The big idea: Trace the family tree of every living thing — every bacterium, mushroom, oak tree and human — back far enough and the branches all meet at one point.
That point is LUCA: the Last Universal Common Ancestor.
LUCA was the single ancestral population of cells from which all life alive today descends. It is not the first cell ever — it is the most recent ancestor that everything still living has in common.
- LUCA
- The Last Universal Common Ancestor — the single ancestral population of cells from which all organisms alive today are descended.
- Common ancestry
- The idea that two or more species descend from the same ancestor; shared inherited features are evidence for it.
- Molecular clock
- Using the steady rate at which DNA/protein sequences accumulate changes to ESTIMATE how long ago two lineages shared an ancestor.
'Last', not 'first': The L in LUCA is Last, not first.
There were almost certainly earlier cells. LUCA is just the most recent ancestor that all surviving life has in common — the place where every modern branch of the tree of life joins up.
LUCA is thought to have lived around 4 billion years ago, probably at hydrothermal vents on the deep-sea floor, which supplied both energy and the minerals early metabolism needed.
Why do biologists believe in LUCA at all? Because of one striking fact: at the deepest level, all life runs on the same machinery.
Read it as an inference. If wildly different organisms all share the same core biochemistry, the simplest explanation is that they inherited it from a common ancestor — they didn't each invent the identical system by chance.
What all life shares (the evidence for LUCA)
- A near-universal genetic code — the same codons mean the same amino acids almost everywhere
- DNA and RNA as the information molecules, copied the same way
- ATP as the shared energy currency that powers reactions
- Ribosomes that translate mRNA into protein
- Common metabolic pathways (e.g. glycolysis) built from the same steps
| Shared feature of all life | Why it points to a single common ancestor |
|---|---|
| A near-universal genetic code | The SAME codons specify the SAME amino acids in almost every organism — bacteria, plants, humans. An arbitrary code being shared is best explained by inheriting it from ONE ancestor. |
| DNA / RNA as the genetic material | Every cell stores and copies information the same way, using the same nucleotide chemistry. |
| ATP as the universal energy currency | Cells from every domain power their reactions with the same molecule (ATP), not a patchwork of unrelated ones. |
| Ribosomes that build proteins | All cells translate mRNA into protein on ribosomes — the core machinery is shared. |
| Common core metabolic pathways | Reactions such as glycolysis appear across all life, using the same intermediates and enzymes. |
How LUCA is dated: We can't dig up LUCA, so its age is estimated, not measured directly.
By comparing the DNA and protein sequences of living organisms and using the molecular clock (sequences change at a roughly steady rate over time), biologists work backwards to when the lineages last shared an ancestor — about 4 billion years ago.
After LUCA, life branched into the prokaryotes (bacteria and archaea). The next great leap was the eukaryotic cell — and the standout feature of eukaryotes is their membrane-bound organelles, especially mitochondria (and, in plants and algae, chloroplasts).
Where did those organelles come from? The endosymbiotic theory answers this with a cause-and-effect chain.
Endosymbiosis — how a mitochondrion was born
- A larger host cell engulfed a smaller free-living aerobic bacterium by endocytosis — but did not digest it.
- The bacterium survived inside the host as an endosymbiont (one organism living inside another, to mutual benefit).
- The bacterium did aerobic respiration for the host (lots of ATP); the host gave it a safe home and nutrients.
- Over many generations the endosymbiont became permanent — it became the mitochondrion.
- The same story with a photosynthetic cyanobacterium gave rise to the chloroplast in plants and algae.
The endosymbiotic origin of mitochondria: a host cell engulfs a free-living aerobic bacterium by endocytosis; the bacterium survives inside as an endosymbiont and, over generations, becomes a mitochondrion.
Interactive diagram
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The fingerprints that prove it: If mitochondria really were once free-living bacteria, they should still carry leftover bacterial features. They do — four of them:
1. Their own circular DNA — like a bacterial chromosome, unlike the host's linear DNA.
2. 70S (prokaryote-type) ribosomes — the bacterial size, not the 80S ribosomes of the rest of the cell.
