Scientists advance probe on how life started on Earth

SCIENTISTS have recorded two major breakthroughs on how life started on Earth even as they baffle at plumes seen reaching high above the surface of Mars.

     A spark from a lightning bolt, interstellar dust, or a subsea volcano could have triggered the very first life on Earth. But what happened next? Life can exist without oxygen, but without plentiful nitrogen to build genes — essential to viruses, bacteria and all other organisms — life on the early Earth would have been scarce. 

    The results were published February 16 in Nature.

      The ability to use atmospheric nitrogen to support more widespread life was thought to have appeared roughly two billion years ago. Now research looking at some of the planet’s oldest rocks finds evidence that 3.2 billion years ago, life was already pulling nitrogen out of the air and converting it into a form that could support larger communities.

     The authors analyzed 52 samples ranging in age from 2.75 to 3.2 billion years old, collected in South Africa and northwestern Australia. These are some of the oldest and best-preserved rocks on the planet. The rocks were formed from sediment deposited on continental margins, so are free of chemical irregularities that would occur near a subsea volcano. They also formed before the atmosphere gained oxygen, roughly 2.3 to 2.4 billion years ago, and so preserve chemical clues that have disappeared in modern rocks.

      Even the oldest samples, 3.2 billion years old – three-quarters of the way back to the birth of the planet – showed chemical evidence that life was pulling nitrogen out of the air. The ratio of heavier to lighter nitrogen atoms fits the pattern of nitrogen-fixing enzymes contained in single-celled organisms, and does not match any chemical reactions that occur in the absence of life.

    Genetic analysis of nitrogen-fixing enzymes has placed their origin at between 1.5 and 2.2 billion years ago.

       Fixing nitrogen means breaking a tenacious triple bond that holds nitrogen atoms in pairs in the atmosphere and joining a single nitrogen to a molecule that is easier for living things to use. The chemical signature of the rocks suggests that nitrogen was being broken by an enzyme based on molybdenum, the most common of the three types of nitrogen-fixing enzymes that exist now. Molybdenum is now abundant because oxygen reacts with rocks to wash it into the ocean, but its source on the ancient Earth – before the atmosphere contained oxygen to weather rocks — is more mysterious.

      The authors hypothesize that this may be further evidence that some early life may have existed in single-celled layers on land, exhaling small amounts of oxygen that reacted with the rock to release molybdenum to the water.

     Future work will look at what else could have limited the growth of life on the early Earth. Lead author Eva Stüeken has begun a University of Washington (UW), United States, postdoctoral position funded by NASA to look at trace metals such as zinc, copper and cobalt to see if one of them controlled the growth of ancient life.

     Also, researchers have found an RNA structure-based signal that spans billions of years of evolutionary divergence between different types of cells, according to a study led by researchers at the University of Colorado School of Medicine at the Anschutz Medical Campus and published in the journal Nature.

     The finding could alter the basic understanding of how two distinct life forms — bacteria and eukaryotes — begin the process of protein synthesis.

        A professor of biochemistry and molecular genetics and corresponding author of the article in Nature, Dr. Jeffrey Kieft, said scientists have long thought that the molecular signals that initiate protein synthesis in bacteria and eukaryotes are mutually exclusive.

      Scientists in Kieft’s lab explored whether a structured RNA molecule from a virus that infects eukaryotic cells could function in bacteria. Surprisingly, they found that it could initiate protein syntheses, a process necessary for life.

     “What we found bridges billions of years of evolutionary divergence,” said Kieft, who is also a Howard Hughes Medical Institute Early Career Scientist. “We wanted to explore whether it was possible to bypass mechanisms that were specific to each domain of life and find a signal capable of operating in both.”

     Eukaryotes are organisms, such as plants, animals and fungi, whose cells contain a nucleus and are enclosed within membranes, while prokaryotes, such as bacteria, do not contain a nucleus.

      In an article that accompanies the study by Kieft and his colleagues, Eric Jan, associate professor in the Department of Biochemistry and Molecular Biology at the University of British Columbia, calls the finding “surprising” and writes that the CU scientists and their colleagues “have shown for the first time that a bona fide signal in an RNA structure promotes protein synthesis in the two domains of life.”

    Meanwhile, plumes seen reaching high above the surface of Mars are causing a stir among scientists studying the atmosphere on the Red Planet.

   On two separate occasions in March and April 2012, amateur astronomers reported definite plume-like features developing on the planet.

      The plumes were seen rising to altitudes of over 250 km above the same region of Mars on both occasions. By comparison, similar features seen in the past have not exceeded 100 km.

     “At about 250 km, the division between the atmosphere and outer space is very thin, so the reported plumes are extremely unexpected,” says Agustin Sanchez-Lavega of the Universidad del País Vasco in Spain, lead author of the paper reporting the results in the journal Nature.