Scientists in 2003 claimed to have found the oldest
evidence of photosynthesis - the most important chemical reaction on
Earth - in 3.7 billion year-old rocks.
Photosynthesis is the process by which plants, algae,
and certain bacteria convert sunlight to chemical energy. Danish
researchers claimed rocks from Greenland showed that life-forms were
using the process about one billion years earlier than had previously
been shown. Details of the research were published in Earth and
Planetary Science Letters.
Professor Minik Rosing and Professor Robert Frei,
both of the University of Copenhagen, Denmark, analysed ancient
seafloor sediments in Isua, on the south-western edge of Greenland,
where they had previously found the earliest evidence of life on
Earth. What this research demonstrated was that Earth had a functioning
biosphere prior to 3.7 billion years ago.
The researchers discovered abundant quantities of the
element uranium in the ancient sediments, which had most likely
precipitated out of ocean water. In a 'reducing' environment where little
or no photosynthesis is taking place, the elements uranium and thorium
would move around together in the ocean as mineral particles.
But the high abundance of uranium relative to thorium
in Isua rocks suggested that uranium had been chemically separated from
thorium. This happens under 'oxidising' conditions in which organisms
release oxygen into the environment. Rosing and Frei concluded that
microbes, much like present-day cyanobacteria, were converting sunlight
to chemical energy through oxygenic, or oxygen-producing, photosynthesis.