Difficulty: Introductory | Prerequisites: The companion notes on Biological Evolution and Natural Selection.
This material picks up where the evolution notes left off. Knowing that life evolves is one thing; knowing when it started, how scientists date events in deep time, and how to read the "family tree" of all life are the practical tools that make evolutionary biology usable. These topics come up in every biology course from introductory level through to palaeontology and molecular systematics.
Early Earth was hot, oxygen-free, and bombarded by radiation, yet life appeared within the first billion years. Scientists date fossils and rocks using relative methods (stratigraphy, superposition) and absolute methods (radiometric dating with isotope half-lives). Phylogenetic trees map evolutionary relationships, with LUCA at the root and branching points representing common ancestors.
Radiometric dating
A method of determining the absolute age of rocks, minerals, or fossils by measuring the ratio of a radioactive parent isotope to its stable daughter product.
Think of it as reading a built-in stopwatch inside the rock: the radioactive atoms tick away at a known rate, and the ratio of what remains tells you how long the clock has been running.
Half-life
The time required for exactly half of a sample of radioactive isotope to decay into its daughter product. Each isotope has a fixed, known half-life (e.g. Carbon-14: ~5,730 years; Uranium-238: ~4.5 billion years).
Parent isotope
The original, unstable radioactive isotope that undergoes decay.
Daughter isotope
The stable product formed when a parent isotope decays.
Relative dating
Methods that determine the order of events (which is older, which is younger) without assigning a specific numerical age.
In simple terms, relative dating tells you "this fossil is older than that one" but not "this fossil is 350 million years old."
Absolute dating
Methods that provide a specific numerical age or age range for a sample, typically in years. Radiometric dating is the most common absolute dating technique.
Fossil
The preserved remains or traces of an organism that lived in the past. Fossils may be actual body parts (bones, teeth, shells) or trace evidence (footprints, burrows, coprolites).
Stratigraphy
The study of rock layers (strata) and their relative positions. Under the principle of superposition, deeper layers are older than shallower ones, assuming the layers have not been disturbed.
Phylogenetic tree (phylogeny)
A branching diagram that depicts the evolutionary relationships among organisms or groups based on shared characteristics and common ancestry.
Node
A branching point on a phylogenetic tree. Each node represents the most recent common ancestor of the groups that branch from it.
LUCA (Last Universal Common Ancestor)
The most recent organism from which all currently living organisms descend. Placed at the root of the tree of life. Likely a single-celled prokaryote that lived roughly 3.5 to 4.1 billion years ago.
Clade
A group consisting of an ancestor and all of its descendants. A valid clade includes everything that branches from a single node on the tree.
Earth formed approximately 4.5 billion years ago. The earliest conditions were radically different from today.
Atmosphere. The early atmosphere contained little to no free oxygen. It was rich in water vapour, carbon dioxide, nitrogen, and possibly methane and ammonia. The ozone layer did not yet exist, so ultraviolet radiation reached the surface largely unfiltered.
Temperature and surface. The young Earth was extremely hot, with widespread volcanic activity. The surface was bombarded by meteorites during the Late Heavy Bombardment (~4.1 to 3.8 billion years ago).
Water. As the planet cooled, water vapour condensed and oceans formed, providing the medium in which the chemistry of life could take hold.
The oldest widely accepted evidence of life comes from fossilised stromatolites (layered structures formed by microbial communities) dating to about 3.5 billion years ago. More recent and still-debated evidence suggests life may have been present as early as 4.1 billion years ago, based on carbon isotope signatures in ancient zircon crystals.
The prevailing scientific framework (though details remain debated) involves several stages:
Simple organic molecules (amino acids, nucleotides) formed from inorganic precursors, driven by energy from lightning, UV radiation, or hydrothermal vents. The Miller-Urey experiment (1953) demonstrated this was chemically plausible.
These molecules polymerised into more complex structures (proteins, nucleic acids).
Self-replicating molecules emerged. The RNA world hypothesis proposes that RNA served as both genetic material and catalyst before DNA and proteins took over those roles.
Protocells, simple membrane-bound compartments containing self-replicating molecules, formed the precursors to true cells.
All known life descends from LUCA, the Last Universal Common Ancestor, a prokaryotic organism that already possessed DNA, RNA, ribosomes, and a basic metabolic toolkit.
A phylogenetic tree is the standard tool biologists use to represent evolutionary relationships. Learning to read one correctly is a core skill in introductory biology.
Root. The base of the tree, representing the most ancient common ancestor of all organisms shown. For the tree of life as a whole, this is LUCA.
