The evolution of photosynthetic organisms like plants, algae and some bacteria completely transformed the composition of Earth’s atmosphere and enabled the evolution of complex life. Before photosynthesis evolved, the atmosphere was very different – it contained virtually no oxygen, which is essential for humans and most animals to breathe. The emergence of photosynthesis filled the atmosphere with oxygen, paving the way for the evolution of complex multicellular organisms. Understanding what Earth was like before photosynthesis can provide insight into how life evolved and the co-evolution of life and atmosphere.
The Early Atmosphere
The early Earth atmosphere, before about 3.5 billion years ago, was very different from today. It is believed to have contained:
- Little to no oxygen – less than 0.001% of current levels
- Higher levels of carbon dioxide – possibly 10-100 times higher than now
- Higher levels of methane and ammonia
- Water vapor
- Nitrogen
- Possible small amounts of hydrogen and inert gases like neon and argon
With no ozone layer, the surface was exposed to intense UV radiation from the sun. The lack of oxygen also meant there was no ozone layer to absorb harmful UV rays.
Why was there no oxygen?
There was virtually no free oxygen because there were no photosynthetic organisms to produce it in large quantities. Oxygen is released as a byproduct of photosynthesis, but photosynthesis had not yet evolved on Earth.
Some free oxygen was produced abiotically – through photolysis of water vapor by UV rays from the sun. However, without photosynthetic organisms, this free oxygen did not accumulate to any significant extent in the atmosphere. Any free oxygen reacted quickly with reducing gases like methane and ammonia or dissolved in the oceans.
Consequences of a low-oxygen atmosphere
The lack of oxygen had major implications for the evolution of life:
- Aerobic respiration could not occur. Only anaerobic metabolism was possible.
- Multicellular life was very limited. Complex organisms require high levels of oxygen for respiration.
- Fires were unlikely. Combustion requires oxygen.
- Oxidative weathering of rocks was minimal, so less mineral nutrients were released into environment.
The atmosphere consisted largely of nitrogen, carbon dioxide and methane – gases that do not support aerobic life. Only microbes that could survive without oxygen thrived on early Earth.
The Emergence of Oxygenic Photosynthesis
The transition to an oxygen-rich atmosphere began around 3 billion years ago with the evolution of cyanobacteria, which could perform oxygenic photosynthesis:
- Water + carbon dioxide + light energy -> glucose + oxygen
Cyanobacteria and later plants and algae produced oxygen as a waste product. Gradually, cyanobacteria populated the oceans and oxygen built up in the atmosphere.
It took a long time for oxygen to build up to current levels – nearly 2 billion years. Oxygen levels increased rapidly about 2.4 billion years ago in what is called the Great Oxidation Event. The oxygen reacted with methane and ammonia, removing them from the atmosphere.
Effects of increased atmospheric oxygen
- Aerobic respiration became possible, enabling more complex life.
- An ozone layer formed, blocking harmful UV radiation.
- Higher rates of weathering increased mineral nutrients.
- New metabolic pathways evolved, like nitrification.
The increase in atmospheric oxygen set the stage for the evolution of complex organisms, shaping the biology we see today. It enabled efficient aerobic metabolism. The oxygen-rich atmosphere created new ecological opportunities.
Composition of the Oceans
Photosynthesis also dramatically impacted the composition of the oceans:
- Oxygen increased in surface waters while deep waters remained anoxic.
- Light surface waters (photic zone) increased productivity.
- Cyanobacteria evolved oxygenic photosynthesis.
- Stromatolites flourished in shallow waters.
However, because there was limited oxygen, many elements remained in their reduced chemical state in the oceans, unlike today:
- More dissolved iron and manganese.
- High concentrations of sulfide.
- More methane.
- Less sulfate.
The build-up of oxygen over time oxidized many of these reduced compounds, changing ocean chemistry. Nutrient cycling was very different without aerobic processes like nitrification occurring.
Impact on ocean life
The oceans before oxygenic photosynthesis contained:
- Anaerobic bacteria – sulfur-reducing, methane-producing, iron-reducing bacteria.
- Archaea.
- Possible primitive eukaryotes.
- Stromatolite communities.
- No aerobic bacteria or multicellular life.
Life was limited to microbial communities in the absence of oxygen. Once cyanobacteria evolved, the oceans became more productive and increasingly oxygenated over time, enabling complex life.
The Carbon Cycle
The carbon cycle was also different without photosynthetic organisms:
- Volcanic outgassing released CO2 into the atmosphere.
- CO2 dissolved in the oceans.
- Organic carbon burial was limited – some through anaerobic bacteria.
- Methanogenesis produced methane.
- No major sink for CO2 without photosynthesis.
This meant the early atmosphere contained very high levels of CO2. Carbon dioxide levels may have been 10-100 times higher than today.
The evolution of photosynthesis created a massive CO2 sink, enabling organic carbon burial and oxygen release. It significantly reduced atmospheric CO2 levels. Carbon cycling was transformed by photosynthesis.
Evidence for high CO2 levels
Evidence for high CO2 levels comes from:
- Isotopic evidence – carbon isotope ratios.
- Lack of carbonate deposits – CO2 acidified the oceans.
- Modeling studies of long-term carbon cycling.
This evidence indicates the early atmosphere was rich in CO2 rather than oxygen and poor in organic carbon.
Climate and Temperature
The increased greenhouse effect from high CO2 levels had major impacts on climate:
- Higher average temperatures.
- No polar ice caps.
- Higher sea levels – continents were likely smaller.
- Possibly more turbulent weather and storms.
The strong greenhouse effect meant the early Earth was much warmer than today, with global average temperatures possibly over 70°C. However, the faint young sun provided less heating, so CO2 maintained warmer temperatures.
Evidence for a warmer early Earth
- Isotopic evidence – Oxygen isotope ratios in sediments.
- Widespread tropical zones and lack of glaciations.
- Evidence of liquid water implies stronger greenhouse effect.
- Geochemical modeling studies.
A combination of proxies and models indicates the early Earth was predominantly warmer than today. Photosynthetic organisms later reduced the greenhouse effect by lowering CO2 levels.
Conclusion
The evolution of photosynthesis utterly transformed the composition of the early Earth’s atmosphere and oceans. It filled the atmosphere with oxygen, enabling the evolution of complex life. It removed greenhouse gases like methane and ammonia. It increased ocean productivity and diversity. The carbon cycle was forever changed by the buried organic carbon. Understanding the pre-photosynthetic world provides insight into the co-evolution of life and environment on Earth, and the transition to an oxygenated world.