IB Biology HLThe Origin of CellsPaper 1 & 2~11 min read
Evidence for the Evolution of Life
Every living thing uses the same genetic code, the same twenty amino acids and the same basic biochemistry. The simplest explanation is that everything alive today is descended from one organism — and we can even estimate when it lived.
📘 What you need to know
Species that evolved from a common ancestor share similar characteristics, such as the bone structure of the vertebrate forelimb.
Organisms with similar DNA sequences are more closely related.
All life is thought to have evolved from the Last Universal Common Ancestor (LUCA), about 4 billion years ago. LUCA sits at the base of the phylogenetic tree.
Evidence for common ancestry: the same biochemistry, the same DNA bases and genetic code, and the same amino acids in all organisms.
Several genes are shared by eubacteria and archaea, suggesting they were inherited from LUCA.
Carbon dating works for samples up to about 60 000 years; radiometric dating is used for older material.
A molecular clock estimates dates from the number of accumulated mutations, assuming a constant rate of change.
LUCA may have evolved near hydrothermal vents, and may have been an autotrophic extremophile.
The Last Universal Common Ancestor
Evidence suggests different species have evolved from a common ancestor, and that they share some similar characteristics with it.
The classic example is the bone structure of the vertebrate forelimb. It is similar in all vertebrate species — a human arm, a bat wing and a whale flipper are built to the same plan — which indicates they inherited that structure from a common ancestor.
DNA provides a useful tool for working out evolutionary relationships. Organisms with similar DNA sequences are more closely related than those with very different sequences.
Follow that logic all the way back and you arrive at a single organism. All life on Earth is thought to have evolved from an ancient common ancestor believed to have existed about 4 billion years ago, known as the Last Universal Common Ancestor, or LUCA. In a phylogenetic tree of life, LUCA is the organism at the very base of the tree.
Everything alive branches from that single purple point. The closer two tips sit on the tree, the more recently they shared an ancestor.
Evidence for a common ancestry
Evidence
Why it points to a shared origin
Same biochemistry in all organisms
The same core reactions and pathways appear everywhere
Same DNA bases and genetic code
The same four bases and the same codons mean the same amino acids in every species
Same amino acids forming proteins
All organisms build proteins from the same set
Genes shared by eubacteria and archaea
Researchers found several genes present in both groups, indicating these were inherited from LUCA
What happened to everything else? It is possible that other organisms evolved at the same time as LUCA, but became extinct due to competition for shared resources. The descendants of LUCA outcompeted them, and went on to shape the tree of life as we know it.
Estimating the timescale
Fossils provide evidence about the history of life on Earth and are often used to determine the timescale over which evolutionary events occurred. Timescales are established using techniques to date the fossils or the rocks they are found in.
Technique
How it works
Range
Carbon dating
Uses the isotope carbon-14
Samples up to approximately 60 000 years old
Radiometric dating
Measures the relative proportions of certain radioactive substances in a sample, such as carbon-13 to carbon-12
Much older material
Molecular clock
Estimates dates from the number of mutations that have accumulated
Used to estimate when species diverged, and when life originated
Older rocks would be expected to contain evidence of more ancient forms of life, so accurately dating those rocks indicates when life may have originated.
How a molecular clock works
The age of an organism’s lineage can also be determined by analysing its genome. DNA changes as mutations occur and accumulate over time.
By estimating the average time for DNA mutations to occur, the relative date when two species branched from a common ancestor can be worked out from the number of mutations between them.
The same applies to changes in the amino acid composition of proteins, since any DNA change translates into a different protein structure.
On the assumption that these changes occur at a constant rate, this forms the basis of a molecular clock, which can be used to estimate when life on Earth originated.
Count the differences between two living species, divide by the mutation rate, and you get an estimate of when they diverged.
Keep in mind that the timescale across which life has been evolving is immense, and that numbers from fossil evidence — and especially from the molecular clock — are estimates, not exact dates. Saying so in an evaluation question is worth a mark.
Evidence from hydrothermal vents
It is possible that LUCA evolved in hydrothermal vents deep in the ocean. Conditions near these vents give organisms the opportunity to generate energy by chemosynthesis, rather than needing sunlight.
The fossil evidence
Scientists have found fossilised structures in sedimentary rocks near deep-sea hydrothermal vents in Quebec, Canada.
These structures are similar to those produced by modern prokaryotes living near vents today.
The fossils are at least 3.77 billion years old, and could be more than 4 billion — among the oldest forms of life ever found.
They are small tubes made of haematite, the mineral form of iron(III) oxide.