3. A double membrane — the inner one is the bacterium's original membrane; the outer one is the host membrane that wrapped around it.
4. They divide by binary fission — splitting in two independently of the cell, exactly as bacteria reproduce.
Why we still see the bacterial origin today: the engulfed bacterium kept its OWN circular DNA, its OWN 70S (prokaryote-type) ribosomes and a DOUBLE membrane, and it still divides by binary fission — the fingerprints of a former free-living cell.
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
| Feature of mitochondria & chloroplasts | Why it is evidence for an engulfed bacterium |
|---|---|
| Their own small, CIRCULAR DNA | Bacteria have a single circular chromosome; eukaryotic nuclei have linear DNA. The organelle keeping circular DNA points to a bacterial origin. |
| 70S (prokaryote-type) ribosomes | These are the bacterial size of ribosome — the rest of the eukaryotic cell uses larger 80S ribosomes. The organelle still runs the bacterial machinery. |
| A DOUBLE membrane | The inner membrane is the bacterium's original membrane; the outer one came from the host's membrane wrapping around it as it was engulfed. |
| Divides by BINARY FISSION | The organelle splits in two independently of the host cell dividing — exactly how free-living bacteria reproduce. |
See how examiners mark answers
Access past paper questions with model answers. Learn exactly what earns marks and what doesn't.
How this is tested: Expect a short Outline/State on what LUCA is (the last common ancestor of all life), and an Explain/Outline asking for the evidence — either the shared biochemistry pointing to LUCA, or the four organelle features that support endosymbiosis.
For an endosymbiosis question, score the marks by listing distinct features (own circular DNA, 70S ribosomes, double membrane, binary fission) and saying why each is bacterial. Don't just say 'mitochondria came from bacteria' — give the evidence.
IB-style question — evidence for the endosymbiotic origin of mitochondria
Mitochondria are thought to have evolved from free-living bacteria that were engulfed by an ancestral host cell. Outline the evidence that supports this endosymbiotic origin. [4]
How to score all four marks
- Own circular DNA. Mitochondria contain their own small, circular DNA, like a bacterial chromosome and unlike the host cell's linear nuclear DNA.
- 70S ribosomes. Mitochondria have 70S (prokaryote-type) ribosomes — the bacterial size — whereas the rest of the eukaryotic cell uses larger 80S ribosomes.
- Double membrane. Mitochondria are bounded by a double membrane: the inner membrane is the original bacterial membrane and the outer one came from the host engulfing it.
- Binary fission. Mitochondria divide by binary fission, independently of the host cell — the way free-living bacteria reproduce. (Award 1 mark per distinct feature, up to 4.)
Final answer
Mitochondria have their own circular DNA (like bacteria), 70S prokaryote-type ribosomes, a double membrane (inner = bacterial, outer = host), and they divide by binary fission independently of the cell — all features of free-living bacteria, supporting an endosymbiotic origin.
✓ Why this scores full marks: It gives four distinct, separately-creditable features and, for each, says why it is bacterial (circular not linear DNA; 70S not 80S; inner-membrane origin; binary fission).
A common way to lose marks is to repeat one idea ('it has bacterial DNA' twice) or to state the conclusion without the evidence — you need four different fingerprints.
| Feature of mitochondria & chloroplasts | Why it is evidence for an engulfed bacterium |
|---|---|
| Their own small, CIRCULAR DNA | Bacteria have a single circular chromosome; eukaryotic nuclei have linear DNA. The organelle keeping circular DNA points to a bacterial origin. |
| 70S (prokaryote-type) ribosomes | These are the bacterial size of ribosome — the rest of the eukaryotic cell uses larger 80S ribosomes. The organelle still runs the bacterial machinery. |
| A DOUBLE membrane | The inner membrane is the bacterium's original membrane; the outer one came from the host's membrane wrapping around it as it was engulfed. |
| Divides by BINARY FISSION | The organelle splits in two independently of the host cell dividing — exactly how free-living bacteria reproduce. |