How our planet formed, changed, and created the earliest microorganisms
The early history of the Earth is a story of enormous changes: from a chaotic, molten body made of dust and planetesimals to a planet capable of supporting complex life. During the first few hundred million years, the Earth experienced relentless bombardment by remaining debris but eventually became stable, with oceans and an atmosphere. This chemical environment created the conditions from which life emerged. Each step shaped the planet's internal structure, surface conditions, and ability to support biological evolution.
Topic 6: Early Earth and the origin of life invites a geological and biological journey through vast time spans, showing how the Earth formed, differentiated, and allowed the earliest microorganisms to develop. From the collision that created the Moon to the microfossils left by microorganisms—these events provide critical insights into life's resilience and the planetary processes that enabled evolution. Below is a brief overview of each main area:
1. Earth's accretion and differentiation
The path from planetesimals in the protoplanetary disk to the proto-Earth involved countless collisions that ultimately formed a molten planet where heavy metals sank to create the core, and lighter silicates rose to form the mantle and crust. This established Earth's layered structure, creating the conditions for tectonics, volcanism, and a protective magnetic field—key features of habitability.
2. Moon formation: the giant impact hypothesis
It is believed that Theia—a Mars-sized body—collided with the young Earth, ejecting material that coalesced into the Moon. This dramatic event determined Earth's rotation, axial tilt, and possibly stabilized the climate. The giant impact hypothesis is supported by the similar isotopic "signature" of Earth and Moon rocks and modeling of circumplanetary disks around young planets.
3. Hadean eon: intense bombardment and volcanism
The Hadean eon (~4.6–4.0 billion years ago) was characterized by extreme conditions—constant asteroid/comet bombardment, frequent volcanic eruptions, and an initially magmatic or partially molten Earth surface. Despite this harsh beginning, a primary crust and oceans eventually formed, indicating the potential for life to arise.
4. Formation of early atmospheres and oceans
Volcanic outgassing (CO2, H2O vapor, SO2, etc.) and delivery of water from comets/asteroids may have created the first stable Earth atmosphere and oceans. Cooling surface allowed water vapor to condense, forming global oceans—a medium where chemical reactions important for life occurred. Geological evidence shows oceans formed very early, stabilizing surface temperature and promoting chemical cycling.
5. Origins of life: prebiotic chemistry
How did non-living molecules form self-replicating systems? Various theories exist, from the primordial soup on the planet's surface to deep ocean hydrothermal vents, where mineral-rich water at the seafloor could have driven energetic chemical gradients. These prebiotic processes are studied in astrobiology, combining geochemistry, organic chemistry, and molecular biology knowledge.
6. Earliest microfossils and stromatolites
The fossil record (e.g., stromatolites—layered microbial community structures) indicates that life existed on Earth as early as 3.5–4.0 billion years ago. These ancient records show that life emerged rapidly once conditions stabilized, possibly just a few hundred million years after the last catastrophic impacts.
7. Photosynthesis and the great oxygenation event
Oxygenic photosynthesis (likely by cyanobacteria) appeared, and Earth's atmosphere ~2.4 billion years ago underwent the "great oxygenation event." The appearance of free oxygen caused the death of many anaerobic organisms but opened the way for aerobic respiration and more complex ecosystems.
8. Eukaryotes and the emergence of complex cells
The transition from prokaryotes to eukaryotes (cells with a nucleus and organelles) marks a major evolutionary leap. According to the endosymbiotic theory, ancient cells engulfed free-living bacteria that eventually became mitochondria or chloroplasts. This innovation enabled more versatile metabolism and the emergence of more complex organisms.
9. "Snowball Earth" hypotheses
Geological evidence suggests Earth may have undergone near-global glaciation ("Snowball Earth") episodes, possibly regulating or altering evolutionary pathways. Such global ice ages reveal how planetary climate feedback mechanisms, continental arrangements, and biosphere impacts determine the planet's climate balance.
10. Cambrian explosion
Finally, about 541 million years ago, the Cambrian explosion occurred, leading to a rapid increase in animal diversity—many modern animal phyla originated here. This highlights how planetary conditions, oxygen levels, genetic innovations, and ecological interactions can trigger a rapid complexity burst on an evolving Earth.
Examining these stages in detail—from molten youth and violent impacts to thriving microbial "mats" and eventually multicellular organisms—Topic 6 describes how geological and biological phenomena combined to form our "living planet." Through geochemical, fossil, and comparative planetology data, we see Earth's "biographic" history as a weave of catastrophes, adaptations, and innovations. Understanding how Earth achieved and maintained habitability provides valuable insights for searching for life elsewhere, revealing a universal principle of matter, energy, and chemistry interactions capable of sustaining biology in the universe.