Estimating When Life First Appeared on Earth
- Earth formed about 4.5 billion years ago (Gya).
- The first living cells and the Last Universal Common Ancestor (LUCA) appeared soon after, but direct evidence is limited.
- Scientists use a combination of fossil, isotopic, and molecular evidence to estimate when life began.
The Formation of Earth and the Pre-Biotic Era
- Formation of Earth (~4.5 Gya):
- Earth emerged from a cloud of gas and dust in the early solar system.
- It was a hostile environment with intense volcanic activity, asteroid impacts, and an atmosphere rich in methane, ammonia, and carbon dioxide but lacking oxygen.
- The Primordial Soup (~4.0-3.8 Gya):
- Early Earth’s conditions enabled the formation of simple organic molecules, which accumulated in oceans, lakes, and pools.
- This chemical "soup" set the stage for the emergence of life’s building blocks.
Earth’s early atmosphere lacked oxygen, which only began to accumulate billions of years later as a byproduct of photosynthesis by early organisms.
The Oldest Evidence of Life: Stromatolites
- The oldest known fossils are stromatolites, layered rock structures formed by cyanobacteria, photosynthetic microorganisms.
- They were found in the Strelley Pool Formation (Western Australia), dating to 3.42 billion years ago.
- Their layered pattern resembles modern cyanobacterial mats.
- Carbon isotope ratios (¹³C/¹²C) in these rocks suggest biological carbon fixation
