# Lecture 1: Origin of Life and Early Earth ## General Biology II — Organismal, Evolution & Ecology --- ## Learning Objectives By the end of this lecture, students will be able to: 1. Describe the conditions on early Earth and explain how they differed from the present day 2. Outline the key hypotheses for the origin of life, including abiogenesis and the RNA world hypothesis 3. Explain the significance of the Miller-Urey experiment and subsequent prebiotic chemistry research 4. Describe the major milestones in early life evolution, from protocells to the first prokaryotes 5. Explain the endosymbiotic theory and the origin of eukaryotic cells 6. Interpret geological and chemical evidence used to date the earliest life forms --- ## Lecture Content ### I. Conditions on Early Earth Earth formed approximately 4.6 billion years ago (Ga) from the accretion of dust and gas within the solar nebula. The planet that emerged looked nothing like the world we inhabit today. Its early atmosphere was a reducing atmosphere, meaning it lacked free oxygen (O2). Instead, volcanic outgassing filled the skies with water vapor (H2O), carbon dioxide (CO2), nitrogen (N2), methane (CH4), ammonia (NH3), and hydrogen (H2). The surface was brutally hot, subjected to relentless bombardment by meteorites and comets during a period known as the Late Heavy Bombardment, which lasted from roughly 4.1 to 3.8 Ga. As the planet gradually cooled between 4.2 and 4.0 Ga, water vapor condensed to form the first oceans -- a development of profound importance, since liquid water is considered essential for life as we know it. With no ozone layer to block ultraviolet radiation and no oxygen to temper the atmosphere's chemistry, early Earth was awash in energy sources capable of driving chemical reactions: lightning discharges, intense UV radiation, volcanic activity and hydrothermal vents, and the steady output of radioactive decay. A timeline diagram of early Earth history from 4.6 Ga to 2.0 Ga. Panel A: Formation of Earth with magma oceans and heavy bombardment (4.6–4.0 Ga). Panel B: Cooling, ocean formation, and first evidence of life — stromatolites and microfossils (3.8–3.5 Ga). Panel C: Rise of cyanobacteria and the Great Oxidation Event (~2.4 Ga). Key geological and atmospheric milestones are labeled along the timeline with color-coded eras. ### II. Abiogenesis: From Simple Molecules to Organic Compounds Abiogenesis refers to the natural process by which life arose from non-living matter. The first serious scientific framework for this transition came from the Oparin-Haldane hypothesis in the 1920s, which proposed that organic molecules could form spontaneously in a reducing atmosphere. Under this model, simple organic compounds accumulated in the early oceans, forming a so-called "primordial soup," while energy sources drove chemical reactions that produced increasingly complex molecules. This hypothesis received dramatic experimental support in 1953, when Stanley Miller and Harold Urey simulated early Earth conditions in the laboratory. They constructed a sealed apparatus containing water, methane, ammonia, and hydrogen, then passed electrical sparks through the gas mixture to simulate lightning. After just one week, analysis of the resulting solution revealed amino acids -- including glycine, alanine, and aspartic acid -- along with other organic molecules. The experiment demonstrated convincingly that abiotic synthesis of organic monomers is possible under reducing conditions. In a fascinating postscript, a re-analysis of the sealed original samples in 2008 revealed even more amino acids than Miller and Urey had originally reported. Subsequent research has expanded on these findings considerably. Jeffrey Bada and others showed that amino acids form under various atmospheric compositions, including less strongly reducing conditions than Miller and Urey assumed. Formaldehyde and hydrogen cyanide (HCN) have emerged as key intermediates in prebiotic synthesis: nucleotide bases such as adenine and guanine can be synthesized from HCN, sugars including ribose can form via the formose reaction from formaldehyde, and lipid-like molecules such as fatty acids form spontaneously under certain conditions. Together, these discoveries demonstrate that all the major building blocks of life -- amino acids, nucleotides, sugars, and lipids -- can arise from simple inorganic precursors. ### III. From Monomers to Polymers and Protocells Even if organic monomers formed readily in the primordial soup, a critical challenge remained: the "concentration problem." In a vast, dilute ocean, how could monomers come together in sufficient concentrations to polymerize into the macromolecules that life requires? Several solutions have been proposed. Evaporation in tidal pools could concentrate molecules on mineral surfaces, while