Past climate change - Climate Change: Biological and Human Aspects - page 74

fromthesame,orfossilfuel,whichitselfisofphotosyntheticorigin)andcarbonateformswithcarbonfrom
inorganicsources.
Thissummaryisalltoobrief.Forfurtherinformation,andinformationonotherisotopesasindicatorsof
pastenvironmentalchangeseeNewtonandBottrell(2007),fromwhichthistextboxispartlytaken.

As a greenhouse gas, carbon dioxide has retained its importance ever since. But
what brought us the oxygen in the first place? What were the early photosynthetic
species? Well, the earliest oxygenic photosynthesisers were most probably organisms
like cyanobacteria (also known as blue-green algae). These early microbial species
were then incorporated into other cells so that there were then, if you like, cells within
cells (this is the endosymbiotic theory). The absorbed cells became organelles such
as chloroplasts (found in plant cells doing photosynthesis) and mitochondria (for
energy-releasing metabolism). These more complex cells with specialised organelles
are called eukaryotes. Fossils have been found of one of the first early eukaryotic
aerobic species, dating from 2100 mya. It was similar to a species of alga living
today, Grypania spiralis (Han and Runnegar, 1992). G. spiralis grows in long, spiral
filaments about 1-2 mm wide. Another good candidate for a modern descendant
of an early eukaryotic oxygenic photosynthesiser has been presented by a large
international team of researchers in 2008 led by Robert Moore and Miroslav Obornik.
It is a microbial marine brown ball related to present-day apicomplexian parasites
(one of which you may know: Plasmodium falciparum , the malaria parasite). This
brown ball is called Chromera velia and its discovery came as a surprise, for although
it is related to the apicomplexians, it is fully photosynthetic. 3
Subsequent to Snowball Earth I, over 2 bya, the anaerobic species were to lose their
domination of the planet to the aerobes that generated and/or used oxygen. Meanwhile,
the huge carbon dioxide greenhouse blanket declined over the next billion years while
the Sun continued to grow stronger. If this greenhouse effect had not declined then the
Earth may have seen a runaway greenhouse effect. In this, warming of the Earth would
have caused more water to evaporate from the oceans. Water vapour is a greenhouse
gas and would have generated further warming, so resulting in a positive-feedback
loop (opposite to the albedo feedback loop that would have cooled the Snowball
Earth).
Where did the primordial carbon dioxide go? The carbon dioxide was chemically
transformed by biological processes into carbonates that were laid down as great beds
of chalk, and by abiotic, purely chemical, processes.
By 800 mya life had diversified so much that multicellular organisms had evolved,
including animals. Indeed, the eon 542 mya to the present day is known as the
Phanerozoic eon, which means the age of visible life. The significance of multicelled
living things was profound for the biosphere. More than one cell means that it is
possible to have a more advanced structure than the simple cellular repetition seen in
algal colonies. A more advanced structure in turn enables specialization of cells for
processing food, respiration and other metabolic functions. This meant that carbon

3

Patrick Keeling wrote a short article explaining this organism's evolutionary significance for Nature in
2008.