Bangalore, June 18 (IANS) The cells that have led to the rise of the most complex life forms on earth, including multicellular organisms such as animals or plants, probably evolved as a result of growing initimacy belween their single-celled relatives , say researchers including one from Bangalore's National Centre for Biological Sciences.

The first living beings on earth were single-cell organisms. Those cells were quite simple, but during the course of evolution they gave way to a more complex cellular lineage - the eukaryotes, or cells with a nucleus.

The first eukaryote is thought to have arisen when prokaryotes - the kingdoms of archaea and bacteria - joined forces. Prokaryotes are single-celled organisms that have no cell nucleus.

But in an Opinion paper published in the journal Trends in Cell Biology, researchers have now proposed that the molecular machinery essential to eukaryotic life was probably borrowed, little by little over time, from those simpler ancestors.

"We are beginning to think of eukaryotic origins as a slow process of growing intimacy - the result of a long, slow dance between kingdoms, and not a quick tryst, which is the way it is portrayed in textbooks," said Mukund Thattai from National Centre for Biological Sciences.

The proposal is based on new genomic evidence derived from a deep-sea vent on the ocean floor.

The eukaryotic cells of plants, animals, and protists are markedly different from those of their single-celled, prokaryotic relatives, the archaea and bacteria. 

Eukaryotic cells are much larger and have considerably more internal complexity, including many internal membrane-bound compartments.

Although scientists generally agree that eukaryotes can trace their ancestry to a merger between archaea and bacteria, there has been considerable disagreement about what the first eukaryote and its immediate ancestors must have looked like. 

As Thattai and his colleagues Buzz Baum and Gautam Dey of University College London explained in their paper, that uncertainty has stemmed in large part from the lack of known intermediates that bridge the gap in size and complexity between prokaryotic precursors and eukaryotes. 

As a result, they said, the origin of the first eukaryotic cell has remained "one of the most enduring mysteries in modern biology."

That began to change last year with the discovery of DNA sequences for an organism, that no one has ever actually seen, living near a deep-sea vent on the ocean floor. 

The genome of the archaeon known as Lokiarchaeum ('Loki' for short) contains more "eukaryotic signature proteins" (ESPs) than any other prokaryote. 

Importantly, among those eukaryotic signature proteins are proteins critical for eukaryotes' ability to direct traffic amongst all those intercellular compartments.

"The genome can be seen as 'primed' for eukaryogenesis. With the acquisition of a number of key genes and lipids from a bacterial symbiont, it would be possible for Loki-type cells to evolve a primitive membrane trafficking machinery and compartmentalisation," Baum said.

The researchers predict that, when Loki is finally isolated or cultured, "it will look more like an archaeon than a proto-eukaryote and will not have internal compartments or a vesicle-trafficking network." 

But its morphology and/or cell cycle might have complexities more often associated with eukaryotes, they noted.​