Radiometric Dating and Fossil Analysis

Lecture 2: Organisms in their Environment

• Lecture date: January 10, 2025
• Course: BES 108D
• Instructor: Dr. Benazir Alam
• Topic 1: Prokaryotes: Bacteria and Archaea, Chapter 25: The History of Life on Earth
• Chemical and physical processes on early Earth led to simple cells through stages:
  • Abiotic synthesis of small organic molecules (e.g., amino acids, nitrogenous bases)
  • Joining of small molecules into macromolecules (e.g., proteins, nucleic acids)
  • Packaging of molecules into protocells with membranes, maintaining internal chemistry different from surroundings
  • Origin of self-replicating molecules enabling inheritance

Synthesis of Organic Compounds on Early Earth

• About 4.6 billion years ago, solar system formed from a rotating disk of gas, rocks, and dust.

• For the first few hundred million years, Earth was bombarded by asteroids/comets, releasing significant heat and preventing seas from forming until 4.2 to 3.9 billion years ago.

• This bombardment ended 4 billion years ago, setting the stage for life's origin.

• Earth's initial atmosphere had little oxygen; it contained: Water vapour, Volcanic chemicals (nitrogen, nitrogen oxides, carbon dioxide, methane, ammonia, hydrogen, hydrogen sulfide).

• As Earth cooled, water vapor condensed to form oceans.

• Much of the hydrogen escaped into space due to low molecular weight and high Earth temperatures.

• Hydrogen could have reacted with oxygen to form water.

• This reduced the amount of hydrogen in the atmosphere.

• In 1920s, Oparin and Haldane independently hypothesized that early atmospheres were reducing environments (electron gaining is common in anaerobic conditions).

• Organic compounds may have formed from simpler molecules using energy from lightning and UV radiation.

• Haldane suggested that early oceans were a solution of organic molecules ("primitive soup").

• Miller-Urey experiments in 1953 demonstrated the possibility of abiotic synthesis of organic molecules in a reducing atmosphere

• First organic compounds may have originated near volcanoes or deep-sea vents.

• Miller-Urey-type experiments showed that organic molecules could form in various atmospheres.

Amino Acid Synthesis in Simulated Volcanic Eruptions

• Comparison of 1953 and 2008 reanalysis experiments.
• Showed higher amounts of amino acids in the 2008 experiment.

Organic Compounds Produced Near Deep-Sea Vents

• Some deep sea alkaline vents produce warm, high pH water (9-11, 40-90°C).
• This could have created a suitable environment for abiotic synthesis of organic molecules and the first cells.
• Early oceans were acidic, resulting in a pH gradient between vents and surrounding water.
• Energy for the synthesis of organic compounds could have come from this pH gradient.

Abiotic Synthesis of Macromolecules

• Amino acids and other key organic molecules (lipids, simple sugars, nitrogenous bases) have been found in meteorites.
• RNA monomers (adenine, guanine, cytosine, and uracil) can also form spontaneously from simpler molecules.
• Scientists have produced polymers of these molecules by dripping solutions of amino acids or RNA nucleotides onto heated surfaces (e.g., sand, clay, rock), without the help of enzymes or ribosomes.
• Minerals in clay or rock can catalyze the polymerization of small molecules into larger chains of proteins or RNA.

Protocells

• A protocell is a theoretical model of an early form of life structure resembling a cell but simpler.
• Protocells are a step in the transition from non-living chemical systems to living organisms.
• Protocells may have been fluid-filled vesicles with a membrane-like structure formed from lipids and other organic molecules.
• Protocells can spontaneously form vesicles in water.
• Adding clay can increase the rate of vesicle formation.
• Vesicles exhibit simple reproduction and metabolism, as well as maintaining an internal chemical environment.

Self-Replicating RNA

• RNA, not DNA, was likely the first genetic material.
• RNA plays a crucial role in protein synthesis and acts as an enzyme-like catalyst (ribozymes).
• Ribozymes can produce complementary copies of short RNA stretches using nucleotide building blocks.
• Natural selection at a molecular level may have favoured stable and fast-replicating ribozyme variations.
• Small RNA molecules could have replicated, storing genetic information about the vesicles that carried them.
• Protocells with replicating RNA would have been primitive cells but with limited genetic information encoding only a few properties.
• RNA could serve as a template for the development of DNA, a more stable and efficiently replicating genetic material.

Transition from RNA to DNA

• Accurate replication was crucial as genomes grew, through duplication and other processes.
• Development of genetic information in protocells led to new life forms.
• The fossil record documents this progression of life.

The Fossil Record

• The fossil record shows changes in life on Earth.
• Sedimentary rocks, deposited in layers (strata), are a primary source of fossils.
• Fossils provide an incomplete picture of evolution; Many organisms did not fossilize, many fossils have been destroyed, and many have not been discovered yet.
• The fossil record is biased towards abundant, widespread species that possessed hard parts.

The Geologic Record

• A record of geological events, illustrating the order (time-line) that life appeared on Earth in the geologic past.
• Provides a timeline for geological/evolutionary events.

How Rocks and Fossils Are Dated

• Sedimentary strata reveal the relative ages of fossils.
• Radiometric dating determines the absolute ages-A radioactive isotope decays to a daughter product at a constant rate.
• Isotopes decay at known and unchanging half-lives independent of external conditions allowing for accurate dating.
• Carbon-14 dating is used to date fossils up to 75,000 years old.
• Older fossils can be dated by examining the surrounding geological layers.

Radiometric Dating

• Living organisms contain carbon-12, and a small amount of a radioactive carbon-14.
• When an organism dies, carbon-14 stops accumulating and decays into nitrogen.
• Measuring the ratio of carbon-14 to carbon-12 reveals the age of the fossil.
• Radiocarbon dating is used to date fossils up to 75,000 years old.
• By analysing sedimentary layers surrounding fossils and volcanic layers, scientists can approximate the absolute age of older fossils.

Oxygen Revolution

• The initial rise of oxygen (O2) in the atmosphere was likely due to oxygenic photosynthetic prokaryotes.
• Later increases in atmospheric O2 could have originated from the development of eukaryotic cells containing chloroplasts.
• This "oxygen revolution" (2.7 - 2.4 billion years ago) caused extinction in many anaerobic prokaryotic groups.
• Survivors adapted to using cellular respiration to harvest energy.