of the archaeal evolutionary tree where eukaryotes belong," said Eugene V. Koonin, an evolutionary biologist and principle investigator at the National Center for Biotechnology Information who was not connected to the research. "Equally important, Lokiarchaeota combine a number of 'eukaryotic-like' features that previously have been found scattered among different archaeal genomes. Taken together, these findings give credence to the evolutionary scenario in which the eukaryotes evolved from an archaeon with a complex cellular organization that might have been capable of engulfing bacteria."

"This is a genuine breakthrough. It's almost too good to be true."

Newcastle University researchers T. Martin Embley and Tom A. Williams who were not connected to the study described the results as "spectacular."

Moving Forward

But it's likely that this is just a beginning for the research team. They found Loki in the extreme conditions of the hydrothermal vent Loki's Castle. These kinds of hostile environments provide rich sources of unknown microorganisms, or, to the team, "microbial dark matter." Ettema and his team believe they may discover more about the evolution of complex cells by exploring the genomics of microbial dark matter.

"In a way, we are just getting started. There is still a lot out there to discover, and I am convinced that we will be forced to revise our biology textbooks more often in the near future." 

The team is now working to study Loki despite the fact that they die off quickly. They are trying to create conditions that will allow Loki to survive, and perhaps even grow, although it yet remains a mystery what the microbes may need.

"It's definitely not easy, but we're not giving up. There are so many questions-this is a whole new biology we have to study" Ettema says. "We'd like to obtain more genomes of more distant cousins, and some of those might actually be closer to us or to the common ancestor than Loki is."

"We could maybe start to get gradual buildup, to build a road map of the journey from single-celled life towards cellular complexity."

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