Beginnings of life | Life on earth and in the universe | Cosmology & Astronomy | Khan Academy

Named after Hades, the Greek god of the underworld, indicative of its hellish conditions.
Characterized by:
- Intense volcanic activity (lava flows).
- Frequent impacts from space objects.
- Completely inhospitable to any known form of life.

Occurred after Earth's initial cooling, around a few hundred million years post-Theia impact (formation of the Moon).
Characterized by:
- Violent impacts from asteroids, likely destroying any primitive life present.
- Caused by orbital changes in Uranus and Neptune, leading to an influx of asteroids hitting the inner planets.
The consensus is that life likely began after this period due to the destruction of potential early life forms.

First life forms are believed to have appeared approximately 3.824 billion years ago.
- These were not complex organisms, but very simple life forms.
Types of early life forms:
- Prokaryotes:
  - Unicellular organisms without nuclei or membrane-bound organelles.
  - DNA exists freely within the cell, not enclosed.
  - "Prokaryote" translates to "before kernel/nucleus".
- Examples include bacteria and archaea, which persist today.
This era likely featured self-replicating molecules that later became organized with membranes.
- RNA may have served as the initial genetic material for information transfer.

Current understanding suggests a common ancestor for prokaryotes and eukaryotes.
Differentiation into:
- Bacteria (e.g., E. coli).
- Archaea and Eukaryotes (cells with nuclei).

The earliest life forms were anaerobic, avoiding oxygen, which was toxic to them.
Energy sources for early life were likely chemical rather than photosynthetic.

The transition to the Archean Eon is placed around 3.8 billion years ago based on evidence from rocks.
Environmental conditions during this time:
- Dominated by nitrogen, methane, and carbon dioxide; little to no oxygen in the atmosphere.
- Continual volcanic activity.
- Life mostly confined to oceans, with land being inhospitable and irradiated.
Stromatolites: Formation from sediment deposits by bacteria, indicative of early life.

Around 3.5 billion years ago:
- Development of photosynthesis by cyanobacteria (blue-green bacteria).
- Initial production of oxygen within oceans.
- Early oxygen bonded with dissolved iron in the ocean, preventing accumulation in the atmosphere.

The increase in oxygen levels:
- Initially reacted with iron and did not accumulate in the atmosphere.
- Eventually, significant oxygen release caused a catastrophic effect on anaerobic life forms, leading to their decline.
- This oxygenation was crucial for the evolution of oxygen-breathing organisms in the future.
The timeline mentioned indicates billions of years until significant life forms would emerge on land.