Unlike bacteria, humans have big, complex cells, packed with nuclei containing DNA and mitochondria that produce energy. All so-called eukaryotes share our cellular complexity: animals, plants, fungi, even single-celled protozoans like amoebae.
Scientists estimate that the first eukaryotes evolved about 2 billion years ago, in one of the greatest transitions in the history of life. But there is little evidence of this momentous event, no missing link that helps researchers trace the evolution of life from bacteria to eukaryotes.
On Wednesday, a team of scientists announced the discovery of just such a transitional form. At the bottom of the Arctic Ocean, they found microbes that have many — but not all — of the features previously only found in eukaryotes. These microbes may show us what the progenitors of complex cellular organisms looked like.
“This is a genuine breakthrough,” said Eugene Koonin, an evolutionary biologist at the National Center for Biotechnology Information who was not involved in the research. “It’s almost too good to be true.”
In the 1970s, scientists got their first major clue about the origins of eukaryotes. Carl Woese, a microbiologist at the University of Illinois, and his colleagues compared genetic material from different species to reconstruct the tree of life. Their analysis indicated that there were three major branches.
One branch included bacteria, among them such familiar species as E. coli. A second branch, which Dr. Woese dubbed archaea, included lesser-known species of microbes that live in extreme environments such as swamp bottoms and hot springs. Eukaryotes, which form the third branch, are more closely related to archaea than bacteria.
Over the past 40 years, as scientists have discovered new species of microbes and developed powerful ways to compare their DNA, the tree of life has come into sharper focus. A number of recent studies have indicated that eukaryotes are not actually a third separate branch. Instead, they evolved from archaea.
Thijs J. G. Ettema, a microbiologist at Upssala University in Sweden, was struck by the fact that the species of archaea most closely related to eukaryotes lived in the deep seafloor. It was possible that even closer relatives might be waiting to be discovered there.
By a stroke of good fortune, Steffen L. Jorgensen, a microbiologist at the University of Bergen, had been digging up sediment from two miles below the surface of the Arctic Ocean. A preliminary look at the sediment revealed archaea living in some layers. Dr. Jorgensen offered Dr. Ettema some of the sediment to investigate more closely.
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