Missing link in evolution of complex life found

A missing link in the evolution of complex life has been found, helping us understand how animals (including humans), plants and fungi evolved from simple microbes. It is a mystery that has long baffled scientists.

Cells are the fundamental building blocks of all life on Earth. While all microscopic life such as bacteria and other microbes are made up of simple cells, all visible life, including us, generally consists of large and complex cell types.

Scientists have long been unable to explain where complex cell types came from – there has been a ‘missing link’. Now, however, researchers from Uppsala University in Sweden, and colleagues in Norway and Austria, say they have found the answer – a new group of microorganisms which represent the missing link in the evolutionary transition from simple to more complex cells.

Missing link

Scientists believe that ‘Loki’ is the missing link between simple and complex life forms on Earth.

In the 1970s, American microbiologist and biophysicist Carl Richard Woese (1928-2012) discovered a new group of microorganisms – the Archaea. He showed that these represented a new domain or Kingdom of Life (a separate branch in the Tree of Life), a finding that at the time stunned the scientific community.

Even though archaeal cells were small and simple like bacteria, scientists found that Archaea were closely related to ‘eukaryotes’, organisms with complex cell types (humans are eukaryotes).

 

For decades, biologists have been unable to explain how the complex cell types of eukaryotes have emerged from the simple Archaea cells.

‘Loki’ – the missing link

Thijs Ettema, who works at the Department of Cell and Molecular Biology, Uppsala University, and colleagues from the Universities of Vienna in Austria and Bergen in Norway, wrote in the academic journal Nature (citation below) about their discovery of a new group of Archaea, the ‘Loki’ (short form for Lokiarchaeota). They claim it is the missing link in the origin of eukaryotes.

Team leader, Dr. Ettema, said:

“The puzzle of the origin of the eukaryotic cell is extremely complicated, as many pieces are still missing. 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.”

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.”

This idea was confirmed when Loki was placed in the Tree of Life.

Co-author Lionel guy, also from Uppsala University, said “Loki formed a well-supported group with the eukaryotes in our analyses.”

Loki shares several eukaryote-like genes

Co-lead author, Anja Spang, a researcher at Uppsala University’s Department of Cell and Molecular Biology, said:

“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.”

Hydrothermal vent

Hydrothermal vent field along the Arctic Mid-Ocean Ridge, near to where ‘Loki’ was found in marine sediments. (Image: R.B. Pedersen, University of Bergen)

Lokiarchaeota’s name is derived from the hostile environment near to where it was found – Loki’s Castle – a hydrothermal vent system located in the Mid-Atlantic Ridge between Norway and Greenland 2,353 metres below the ocean’s surface.

Steffen Jørgensen, who works at the University of Bergen in Norway, said:

“Hydrothermal vents are volcanic systems located at the ocean floor. The site where Loki is heavily influenced by volcanic activity, but actually quite low in temperature.”

Co-lead author Jimmy Saw, from Uppsala University’s Department of Cell and Molecular Biology, said “Extreme environments generally contain a lot of unknown microorganisms, which we refer to as microbial dark matter.”

By studying microbial dark matter with new genomic techniques, Dr. Ettema and colleagues hope to gain further insight into how complex cells evolved.

“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”, said Dr. Ettema.”

Citation: Complex archaea that bridge the gap between prokaryotes and eukaryotes,” Anja Spang, Jimmy H. Saw, Steffen L. Jørgensen, Katarzyna Zaremba-Niedzwiedzka, Joran Martijn, Anders E. Lind, Roel van Eijk, Christa Schleper, Lionel Guy & Thijs J. G. Ettema. Nature. Published 6 May, 2015. DOI: 10.1038/nature14447.

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