The Precambrian, which lasted approximately 4 billion years from the formation of Earth, was the time when Earth’s environment developed and life originated and evolved. Changes in atmospheric composition and the evolution of organisms were closely connected.
Basic Knowledge
Earth formed approximately 4.6 billion years ago through collisions and accretion of planetesimals in the protoplanetary disk. The earliest Earth was in a magma ocean state, in which rock was globally molten. As it cooled, evidence from zircon crystals shows that liquid water existed by approximately 4.4 billion years ago.
The oldest evidence of life is chemical biomarkers (anomalies in carbon-isotope ratios) preserved in strata from approximately 3.8–4.0 billion years ago. Early life consisted of prokaryotes (organisms without cell nuclei) and was anaerobic, not requiring oxygen. Around 2.7 billion years ago, cyanobacteria appeared and began releasing oxygen into the atmosphere through photosynthesis.
During the Great Oxidation Event (GOE), approximately 2.4–2.2 billion years ago, atmospheric oxygen levels rose sharply. At first, oxygen combined with iron ions in seawater and precipitated as banded iron formations (BIFs). Later, oxygen accumulated in the atmosphere after the oceans became saturated with oxygen. By approximately 2.1 billion years ago, eukaryotes (organisms with cell nuclei) had appeared. Around 0.6–0.7 billion years ago, global freezing events (Snowball Earth) occurred several times, followed by rapid diversification of multicellular organisms.
Explain the process by which banded iron formations (BIFs) are formed.
Answer: Oxygen released by cyanobacteria through photosynthesis reacted with dissolved ferrous iron ions (Fe²⁺) in seawater and precipitated as iron oxide (Fe₂O₃). Alternating periods of high and low oxygen release produced the banded pattern.
- Formation of Earth: approximately 4.6 billion years ago; planetesimal accretion; magma ocean
- Evidence of the oldest liquid water: zircon crystals from approximately 4.4 billion years ago
- Oldest life: approximately 3.8–4.0 billion years ago (chemical evidence)
- Appearance of cyanobacteria: approximately 2.7 billion years ago; oxygen released through photosynthesis
- Great Oxidation Event (GOE): approximately 2.4–2.2 billion years ago; sharp increase in atmospheric oxygen
- Banded iron formations (BIFs): iron–oxygen precipitates from the GOE; major source of iron ore
- Snowball Earth: approximately 0.6–0.7 billion years ago; global freezing
Points to Note
① The Great Oxidation Event resulted from oxygen accumulating through photosynthesis by cyanobacteria; it was a gradual process rather than a single event. ② BIFs are evidence that oxygen increased, but atmospheric oxygen accumulated somewhat later because oxygen was initially consumed by iron in the oceans. ③ The rapid increase in life after Snowball Earth is explained by the sudden increase in nutrients after global ice melted and by environmental selection pressure.
Practice
- Describe the effect of cyanobacteria on the composition of Earth’s atmosphere.
- Approximately how many billion years ago did the Great Oxidation Event (GOE) occur?
- What kind of phenomenon was Snowball Earth?