clay minerals such as montmorillonite can catalyze the polymerization of amino acids and nucleotides by providing a charged surface that organizes and concentrates monomers. The hydrothermal vent hypothesis offers another compelling scenario. Deep-sea alkaline hydrothermal vents, such as those at the Lost City formation, provide steep chemical gradients between the vent fluid and surrounding seawater. Iron-sulfur minerals at these sites catalyze organic reactions, and the natural proton gradients across thin mineral membranes could have driven early metabolism -- much as proton gradients power ATP synthesis in modern cells. Many researchers now favor hydrothermal vents as one of the most plausible sites for life's origin. A key step toward true cellularity was the emergence of protocells -- self-assembling lipid vesicles that form spontaneously when amphipathic molecules (those with both hydrophilic heads and hydrophobic tails) are placed in water. These molecules naturally arrange themselves into bilayer membranes, creating enclosed compartments. Protocells can grow by incorporating additional lipids from their environment, divide through simple mechanical forces, and encapsulate macromolecules such as RNA. In this way, they provide a contained environment for chemical reactions -- the essential principle of compartmentalization. Laboratory experiments by Jack Szostak have demonstrated that simple protocells can indeed undergo division, lending experimental credibility to this model. A diagram illustrating protocell formation. Panel A: Amphipathic fatty acid molecules in water spontaneously forming micelles and vesicles (bilayer cross-section shown). Panel B: A protocell encapsulating RNA molecules, with arrows showing growth by incorporation of additional lipids from the environment. Panel C: Mechanical division of a protocell into two daughter protocells. Labels indicate hydrophilic heads, hydrophobic tails, aqueous interior, and encapsulated RNA strands. ### IV. The RNA World Hypothesis One of the deepest puzzles in origin-of-life research is the chicken-and-egg problem of information and catalysis. In modern cells, DNA stores genetic information but requires protein enzymes to replicate, while proteins perform catalytic work but require DNA to encode their amino acid sequences. Which came first? The RNA world hypothesis offers an elegant resolution by proposing RNA as the first self-replicating molecule. RNA occupies a unique position because it can serve both roles: like DNA, it can store genetic information in its nucleotide sequence, and like proteins, it can catalyze chemical reactions. Catalytic RNA molecules, called ribozymes, were discovered by Thomas Cech and Sidney Altman, a breakthrough that earned them the Nobel Prize in 1989. Perhaps the most striking piece of evidence for the RNA world is the ribosome itself, which is fundamentally a ribozyme -- the peptide bond that links amino acids together is catalyzed by ribosomal RNA, not by protein. Additional support comes from the fact that RNA nucleotides such as ATP, GTP, NAD+, and coenzyme A serve as cofactors throughout modern metabolism, hinting at a time when RNA played a far more central role in cellular chemistry. In vitro evolution experiments have even generated RNA molecules capable of self-replication, albeit with limited fidelity. The RNA world hypothesis is not without challenges. Abiotic synthesis of RNA is difficult because ribose is chemically unstable and phosphodiester bond formation requires substantial energy input. Alternative hypotheses suggest that "metabolism-first" scenarios -- self-sustaining chemical networks that preceded replication -- may have preceded the RNA world, or that simpler nucleic acid analogs such as PNA (peptide nucleic acid) or TNA (threose nucleic acid) may have served as precursors to RNA. Regardless of these uncertainties, the eventual transition from RNA to DNA as the primary information storage molecule makes chemical sense: DNA is more stable than RNA thanks to its deoxyribose sugar and double-stranded architecture. Reverse transcriptase-like enzymes may have facilitated this RNA-to-DNA transition, after which DNA assumed the role of long-term genetic repository while RNA retained its catalytic and messenger functions. ### V. First Cells and Early Life The oldest evidence of life on Earth comes from multiple lines of geological evidence. Stromatolites in the Pilbara region of Western Australia, dating to approximately 3.5 Ga, are layered sedimentary structures formed by ancient microbial mats. Carbon isotope signatures in rocks from Greenland, dating to 3.7-3.8 Ga, carry chemical fingerprints consistent with biological carbon fixation. Microfossils from the Apex Chert in Australia, approximately 3.46 Ga old, represent potentially the earliest