Stromatolites represent some of the most ancient evidence of life on Earth, appearing in the fossil record as far back as 3.5 billion years ago. These layered rock structures were primarily built by colonies of microscopic organisms, specifically photosynthetic cyanobacteria. These microbial communities played a foundational role in shaping the early planet, influencing its geology and setting the stage for future biological developments.
The Revolutionary Gas
The gas released by ancient stromatolites (cyanobacteria) was oxygen. This occurred through oxygenic photosynthesis. During photosynthesis, cyanobacteria utilized sunlight, water, and carbon dioxide to create organic compounds, producing oxygen as a byproduct. This metabolic innovation introduced a reactive gas largely absent from the planet’s early atmosphere. The continuous activity of these organisms steadily accumulated oxygen, which fundamentally altered the planet’s environment.
Earth’s Atmospheric Transformation
The accumulation of oxygen released by stromatolites led to a shift in Earth’s atmosphere, a period known as the Great Oxygenation Event (GOE). This event, which began around 2.4 to 2.5 billion years ago, marked the transition from an anoxic, or oxygen-free, environment to an oxygen-rich one. Prior to the GOE, any free oxygen produced was quickly consumed by reactions with dissolved minerals and volcanic gases.
Geological evidence for this transformation includes the widespread formation of banded iron formations (BIFs). These distinctive sedimentary rocks, found globally, consist of alternating layers of iron oxides and iron-poor chert. As oxygen levels rose, it reacted with dissolved iron in the ancient oceans, causing insoluble iron oxides to precipitate and settle on the seafloor, creating these characteristic red and black bands. The presence and eventual decline of BIFs indicate that the oceans became saturated with oxygen, allowing it to then build up in the atmosphere.
Life’s Evolutionary Turning Point
The rising levels of atmospheric oxygen had complex consequences for the evolution of life. For many existing anaerobic life forms, which thrived in the absence of oxygen, this new oxygen-rich environment was toxic. This led to widespread extinctions among these early organisms, marking one of Earth’s first major mass extinction events.
However, the increasing oxygen also created opportunities for new forms of life to emerge. Aerobic organisms, capable of utilizing oxygen for respiration, evolved more efficient metabolic pathways, extracting more energy from nutrients than their anaerobic counterparts. This shift enabled the development and diversification of more complex life forms, ultimately paving the way for the emergence of multicellular organisms and the vast biodiversity seen today.