Origins and Evolution of Life on Earth
Overview of the Origins and History of Life on Earth
Geological Time Scale
Origin of Earth: ~4.55 billion years ago
Most life was microscopic until around 1 billion years ago.
Multi-cellularity began ~1.5 billion years ago leading to complex organisms.
Eons and Major Life Distinctions
Four primary eons: Precambrian and Phanerozoic are major divisions.
Precambrian: Small, mostly unicellular organisms.
Phanerozoic: Explosion of multicellular life around 543 million years ago.
Origin of Life Hypotheses
Life likely emerged between 4-3.5 billion years ago.
Earliest fossils resemble cyanobacteria.
Cyanobacteria perform photosynthesis, producing oxygen.
Miller-Urey Experiment
Simulated early Earth conditions using hydrogen, methane, ammonia, and water.
Produced amino acids, supporting the idea that life's building blocks could form under primordial conditions.
Four Stages of the Origin of Life
Abiotic Synthesis of Organic Compounds
Organic precursors assembled into larger molecules.
Polymer Formation
Interaction on surfaces of clay (Montmorillonite) catalyzed formation of polymers like RNA and proteins.
Membrane Formation
Early cell-like structures called protobionts formed with boundaries from lipid layers.
Self-replication and Evolution
RNA is posited as the first genetic material capable of self-replication.
Early Earth Conditions
Hot, gassy atmosphere with high volcanic activity.
Shift to cooler conditions ~2.5 billion years ago, leading to the emergence of eukaryotes (1.8 billion years ago).
Mass Extinctions
Five major mass extinctions, including KT event affecting dinosaurs.
Current potential sixth mass extinction linked to human activity.
Early Prokaryotic Cells
First prokaryotes appeared ~3.8-3.5 billion years ago.
Mainly anaerobic, heterotrophs or autotrophs.
Cyanobacteria as first known photosynthesizers, changing atmospheric composition.