cellular fossils, though their biological origin remains debated. The first cells were almost certainly prokaryotic, lacking a membrane-bound nucleus. They were anaerobic, since no free oxygen was yet available, and likely derived their energy from chemical sources -- either as chemolithoautotrophs using inorganic electron donors or as chemoorganoheterotrophs feeding on available organic molecules. Early metabolic evolution proceeded through several transformative stages. Anaerobic chemotrophy, using electron donors like H2, H2S, and Fe2+, likely came first. Photosynthesis then evolved in two major steps: anoxygenic photosynthesis appeared first, using H2S as an electron donor and producing sulfur rather than oxygen (as seen today in purple sulfur bacteria), followed by oxygenic photosynthesis in cyanobacteria around 2.7-2.4 Ga, which used water as an electron donor and released O2 as a byproduct. The accumulation of oxygen from cyanobacterial photosynthesis triggered the Great Oxidation Event approximately 2.4 Ga. This was a catastrophe for obligate anaerobes, which were poisoned by the reactive gas, but it opened the door to aerobic respiration -- a far more efficient means of ATP production. The rising oxygen levels also led to the formation of the ozone layer (O3), which began shielding Earth's surface from the ultraviolet radiation that had bathed it for billions of years. A cross-section diagram comparing anoxygenic and oxygenic photosynthesis. Panel A: Anoxygenic photosynthesis in purple sulfur bacteria — a single photosystem using H2S as electron donor, producing sulfur (S) as byproduct, no O2 released. Panel B: Oxygenic photosynthesis in cyanobacteria — two photosystems (PSI and PSII) using H2O as electron donor, releasing O2 as byproduct. Arrows show electron flow, light energy input, and ATP/NADPH production in both panels. ### VI. Endosymbiotic Theory and the Origin of Eukaryotes Eukaryotic cells appeared in the fossil record between approximately 1.8 and 2.1 Ga, representing a dramatic leap in cellular complexity. The endosymbiotic theory, championed by Lynn Margulis in 1967, provides the most widely accepted explanation for how this complexity arose. According to this theory, mitochondria evolved from an aerobic alpha-proteobacterium that was engulfed by an ancestral archaeal host cell, while chloroplasts evolved from a cyanobacterium engulfed by a eukaryotic cell that already possessed mitochondria. In both cases, the engulfed prokaryotes were not digested but instead became permanent endosymbionts, eventually losing their independence while retaining essential metabolic functions. The evidence supporting endosymbiosis is extensive and mutually reinforcing. Mitochondria and chloroplasts both possess double membranes, with the inner membrane corresponding to the original prokaryotic membrane. Both organelles retain their own circular DNA genomes, which phylogenetic analysis places closest to alpha-proteobacteria (for mitochondria) and cyanobacteria (for chloroplasts). Both have their own 70S ribosomes of the bacterial type, rather than the 80S ribosomes found in the eukaryotic cytoplasm, and both replicate by binary fission independently of the host cell cycle. Their size is comparable to free-living bacteria, and their translation machinery is inhibited by bacterial antibiotics such as chloramphenicol -- a sensitivity that would make no sense unless these organelles descended from bacteria. Over evolutionary time, many genes have migrated from the endosymbiont genomes to the host nuclear genome, a process called endosymbiotic gene transfer. The proteins encoded by these relocated genes are synthesized in the cytoplasm and then imported back into the organelle via transit peptide sequences. Beyond endosymbiosis, the evolution of eukaryotic cells involved additional innovations: the endomembrane system (including the endoplasmic reticulum, Golgi apparatus, and lysosomes), the nuclear envelope (possibly derived from invagination of the plasma membrane), and the cytoskeleton, which enabled phagocytosis, intracellular transport, and the complex cell division machinery that characterizes eukaryotic life. A stepwise diagram illustrating the endosymbiotic origin of mitochondria and chloroplasts. Step 1: An ancestral archaeal-like host cell with infoldings of the plasma membrane (forming a primitive endomembrane system and nuclear envelope). Step 2: Engulfment of an aerobic alpha-proteobacterium — shown entering the host via phagocytosis, becoming a mitochondrion with a double membrane. Step 3: In a separate lineage, a mitochondria-containing eukaryote engulfs a photosynthetic cyanobacterium, which becomes a chloroplast. Arrows indicate gene transfer from endosymbiont to host nucleus. Final panel shows a modern plant cell with both organelles labeled. ---