Earth's Early Atmosphere and Evolution of Life - Study Notes

Early Atmosphere and Origin of Organics

Experiments simulated the conditions of Earth's early atmosphere and provided energy to drive reactions.
Outcomes showed production of organic compounds that were more complex than the starting gases and resembled components of modern macromolecules inside organisms.
Another possible source of early organics is delivery by impacts from meteors/asteroids, bringing organics from outside Earth.
A famous meteorite, the Murchison meteorite, fell to Earth and, after analysis, contained amino acids, suggesting extraterrestrial delivery of prebiotic materials.
Amino acids are fundamental building blocks; nature uses 20 different amino acids to form proteins, which are the workhorses of cellular machinery.

Extraterrestrial Delivery of Organics

Meteorite delivery provides amino acids and other organics, supporting the idea that Earth’s organics could have extraterrestrial origins.
The transcript notes amino acids observed in the meteorite and links to laboratory experiments that also yield amino acids from simple starting materials.

Early Life: Fossil Evidence and Timeline

About a billion years after Earth formed, fossil evidence appears for very simple, worm-like single-celled organisms.
Fossils indicate cells and simple multicellular-like forms preserved in ancient rocks, estimated to be around
ext{age} \sim 3.8 \times 10^9 \text{ years ago}.
Modern organisms retain some resemblance to these ancient fossils, e.g., cyanobacteria show similar forms when viewed under a microscope.
Fossil captures of cell division illustrate early cellular life.

Absence of Oxygen in Earth's Early Atmosphere

For roughly the first $\sim 1.5 \times 10^9$ years, the atmosphere lacked oxygen.
This absence of oxygen is significant because it shaped the metabolic strategies available to early life.
Oxygen is essential for efficient energy production in many organisms; lack of oxygen meant primitive life relied on anaerobic processes.

Energy and Oxygen: Why Oxygen Matters Today

In bacteria, glucose metabolism without oxygen yields about 2 \text{ ATP} (anaerobic respiration or glycolysis).
In animals (and other aerobic organisms), glucose in the presence of oxygen yields about 32 \text{ ATP}, enabling higher energy demands.
The increased ATP production supports greater biological complexity, including brain function, since the brain uses roughly 20\% of total calories consumed.
The appearance of oxygen in the atmosphere enabled much more energy-efficient metabolism and the evolution of more complex life.

Oxidation and Evidence for Oxygen

Oxidation of metals like iron serves as evidence for the rise of oxygenic conditions, as oxidized iron (rust) forms in the presence of oxygen.
The appearance of photosynthesis is tied to the release of oxygen as a byproduct.
Photosynthesis begins by splitting water to remove hydrogens and electrons; the remaining oxygen becomes a waste product for photosynthetic organisms.
Cyanobacteria are photosynthetic; their activity introduced oxygen into the atmosphere, enabling later aerobic life.

Phylogenetic Tree and Last Universal Common Ancestor (LUCA)

A phylogenetic tree distinguishes prokaryotes (bacteria and archaea) from eukaryotes.
All life shares a Last Universal Common Ancestor (LUCA) from which both prokaryotic and eukaryotic lineages diverged.
A shared feature across life is the use of DNA as the genetic material and the same basic code translating DNA into proteins.
The central dogma: DNA (genes) -> RNA -> protein, via transcription and translation.
The genetic code is read in triplets (codons). Each codon (three nucleotides) specifies one amino acid.
Example: a codon like ATC would designate a particular amino acid; GTA would designate a different amino acid. The code is shared between bacteria and humans, so a gene from a human can be expressed in bacteria to make a functional protein.

Central Dogma and Universality of the Genetic Code

DNA contains genes (the code red in the transcript) and is transcribed to RNA, which is translated by ribosomes into proteins.
The three-nucleotide codons specify amino acids, and there are 20 amino acids used in nature.
The genetic code is universal across life, supporting the idea of a common origin.

Prokaryotes vs Eukaryotes: Cellular Organization

Prokaryotes (bacteria/archaea) are simpler and typically lack a nucleus and membrane-bound organelles.
Eukaryotes have a nucleus enclosed by a membrane and numerous membrane-bound organelles, enabling compartmentalization of functions.
Both have ribosomes and a plasma membrane, but bacteria also have a cell wall; eukaryotic cells generally do not.
Yeast (a eukaryote) is a single-celled organism more similar internally to animal/plant cells than bacteria are.
Eukaryotic cells are typically about 10× larger than prokaryotic cells due to internal membrane-bound organelles.

Endosymbiotic Theory: Mitochondria and Chloroplasts

Mitochondria and chloroplasts share several features with bacteria: double membranes, own ribosomes, circular DNA, and the ability to replicate independently.
Endosymbiotic theory posits that mitochondria originated from free-living bacteria that were engulfed by a host cell and remained, providing energy advantages via aerobic respiration.
Chloroplasts originated from another engulfed photosynthetic bacterium, enabling plant cells to perform photosynthesis.
The evidence supports that mitochondria and chloroplasts were once independent prokaryotic cells that entered into a symbiotic relationship with early eukaryotic cells.

