An international team of researchers has proposed a new theory that aerosol particles in the ancient atmosphere may have played a role in generating the chemical building blocks for life on Earth. Indeed, they say, the aerosols may have had many characteristics of a primitive form of "cell".
The study will be published in the proceedings of the National Academy of Sciences by scientists from NOAA's Aeronomy Laboratory in Boulder, Colo., Oxford University, England, and the University of Colorado in Boulder.
Researchers Adrian Tuck of NOAA, Christopher Dobson of Oxford, and Barney Ellison and Veronica Vaida of the University of Colorado suggest that aerosol particles, which are produced by waves at the ocean surface, could have picked up an organic outer coating as the bubbles burst in the air.
Aloft in the atmosphere for days, months or even longer, the particles may have been like minuscule "chemical reaction chambers," exposing their interior contents to a wide range of temperatures, humidities, and sunlight exposures.
Through evaporation, simple organic materials inside the aerosols could have become more concentrated. "Encounters with other atmospheric particles, such as those derived from meteorites, could have enabled the particles to pick up trace amounts of metallic species that facilitated chemical reactions within."
This combination of factors could have promoted formation of the more complex organic molecules that are fundamental to life, such as proteins and nucleic acids. The lack of oxygen and ozone in the early atmosphere would have aided the process. The new theory helps to explain many aspects of the simplest organisms of ancient Earth.
The authors show that aerosol particles have many characteristics that are similar to bacteria and other single-celled organisms. For example, they are similar in size, they possess a watery solution in their interior, and their salinity values differ from those of the ocean.
Further, the new theory shows how some of the more unusual aspects of cells may have developed. For example, the presence of a double outer membrane in cells has been difficult to explain.
The new study shows how the airborne particles would have formed two outer layers of organic material, one layer when they first left the ocean surface and a second layer when they returned from the atmosphere back to the ocean.
Oxford University's Dobson says that coagulation and division of the particles could be viewed as an early analog of reproduction and replication processes in cells.
The large populations of aerosols, recycling over geological time, would have allowed natural selection of successful chemical populations of molecules to evolve.
According to Adrian Tuck, the study was sparked by recent NOAA observations that individual atmospheric particles are not just seawater or sulphuric acid, but instead contain a large amount of organic material.
In theorizing how the organic material could have gotten into the atmosphere, the researchers conceived of the oceanic mechanism and the possible connection to precursors of living systems on Earth.
Many previous theories of the origin of single-celled life forms have suggested that the prebiotic chemistry occurred in terrestrial waters, such as tidal pools, but questions had remained about how sufficient energy and concentrations of chemicals could have been achieved.
It was also not readily apparent how single-celled organisms could have formed, or why they had the size they did.
The authors of today's study state their new atmospheric aerosol theory helps to answer many key questions, but that experiments in large reaction chambers could provide useful tests of their ideas.
NOAA Aeronomy Laboratory
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