Carbonate and other carbonaceous material in the rocks indicates that oxidation and other biological activities occurred there, and that these ancient bacteria had a similar biochemistry to modern iron-oxidising bacterial communities near vents.
What LUCA may have been like
Analysis of sequence data from modern species living near hydrothermal vents indicates they all share a common ancestor. Based on the properties and functions of their amino acid sequences, LUCA may have had these characteristics:
Characteristic
What it means
Anaerobic
Able to survive without oxygen — consistent with an early Earth that had no free oxygen
Converted carbon dioxide into glucose
It made its own food, so it was autotrophic
Used hydrogen as an energy source
Not sunlight — this is chemosynthesis, which fits a deep-ocean vent
Converted nitrogen into ammonia
For the synthesis of amino acids
Thermophilic
Survived at very high temperatures
Taken together, fossil evidence and genetic analysis indicate LUCA may have been an autotrophic extremophile living in hydrothermal vents, in an environment rich in hydrogen, carbon dioxide and iron.
Stay tentative. This is not the only hypothesis for the origin of life. Scientists will continue to gather and analyse data that may support or refute existing theories.
Worked examples
WE 1
Outline the evidence for a last universal common ancestor
Outline the evidence that all living organisms share a common ancestor. (4 marks)
Point 1: shared biochemistry
All organisms share the same basic biochemistry and use the same set of amino acids to build proteins.
Point 2: shared genetic code
All organisms use the same DNA bases and the same genetic code, so the same codon means the same amino acid in every species.
Point 3: shared genes
Several genes are found in both eubacteria and archaea, indicating they were inherited from LUCA.
Point 4: shared structures
Anatomical similarities such as the bone structure of the vertebrate forelimb show inheritance from a common ancestor.
Same chemistry, same code, shared genes, shared structuresgive molecular and anatomical evidence — answers using only one type rarely reach full marks
WE 2
Explain how a molecular clock is used
Explain how analysis of a genome can be used to estimate when two species diverged. (3 marks)
Point 1: what changes over time
DNA changes as mutations occur and accumulate, and these changes also alter the amino acid composition of proteins.
Point 2: the calculation
By estimating the average time for mutations to occur, the number of differences between two species can be converted into a time since they branched from a common ancestor.
Point 3: the assumption
This assumes mutations occur at a constant rate, which is the basis of the molecular clock, so results are estimates rather than exact dates.
More accumulated differences = longer since the two lineages splitstate the constant-rate assumption — it is usually the third mark
WE 3
Suggest why LUCA may have lived near hydrothermal vents
Suggest why scientists believe LUCA may have evolved near deep-sea hydrothermal vents. (3 marks)
Point 1: the energy source
Vents allow organisms to generate energy by chemosynthesis rather than sunlight, which suits a deep-ocean environment.
Point 2: the fossil evidence
Fossilised haematite tubes at least 3.77 billion years old have been found in sedimentary rocks near vents in Quebec, similar to structures made by modern vent prokaryotes.
Point 3: the genetic evidence
Sequence analysis suggests LUCA was anaerobic, thermophilic and used hydrogen as an energy source — all conditions found at vents.
An autotrophic extremophile in a hydrogen, carbon dioxide and iron rich environmentgive both fossil and genetic evidence, and remember this is one hypothesis among several
💡 Exam tips
Know what LUCA stands for and where it sits on a phylogenetic tree — at the base.
Learn the three dating techniques and their limits: carbon-14 up to about 60 000 years, radiometric for older, molecular clock for divergence.
Always say fossil and molecular dates are estimates.
Use the correct group names: eubacteria, archaea, eukaryotes.
Learn the five LUCA characteristics — anaerobic, autotrophic, hydrogen energy source, nitrogen fixing, thermophilic.
Remember the number 3.77 billion years for the Quebec vent fossils.
⚠ Common mistakes
Saying LUCA was the first living organism. It is the last common ancestor of everything alive now — other organisms may have existed alongside it and gone extinct.
Using carbon dating for very old fossils. Carbon-14 only works up to about 60 000 years.
Treating molecular clock dates as exact. They rest on an assumed constant mutation rate.
Saying LUCA used sunlight. It is thought to have used hydrogen as an energy source.
Confusing autotrophic with heterotrophic. LUCA converted carbon dioxide into glucose, so it made its own food.
Presenting the vent hypothesis as settled. It is one hypothesis among several.
That completes The Origin of Cells. The three notes tell one continuous story: chemistry produced molecules, molecules assembled into something enclosed and self-copying, and everything alive today traces back to one of those early survivors.
Want this explained one-to-one?
Book a free session with an experienced IB Biology tutor and get your trickiest topics made simple.
