Bacteria took ~1 billion years to emerge on Earth, but why did it take another ~3 billion years for animals to appear?
Scientists analysed isotopes of Chromium in rocks as a measure O2 levels in the atmosphere of the early Earth.
Reduced-Cr(III) indicates low O2 levels, oxidised-Cr(IV) indicates high O2 levels.
Atmospheric O2 levels dramatically increased ~700-800 million years ago, just before the emergence of animals in the Cambrian explosion.
The rise in O2 levels (and decrease in CO2) was mostly caused by photosynthesis in cyanobacteria (and later algae).
This was a crucial factor for helping animal life emerge on Earth.
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Background
The Earth is ~4.54 billion years old.
The first life forms were bacteria, then cyanobacteria (photosynthesising microorganisms), emerging around ~3-3.6 billion years ago.
Multi-cellular animals (metazoans) took much longer to appear, sometime around ~540 million years ago. This period is called the Cambrian explosion (lasting ~25 million years), when many new animal species emerged (e.g. sponges, molluscs, crustaceans, worms).
If unicellular life took ~1 billion years to emerge, why did it take another ~3 billion years for animals to appear?
Materials and Methods
A team of scientists (including Peter McGoldrick from the University of Tasmania) measured oxygen levels in the early Earth’s atmosphere by analysing isotopes of Chromium found in rocks of different ages from Australia, China and North America. High levels of reduced Cr(III) indicate low atmospheric oxygen levels, while higher levels of oxidised Cr(IV) indicates high atmospheric oxygen levels.
Results
Early Earth atmosphere had very low O2 levels (high CO2 levels).
Atmospheric O2 levels dramatically increased ~700-800 million years ago, just before the emergence of animals in the Cambrian explosion.
Discussion
The rise in atmospheric O2 levels was a crucial factor for helping animal life emerge on Earth.
The rise in O2 levels (and decrease in CO2) was mostly caused by photosynthesis in cyanobacteria (and algae).
Genetic and developmental innovations almost certainly contributed to emergence of animals as well.
Article
Low mid-proterozoic atmospheric oxygen levels and the delayed rise of animals
Planavsky et al., 2014 Science 346:635-8
Keywords
Life, Earth, bacteria, cyanobacteria, photosynthesis, oxygen, O2, CO2, carbon dioxide, atmosphere, Cambrian, animal, chromium, isotope, reduced, oxidised, rocks
Subject
Science, chemistry, ST1-11LW, ACSSU211, ST2-10LW, ACSSU044, ST3-11LW, ACSSU094, SC4-12ES, ACSSU153, ACSSU115, SC4-14LW, ACSSU111, ACSSU149, ACSSU150, SC5-13ES, ACSSU189, SC5-14LW, ACSSU175, SC5-15LW, ACSSU185, SC5-17CW, ACSSU178, ACSSU179