Source: Lecture notes / textbook readings
Difficulty: Introductory | Prerequisites: Characteristics of Life and Evolution notes
This unit places life in the context of Earth's 4.5-billion-year history. You will learn how scientists determine the ages of rocks and fossils, how Earth's atmosphere changed from reducing to oxidising (and why that matters for life), and how phylogenetic trees map evolutionary relationships. These topics tie together the "when" and "how" of evolution. If you are comfortable with natural selection and the characteristics of life, you have the background you need.
Earth is about 4.5 billion years old; life appeared at least 3.5 billion years ago. Scientists date rocks and fossils using relative dating (position in sediment layers) and absolute dating (radiometric methods such as carbon-14 and potassium-argon decay). Early Earth had a reducing atmosphere with no free oxygen; cyanobacteria changed that through the Great Oxygenation Event, enabling aerobic life. Phylogenetic trees map the evolutionary relationships among species, rooted at LUCA (the Last Universal Common Ancestor).
Fossil record
The totality of fossils discovered and the information they provide about the history of life on Earth. Think of it as life's incomplete but revealing archive.
Geologic time scale
The system used to divide Earth's history into eons, eras, periods and epochs, based on major geological and biological events.
Relative dating
A method for estimating whether a fossil or rock layer is older or younger than another, based on its position in the sediment. Deeper layers are generally older. It does not give a numerical age.
Absolute dating
A method that provides a numerical age (in years) for a rock or fossil, typically using radiometric techniques.
Radiometric dating
A technique that measures the decay of radioactive isotopes in a sample to calculate its age. In simple terms, it uses a natural clock built into certain atoms.
Half-life
The time it takes for half of a radioactive isotope in a sample to decay into its daughter product. Each isotope has a fixed, known half-life.
Carbon-14 (C-14) dating
A radiometric method that measures the ratio of radioactive C-14 to stable C-12 in organic material. Effective for samples up to about 50,000 years old.
Potassium-argon (K-Ar) dating
A radiometric method that measures the decay of potassium-40 to argon-40 (half-life: 1.25 billion years). Used for dating ancient rocks and, by extension, the fossils found in or near them.
Great Oxygenation Event (GOE)
The period (roughly 2.4 billion years ago) when cyanobacteria produced enough oxygen through photosynthesis to transform Earth's atmosphere from reducing to oxidising.
Reducing atmosphere
An atmosphere with little or no free oxygen, rich in gases such as methane, ammonia and hydrogen. Earth's early atmosphere was reducing.
Oxidising atmosphere
An atmosphere with significant free oxygen, which enables aerobic respiration. Earth's current atmosphere.
LUCA (Last Universal Common Ancestor)
The hypothetical organism from which all current life on Earth descends. It sits at the root of every phylogenetic tree.
Phylogenetic tree
A branching diagram showing inferred evolutionary relationships among species. Nodes represent common ancestors; branches represent lineages diverging over time.
Cyanobacteria
Photosynthetic bacteria that were among the first organisms to produce oxygen, triggering the Great Oxygenation Event.
Earth formed approximately 4.5 billion years ago. Its early atmosphere was reducing: rich in methane, ammonia, water vapour and hydrogen, with essentially no free oxygen. These conditions favoured the spontaneous formation of complex organic molecules, the chemical precursors to life.
The earliest evidence of life dates to around 3.5 billion years ago (some estimates push this to 4.1 billion years). The first organisms were anaerobic, single-celled prokaryotes that did not require oxygen.
Cyanobacteria evolved the ability to perform oxygenic photosynthesis, using sunlight to split water molecules and releasing oxygen as a byproduct. Over hundreds of millions of years, this oxygen accumulated in the atmosphere.
The Great Oxygenation Event (roughly 2.4 billion years ago) transformed Earth's atmosphere from reducing to oxidising. This had two major consequences:
It was catastrophic for many anaerobic organisms, to which oxygen was toxic.
