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Newly discovered ‘missing link’ shows how humans could evolve from single-celled organisms

An image of a hydrothermal vent field along the Arctic Mid-Ocean Ridge, close to where Loki was found in marine sediments. (Centre for Geobiology /University of Bergen, Norway/ by R.B. Pedersen)

A new microbe found a mile and a half below the Atlantic Ocean just gave scientists a major clue on the origins of life. Named Lokiarchaeota -- Loki for short -- the tiny organism is single-celled, but more like a multicelled organism than any ever found before. Scientists are calling it the missing link between single cells and complex life. Their findings were published Wednesday in Nature.

First, some background: Since the 1970s, life has traditionally been divided into three domains: complex organisms, such as plants and animals, and the two domains of simple-celled life, bacteria and archaea, which lack organelles like the mitochondria and cell nuclei. But in recent years, scientists have suggested that the real split may be between bacteria and archaea. Perhaps, research has suggested, multicelled organisms actually descended from the archaea, specifically the TACK superphylum. In fact, the prevailing theory is that complex life emerged vis-à-vis a symbiotic relationship between an archaea organism and a bacterial organism, so I guess you could say we're just the best of both worlds.

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The newly-discovered Loki has genes that give eukaryotes -- the domain that includes multicellular life -- some of their unique features. Because of this, the researchers who discovered it believe it may share a common ancestor with us. Loki is single-celled and lacks the physical characteristics of a eukaryotic cell, but it seems to have a toolkit that could theoretically allow the evolution of a complex organism.

According to lead author Thijs Ettema of Uppsala University, Loki's genomic structure seemed to good to be true when it was first spotted in sediment dredged up from the sea.

"When we started to have a more in-depth look at the genes of this new Loki genome, we found that something was strange about it quite early on," Ettema said. "We found genes that were much more like eukaryotes."

At first he thought it had to be a mistake, an artifact of some kind of genomic contamination.

"First we had to convince ourselves it was true," he said. "And once we were certain, we did further analysis of the genes. And it turns out that they're quite special."

Ettema and his team don't know what Loki uses these familiar genes for. In the deep sea environment it comes from, life is slow due to lack of nutrients and solar energy. It's possible that an organism like Loki might only divide its cells once every 10 years, so it's not like they found a whole gaggle of them to study. But in complex organisms, these genes make cell walls and organelles possible.

The team believes that complex organisms share a common ancestor with Loki.

"The implication is that these genomic tools were present in the lost common ancestor, and it had this genomic starter kit that helped eukaryotes become complex," Ettema said. "Some 2 billion years ago, we took a right turn, and Loki's ancestors took a left. Loki remained in these sediments, and it never made it into a complex organism. Instead it specialized in living in these deeply buried sediments."

"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, a principle investigator at the National Center for Biotechnology Information who wasn't involved in the study. "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," he said.

In an article written to accompany the paper, Newcastle University researchers T. Martin Embley and Tom A. Williams (neither of whom were part of the research team) called the results "spectacular" and the paper "technically outstanding."

But Loki isn't the grand prize: The organism just brings us one step closer to the common ancestor we're seeking. That ancient, single-celled organism would have had an unprecedented potential for complexity, and without it multicelled life -- let alone humanity -- could never have evolved.

"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," Ettema said. "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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