Branches. Lines connecting nodes to tips. The length of a branch may represent time or the amount of evolutionary change, depending on the tree.
Nodes (internal branching points). Each node represents the most recent common ancestor shared by everything that descends from it. A node is where one lineage split into two.
Tips (terminal nodes). The endpoints of the tree, representing the organisms or groups being compared (usually living species, but sometimes extinct ones).
Time markers. Some trees include a time axis (often along the bottom or left side), showing when divergence events occurred.
Closeness of relationship is determined by how recently two groups share a common ancestor (their nearest node), not by how close together they appear on the page. Two groups on opposite sides of the tree may still be sister taxa if they share the most recent node.
Rotating branches around a node does not change the meaning of the tree. The branching pattern (topology) is what matters, not the order of tips from left to right.
A clade is any group that includes a common ancestor and all of its descendants. If you can "cut" one branch and remove an entire group, that group is a clade.
The tree of life at its broadest level divides into three domains, all descending from LUCA:
Bacteria. Single-celled prokaryotes. The most abundant and diverse domain.
Archaea. Single-celled prokaryotes that superficially resemble bacteria but are genetically and biochemically distinct. Many thrive in extreme environments (hot springs, salt lakes), though they are found everywhere.
Eukarya. Organisms whose cells contain a membrane-bound nucleus. Includes protists, fungi, plants, and animals.
Relative dating establishes the order of events without assigning numerical ages.
Law of superposition. In undisturbed layers of sedimentary rock, the oldest layer is at the bottom and the youngest is at the top.
Law of original horizontality. Sedimentary layers are originally deposited roughly horizontally. Tilted or folded layers indicate later geological forces.
Law of cross-cutting relationships. A geological feature (a fault, an intrusion) that cuts through existing rock must be younger than the rock it cuts.
Index fossils. Fossils of organisms that were widespread, abundant, and existed for a short geological time span. Finding an index fossil in a rock layer places that layer within a known time range.
Relative dating is quick and useful for field work, but it gives you a sequence, not a number.
Radiometric dating assigns a numerical age to a sample by measuring the decay of radioactive isotopes.
How it works:
Radioactive parent isotopes decay into stable daughter isotopes at a fixed, known rate (the half-life). By measuring the ratio of parent to daughter isotopes in a sample, scientists calculate how many half-lives have passed and therefore how old the sample is.
The half-life formula:
After n half-lives, the fraction of parent isotope remaining = (1/2)^n
Worked example: A sample originally contained 100 g of Carbon-14. It now contains 25 g. How many half-lives have passed, and how old is the sample?
100 g to 50 g = 1 half-life
50 g to 25 g = 2 half-lives
Carbon-14 half-life = 5,730 years
Age = 2 x 5,730 = 11,460 years
Common isotope systems:
Carbon-14 (half-life ~5,730 years). Used for organic material up to about 50,000 years old. Useful for archaeology and recent palaeontology.
Potassium-40 / Argon-40 (half-life ~1.25 billion years). Used for volcanic rocks and minerals. Good for dating very old geological formations.
Uranium-238 / Lead-206 (half-life ~4.5 billion years). Used for the oldest rocks on Earth and meteorites. This is how the age of the Earth itself was determined.
Limitations:
Radiometric dating works on igneous and metamorphic rocks (which contain radioactive minerals), not directly on sedimentary rocks or fossils. Fossils are dated by bracketing them between datable rock layers above and below.
Carbon-14 dating only works on material that was once alive and is limited to roughly the last 50,000 years. Anything older requires a longer-lived isotope system.
Radiometric dating is how we know the Earth is approximately 4.5 billion years old, a figure derived from uranium-lead dating of meteorites and the oldest terrestrial minerals. This same technique dates geological events like mass extinctions, mountain-building episodes, and the appearance of major life forms in the fossil record.
Carbon-14 dating is the workhorse of archaeology. It has been used to date everything from the Dead Sea Scrolls to Otzi the Iceman to the earliest evidence of agriculture. Understanding its limitations (only organic material, only the last ~50,000 years) is essential for interpreting archaeological claims.
Students often think Carbon-14 dating can be used on dinosaur fossils. It cannot. Carbon-14 is only usable to about 50,000 years; dinosaurs went extinct 66 million years ago. Dinosaur-bearing rocks are dated using longer-lived isotope systems like potassium-argon or uranium-lead.
"Relative dating" does not mean "approximate." It means establishing a sequence (older/younger) without assigning a number. Relative dates can be very precise about order.