It enabled the evolution of aerobic respiration, a far more efficient way of extracting energy, and ultimately paved the way for complex multicellular life.
Fossils are the preserved remains or traces of organisms from the past. They provide direct evidence of how life has changed over geological time.
The fossil record is incomplete. This is because fossilisation requires specific conditions:
Hard parts (bones, shells, teeth) preserve much more readily than soft tissue.
Wet, low-oxygen environments (swamps, riverbeds, ocean floors) slow decay.
Rapid burial in sediment protects remains from scavengers and decomposition.
Soft-bodied organisms (jellyfish, worms) are vastly underrepresented in the fossil record. Despite these gaps, the record clearly shows a progression from simple single-celled life to complex multicellular forms.
Fossils are classified and organised using the geologic time scale, which divides Earth's history into eons, eras, periods and epochs based on major geological and biological events.
Relative dating tells you whether a fossil or rock layer is older or younger than another, based on its position in the sediment. The principle of superposition: in undisturbed layers, deeper = older. Relative dating does not assign a numerical age.
Absolute dating gives a numerical age in years. The primary technique is radiometric dating.
Radiometric dating exploits the fact that radioactive isotopes decay at known, constant rates. By measuring the ratio of parent isotope to daughter product in a sample, scientists calculate how long the decay has been running.
The key concept is the half-life: the time for half the parent isotope to decay. After one half-life, 50% remains; after two, 25%; after three, 12.5%, and so on.
Two commonly tested isotopes:
Carbon-14. Half-life: approximately 5,730 years. Used for dating organic material (wood, bone, shell) up to about 50,000 years old. It works by comparing the ratio of C-14 to stable C-12: in a living organism, the ratio is constant; after death, C-14 decays and the ratio drops.
Potassium-40. Half-life: 1.25 billion years. Decays to argon-40. Used for dating ancient volcanic rocks, and by association the fossils found near them. Essential for dating events millions or billions of years in the past.
Radiometric dating is validated through corroborative evidence: cross-checking with other dating methods, known ages of historical artefacts, and consistency across independent labs.
Age of the universe: approximately 13.7 billion years
Age of the solar system: approximately 4.55 billion years
Age of Earth: approximately 4.5 billion years
Earliest evidence of life: approximately 3.5 billion years ago (possibly 4.1 billion)
Great Oxygenation Event: approximately 2.4 billion years ago
Phylogenetic trees are branching diagrams that represent the evolutionary relationships among species. Key components:
Root (LUCA). The base of the tree represents the Last Universal Common Ancestor of all current life.
Nodes. Each branching point represents a common ancestor where a lineage diverged into two or more descendant lines.
Branches. Lines connecting nodes, representing lineages evolving over time.
Time markers. Some trees include a time axis showing when divergence events occurred.
Shared traits. Features common to groups of species are mapped onto the tree to show where those traits evolved.
Phylogenetic trees are built from multiple data sources: morphological comparisons, DNA and protein sequences, and the fossil record. More closely related species share a more recent common ancestor (a nearer node on the tree).
Carbon-14 dating is used routinely in archaeology to date wooden structures, textiles and human remains up to about 50,000 years old. Potassium-argon dating is how geologists date volcanic eruptions and, by extension, the sedimentary layers and fossils sandwiched between lava flows. Together, these techniques underpin our understanding of human prehistory, mass extinction timelines, and climate history.
Students often think carbon-14 dating can date anything, including dinosaur bones. It cannot. C-14 is only effective up to about 50,000 years. Dinosaurs went extinct 66 million years ago; their fossils require methods like potassium-argon dating.
Students sometimes assume the fossil record should be complete. It is inherently incomplete because fossilisation requires rare, specific conditions.
Students may confuse relative and absolute dating. Relative dating tells you order (older vs younger), not a number. Absolute dating gives a numerical age.