Energy Transformation in Cells: Mitochondria and Photosynthesis

Mitochondria: energy-transforming organelles that use oxygen to convert glucose into ATP via cellular respiration.
In bacteria, glucose yields about 2 \text{ ATP} per molecule of glucose in anaerobic pathways.
In eukaryotes, mitochondrial respiration yields significantly more ATP by fully oxidizing glucose in the presence of oxygen.
Chloroplasts: energy-transforming organelles in plants that perform photosynthesis, converting light energy and inorganic carbon (CO2) into chemical energy and sugars.
The combination of mitochondria and chloroplasts underpins the energy economy of most life on Earth.

Lysosome: Internal Membrane-Bound Organelle and Protein Recycling

Lysosomes are membrane-bound organelles containing enzymes that degrade misfolded, damaged, or unneeded proteins and other macromolecules.
Proteins degraded in lysosomes are broken down into amino acids, which can be reused to synthesize new proteins.
Localization of degradative enzymes within lysosomes prevents nonspecific digestion of cellular components.
Without compartmentalization, degradative enzymes would damage the entire cell.

Internal Complexity of Eukaryotic Cells

Eukaryotic cells contain many compartments (organelles) inside a membrane network, enabling specialized functions to take place in defined locations.
Key organelles include the nucleus, endoplasmic reticulum, mitochondria, chloroplasts (in plants), lysosomes, Golgi apparatus, and more.
The concept of compartmentalization explains how higher organisms can sustain greater complexity and energy demands.

Scale: Atoms, Molecules, and Life Building Blocks

Life is built from a small set of elements; about 96\% \text{ to } 97\% of the body weight is made up of four elements: \text{O}, \text{C}, \text{H}, \text{N} (oxygen, carbon, hydrogen, nitrogen).
Trace elements are required in smaller amounts but are essential for biological processes.
Chemistry of life begins with atoms and simple compounds that assemble into larger molecules and eventually into cells.
The periodic table provides the basis for building complex biomolecules from a limited set of elements.

Atoms, Subatomic Particles, and Isotopes

An atom consists of a nucleus containing protons (positive charge, mass ~1) and neutrons (neutral, mass ~1), surrounded by electrons (negative charge, negligible mass) in orbit or probability clouds.
Atomic number (the number of protons) determines the element.
Atomic mass equals the total number of protons and neutrons.
Isotopes are atoms of the same element with different numbers of neutrons (e.g., Carbon-12, Carbon-13, Carbon-14).
Example: Helium nucleus contains 2\text{ protons} and 2\text{ neutrons} (mass number 4).
Electron mass is tiny compared to protons and neutrons, and electrons are responsible for chemical bonding.

Isotopes and Dating: Carbon-14 and Carbon Dating

Carbon-14 is a radioactive isotope with a half-life of t_{1/2} = 5700\ \text{years}.
Carbon dating uses the decay of ^{14}\text{C} relative to the stable ^{12}\text{C} to estimate the age of once-living materials.
The ratio of ^{12}\text{C} to ^{14}\text{C} decreases over time as ^{14}\text{C} decays, allowing age estimation of organic remains.
The dating method is based on measuring the remaining ^{14}\text{C} and comparing it to the initial ratio when the material formed.
Laboratory techniques distinguish isotopes by mass differences, enabling reliable separation and analysis (e.g., in carbon dating).

Medical and Diagnostic Uses of Isotopes: PET Scans

Radioactive isotopes can be incorporated into biologically active molecules for medical diagnostics.
A common application is using a radioactive form of glucose, which accumulates in tumors due to their high glucose uptake.
Imaging methods like PET scans detect the radiation from the decay of the radioactive glucose to visualize tumor locations.

Quick Recap: Linking Concepts Across the Lecture

Early Earth had no oxygen; oxygenation began with photosynthesis, enabling aerobic respiration and greater energy production.
Energy availability underpins biological complexity, including brain function, and drives evolutionary innovations.
The genetic code is universal, supporting a common origin of life; DNA is transcribed to RNA and translated into proteins at ribosomes.
Prokaryotes and eukaryotes share core molecular biology, but eukaryotes gained complexity through internal membranes and organelles.
Endosymbiotic theory explains how mitochondria and chloroplasts originated as free-living bacteria that became integrated into eukaryotic cells.
The scale of life spans from atoms to molecules to cells; a small set of elements builds all biomolecules, with oxygen, carbon, hydrogen, and nitrogen comprising the bulk of body weight.
Isotopes provide tools for dating (carbon dating) and for diagnostics ( PET imaging with radiolabeled substrates).

Break and Course Schedule Note

A short break was announced in the lecture; the next session continues with further topics.