The Origin of Cells

Pasteur's experiment (1860s). Pasteur boiled broth in swan-neck flasks, sealing it from airborne microbes while allowing air in. No growth occurred until the neck was broken — disproving spontaneous generation. Today, cells arise only from pre-existing cells.

But the FIRST cell could not have come from a cell. The first cells must have arisen from non-living chemistry, ~3.5–4 billion years ago. Required steps:

1. Synthesis of simple organic molecules (amino acids, nucleotides, sugars) from inorganic precursors in a reducing atmosphere.
2. Polymerisation into polypeptides and polynucleotides (catalysed by mineral surfaces).
3. Self-replicating molecules — RNA capable of templated copying.
4. Membrane-bound compartments — lipid vesicles that enclose the chemistry, allowing concentration gradients.

Miller-Urey experiment (1953). Stanley Miller passed sparks through a flask of CH₄, NH₃, H₂ and H₂O (simulating early atmosphere). After a week, the flask contained amino acids and other organic molecules — supporting step 1.

The RNA world hypothesis. Modern life uses DNA to store information and protein enzymes to catalyse. But which came first? Neither — RNA can do BOTH:

• Stores information (as in modern mRNA).
• Catalyses reactions (ribozymes — including the ribosome's peptidyl transferase activity).

The RNA world hypothesises an early stage where RNA was both gene and enzyme. DNA arose later as a more stable storage molecule; protein arose later as a more efficient catalyst.

Hydrothermal vents and other settings. Deep-sea alkaline vents provide thermal gradients, mineral surfaces, and chemical disequilibrium — plausible cradles for the first life. Hot springs, tidal pools, and clay surfaces are other candidates.

LUCA — Last Universal Common Ancestor. All extant life shares features that imply a single common ancestor ~3.5–4 Ga:

• DNA as genetic material.
• Same genetic code (with minor variations).
• ATP as energy currency.
• 20 amino acids in proteins.
• L-amino acids and D-sugars (chirality).

LUCA was almost certainly a prokaryote, probably resembling modern thermophilic archaea.

Endosymbiotic theory (Lynn Margulis, 1967). Eukaryotic cells arose when a larger anaerobic prokaryote engulfed:

• An aerobic bacterium → became the mitochondrion.
• A photosynthetic cyanobacterium (in plant lineage only) → became the chloroplast.

Evidence for endosymbiosis:

Evidence	What it shows
Mitochondria and chloroplasts have their OWN circular DNA	Prokaryote-like genome
Their ribosomes are 70S (prokaryote-type)	Prokaryotic protein synthesis machinery
They reproduce by binary fission, independently of the host cell	Prokaryotic cell division
Double membrane (the inner from prokaryote, outer from engulfing vacuole)	Engulfment origin
Their DNA is more similar to bacterial DNA than to nuclear DNA	Phylogenetic origin

Why it succeeded. The engulfed aerobic bacterium produced ATP via oxidative phosphorylation; the host gained efficient respiration. Both partners benefited — a stable mutualism that became permanent.

The endosymbiotic event happened ~2 billion years ago, near the rise of atmospheric oxygen (Great Oxidation Event), and is a single example of one of evolution's largest transitions.