Students sometimes think Earth's atmosphere has always contained oxygen. It has not. Free oxygen appeared in significant quantities only after the Great Oxygenation Event, roughly 2.4 billion years ago.
⚠️ Be able to explain the difference between relative and absolute dating, with an example of each.
⚠️ Know the half-life concept and be ready to calculate how much parent isotope remains after a given number of half-lives.
⚠️ Know the effective range of carbon-14 dating (up to ~50,000 years) and when to use potassium-argon instead.
⚠️ Be ready to describe the Great Oxygenation Event: what caused it (cyanobacteria), what changed (atmosphere became oxidising), and why it mattered (enabled aerobic life).
⚠️ Know the key numbers: age of universe (~13.7 billion years), age of Earth (~4.5 billion years), earliest life (~3.5 billion years).
⚠️ Be able to read a simple phylogenetic tree: identify the root, nodes, branches, and which species are most closely related.
True or false: Carbon-14 dating can be used to date a 200-million-year-old fossil. (False. C-14 is effective only up to about 50,000 years.)
Fill in the blank: After three half-lives, __________ % of the original parent isotope remains. (12.5%.)
True or false: Earth's atmosphere has always contained significant free oxygen. (False. Free oxygen accumulated after the Great Oxygenation Event.)
Fill in the blank: The base of a phylogenetic tree represents the __________. (Last Universal Common Ancestor, LUCA.)
True or false: Relative dating gives a numerical age in years. (False. It gives only the order, older vs younger.)
Q: Explain the difference between relative dating and absolute dating.
A: Relative dating determines whether a fossil or rock layer is older or younger than another based on its position in sedimentary layers (deeper = older), but does not assign a numerical age. Absolute dating uses radiometric methods to determine a specific age in years.
Q: A sample of wood contains 25% of its original carbon-14. Approximately how old is it?
A: After one half-life (5,730 years), 50% remains. After two half-lives, 25% remains. So the sample is approximately 11,460 years old (2 x 5,730).
Q: Why is carbon-14 dating unsuitable for a fossil that is 100 million years old?
A: Carbon-14 has a half-life of about 5,730 years. After roughly 50,000 years, so little C-14 remains that it is undetectable. For a 100-million-year-old fossil, a method with a much longer half-life is needed, such as potassium-argon dating (half-life: 1.25 billion years).
Q: Describe the Great Oxygenation Event and explain its significance for the evolution of life.
A: Cyanobacteria evolved oxygenic photosynthesis, releasing oxygen as a byproduct. Over time, oxygen accumulated in the atmosphere, transforming it from reducing to oxidising. This was toxic to many anaerobic organisms but enabled the evolution of aerobic respiration, which is far more energy-efficient. Aerobic metabolism, in turn, supported the development of larger, more complex multicellular organisms.
Q: On a phylogenetic tree, how do you determine which two species are most closely related?
A: The two species that share the most recent common ancestor (the nearest node connecting them, with no other branching in between) are the most closely related.
This material connects to the evolution and natural selection unit: the fossil record and radiometric dating provide the physical evidence and timeline that support evolutionary theory. It also connects to the characteristics of life (metabolism, reproduction, adaptation), since the transition from a reducing to an oxidising atmosphere changed which metabolic strategies were viable. Phylogenetic trees will reappear in any unit on taxonomy, biodiversity or comparative genomics.
fossil record, geologic time scale, relative dating, absolute dating, radiometric dating, half-life, carbon-14 dating, C-14, potassium-argon dating, K-Ar dating, radioactive decay, parent isotope, daughter isotope, Great Oxygenation Event, GOE, cyanobacteria, photosynthesis, reducing atmosphere, oxidising atmosphere, aerobic respiration, anaerobic organisms, phylogenetic tree, cladogram, LUCA, Last Universal Common Ancestor, nodes, branches, common ancestor, age of Earth, age of universe, origin of life, fossilisation, sedimentary layers, superposition, BIO 101, intro to biology