Slimy Microbes May Have Carpeted Earth 3.2 Billion Years Ago

oldest rocks in the desert in northwestern Australia. — Scientists collected the oldest rock samples, dating back 3.2 billion years, in the desert in northwestern Australia.

(Image credit: R. Buick / Univ. of Washington)

A layer of living scum only a cell thick may have covered parts of Earth more than 3 billion years ago, surviving with the help of nitrogen that these slimy microbes pulled from the atmosphere, a new study finds.

This finding suggests that nitrogen may have helped some planets, such as Mars, support life, researchers say.

Although life can exist without oxygen — and did, in the earliest days of life on Earth — without nitrogen, such organisms would be scarce. Nitrogen is needed to create proteins, DNA and RNA, and necessary for plants to grow and photosynthesize. It is an essential nutrient for all life on Earth and must have been available since its origins, researchers said. [7 Theories for the Origin of Life on Earth]

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Nitrogen molecules do not easily react with other chemicals to form new compounds. The vital process that converts nitrogen into biologically useful molecules that life can use is known as nitrogen fixation. Nitrogen fixation involves breaking the powerful chemical bonds that hold nitrogen atoms in pairs in the atmosphere and using the resulting single nitrogen atoms to help create biologically useful molecules. Microbes that live in the roots of legume plants and in soils are key to modern nitrogen fixation.

Past genetic analysis of nitrogen-fixing enzymes, which would have been produced by the microbes, placed their origin at between 2.2 billion and 1.5 billion years ago. Now, scientists looking at some of the planet's oldest rocks have found evidence that life was already practicing nitrogen fixation about 3.2 billion years ago, nearly three-quarters of the way back to the birth of the planet. [Photo Timeline: The Birth of Planet Earth]

"People always had the idea that the really ancient biosphere was just tenuously clinging on to this inhospitable planet, and it wasn't until the emergence of nitrogen fixation that suddenly the biosphere became large and robust and diverse," study co-author Roger Buick, a geologist and astrobiologist at the University of Washington in Seattle, said in a statement. "Our work shows that there was no nitrogen crisis on the early Earth, and therefore it could have supported a fairly large and diverse biosphere."

Molybdenum is now abundant because oxygen reacts with rocks to convert molybdenum into a soluble form that can wash into the ocean. However, its source on the ancient, oxygen-poor Earth, is uncertain. The researchers suggested that their findings may be evidence that some early life may have existed as single-celled layers on land. Those microbes would have emitted small amounts of oxygen as a byproduct when they broke down nitrogen, the researchers said. Then, the emitted oxygen would have reacted with any molybdenum in the rock, thus helping to release the element into the water.

Charles Q. Choi is a contributing writer for Live Science and Space.com. He covers all things human origins and astronomy as well as physics, animals and general science topics. Charles has a Master of Arts degree from the University of Missouri-Columbia, School of Journalism and a Bachelor of Arts degree from the University of South Florida. Charles has visited every continent on Earth, drinking rancid yak butter tea in Lhasa, snorkeling with sea lions in the Galapagos and even climbing an iceberg in Antarctica.