Life on Earth may have flourished earlier than biologists previously believed, a new study reveals.
Biologists are uncertain exactly how life began on Earth, although some believe that life on our planet began long ago, when reactions between organic chemicals were triggered by lightning. Other researchers contend the first life on the planet developed deep under the sea, near hydrothermal vents. The introduction of complex carbon-based molecules from comets may have also triggered the rise of the first life on the planet.
The atmosphere of early Earth was far different than it is today, with little oxygen, although many life forms can exist without this gas. Also, nitrogen, essential for the manufacture and operation of genes, could not be extracted from the atmosphere by early lifeforms. This led biologists to believe that life would have been rare on Earth until roughly two billion years ago, when organisms first adopted "nitrogen fixation," gathering the vital gas from the air.
Rocks dating from 3.2 to 2.75 million years ago, collected from around northwestern Australia and South Africa, were examined for clues about ancient life. These are some of the oldest, and best-preserved rocks on Earth, which formed well before the Great Oxygenation Event 2.4 billion years ago, which filled the atmosphere with the life-giving gas. Chemical clues are available in these rocks that are wiped out in more modern rocks.
Analysis of the ancient rocks revealed that even 3.2 billion years ago, life forms were already pulling nitrogen out of the atmosphere. Ratios of different forms of nitrogen atoms were found to be consistent with biological processes, and not chemical reactions.
Nitrogen atoms in gaseous form are bonded in pairs, by a powerful triple bond which is difficult to break. However, for life to take advantage of nitrogen, single atoms must join with other molecules. In the modern world, enzymes based on molybdenum mediate the process, lowering the energy required to split atmospheric nitrogen apart. That element is fairly common on Earth today, as oxygen reacts with rocks to wash the metal into oceans. Chemical analysis of the ancient rocks suggests that molybdenum was also used by ancient life forms, although the source of the material remains in question. It is possible life may have existed in thin layers on rocks, producing small amounts of oxygen to gather the element.
"We'll never find any direct evidence of land scum one cell thick, but this might be giving us indirect evidence that the land was inhabited. Microbes could have crawled out of the ocean and lived in a slime layer on the rocks on land, even before 3.2 billion years ago," Roger Buick of the University of Washington said.
The early start for life on Earth could have implications beyond our own planet, as the discovery could provide evidence that life on other planets may have developed quickly, as well, making alien life more likely.
Investigation of the origin of life on Earth was detailed in the journal Nature.