On a phylogenetic tree, students frequently assume that the species at the tips evolved "from" each other (e.g. humans evolved from chimpanzees). They did not. The tips are modern species; they share a common ancestor at the node where their branches diverge.
Students sometimes think that the amount of horizontal space between two tips on a tree indicates how closely related they are. It does not. Relationship is determined by the recency of the shared node, not the position on the page.
⚠️ Half-life calculations are a near-certainty on the exam. Be comfortable working through at least two or three half-lives in either direction (given a starting amount, find the remaining amount, or vice versa).
⚠️ Know which dating technique to use for which situation. Carbon-14 for recent organic material; potassium-argon or uranium-lead for ancient rocks.
⚠️ Be able to read a phylogenetic tree and identify the most recent common ancestor of any two groups. Practice rotating branches around nodes to confirm you understand topology.
⚠️ Know the conditions of early Earth (no free oxygen, intense UV, volcanic activity) and be able to explain why those conditions are relevant to the origin of life.
⚠️ Distinguish between relative and absolute dating clearly. Exam questions often present a scenario and ask which method is appropriate.
True or False: Carbon-14 dating can determine the age of a 200-million-year-old dinosaur bone.
Answer: False. Carbon-14 is only effective to about 50,000 years.
Fill in the blank: In undisturbed sedimentary rock, the oldest layers are at the ________.
Answer: Bottom (law of superposition).
True or False: On a phylogenetic tree, the tips represent ancestral species.
Answer: False. The tips represent the organisms being compared (usually modern species). Ancestral species are at the nodes.
Fill in the blank: After three half-lives, the fraction of original parent isotope remaining is ________.
Answer: 1/8 (one-eighth). Each half-life halves what remains: 1/2, then 1/4, then 1/8.
True or False: The early Earth's atmosphere was rich in free oxygen.
Answer: False. The early atmosphere had little to no free oxygen.
Q: A rock sample contains 12.5% of its original Potassium-40. The half-life of Potassium-40 is 1.25 billion years. How old is the rock?
A: 12.5% = 1/8 of the original amount. 1/8 = (1/2)^3, so three half-lives have passed. Age = 3 x 1.25 billion = 3.75 billion years.
Q: You find a fossil in a sedimentary layer between two layers of volcanic ash. The lower ash layer is dated to 300 million years ago and the upper ash layer to 280 million years ago. What can you say about the age of the fossil?
A: The fossil is between 280 and 300 million years old. You cannot give a more precise age because radiometric dating was applied to the volcanic ash (igneous material), not the fossil itself. The sedimentary layer, and therefore the fossil, is bracketed between the two dates.
Q: On a phylogenetic tree, Species A and Species B share a node. Species C branches off earlier. Which two species are most closely related, and why?
A: Species A and Species B are most closely related because they share the most recent common ancestor (the node where they diverge). Species C split from the lineage earlier, so it is more distantly related to both A and B.
Q: Why can relative dating and absolute dating be described as complementary rather than competing methods?
A: Relative dating establishes the sequence (which is older, which is younger), while absolute dating provides specific numerical ages. Together, they build a detailed chronology. For example, stratigraphy shows the order of rock layers, and radiometric dating of volcanic layers within the sequence assigns ages to key points.
Q: Explain why the early Earth's lack of free oxygen was important for the origin of life.
A: Oxygen is highly reactive and would have destroyed the fragile organic molecules forming in the primordial environment. An oxygen-free (reducing) atmosphere allowed amino acids, nucleotides, and other building blocks to accumulate and interact without being oxidised. Life itself eventually produced the oxygen in our atmosphere through photosynthesis.
This material connects to the evolution and natural selection notes: the phylogenetic tree is the visual framework for understanding how natural selection has produced the diversity of life over billions of years. It also connects forward to ecology and biodiversity (why do different ecosystems contain the organisms they do?) and to molecular biology (DNA sequence data is now the primary basis for constructing phylogenetic trees).
The dating techniques covered here are foundational for geology and earth science courses as well as biology.
Radiometric dating, half-life, Carbon-14 dating, radiocarbon dating, potassium-argon dating, uranium-lead dating, parent isotope, daughter isotope, absolute dating, relative dating, law of superposition, stratigraphy, index fossils, phylogenetic tree, phylogeny, cladogram, clade, node, LUCA, last universal common ancestor, tree of life, three domains, Bacteria, Archaea, Eukarya, early Earth, origin of life, Miller-Urey experiment, RNA world hypothesis, protocells, abiogenesis, fossil record, deep time, intro to biology, Penn State BIO