Origin of Cells (HL)

A2.1

What conditions were present on early Earth that supported the development of life?

High temperatures (75–95°C), no solid landmasses, reducing atmosphere (rich in methane, ammonia, water vapour, CO₂), no oxygen or ozone layer, high solar and cosmic radiation, volcanic activity, and electrical storms.

What is meant by a "reducing atmosphere"?

An atmosphere rich in gases like methane and ammonia that donate electrons, enabling chemical reactions that can form complex carbon compounds.

Define the term "primordial soup."

The early oceans of Earth, rich in simple carbon-based compounds, where the first life forms are thought to have originated.

What are the eight processes of life?

Metabolism, Response to stimuli, Homeostasis, Movement, Growth, Reproduction, Excretion, Nutrition (MR HM GREN).

Why are viruses not considered living organisms?

They cannot reproduce independently, lack metabolism, and rely on host cells for many life processes.

What are the steps believed necessary for the evolution of the first cells?

1. Formation of simple organic molecules; 2. Catalysis of chemical reactions; 3. Assembly into larger molecules (e.g., RNA, phospholipids); 4. Self-replication of molecules like RNA; 5. Formation of membrane-bound compartments.

What evidence supports the idea that life could have formed from non-living matter?

The Miller–Urey experiment (1952), which produced amino acids from inorganic gases under simulated early Earth conditions.

What are the limitations of the Miller–Urey experiment?

Later research suggests early Earth’s atmosphere was not as reducing as they assumed; it may not have accurately reflected pre-biotic conditions.

What is the significance of vesicle formation in the origin of cells?

Fatty acids coalesced into spherical bilayers (vesicles), creating compartments that allowed internal chemical environments to differ from external ones—essential for cellular function.

What is the 'RNA first' hypothesis?

Suggests RNA acted as both genetic material and a catalyst in the first cell-like structures before DNA and proteins took over their modern roles.

Provide evidence for the 'RNA first' hypothesis.

• RNA can self-replicate using ribozymes; - RNA has catalytic functions; - Ribozymes are still used in ribosomes for protein synthesis.

What are other hypotheses for the origin of life besides ‘RNA first’?

Miller–Urey hypothesis, Metabolism first, Sulfur world hypothesis, Lipid world hypothesis.

Why can't life spontaneously form today like it might have on early Earth?

Earth’s atmosphere now contains oxygen, lacks the same reducing gases, and has a protective ozone layer that prevents the same kind of chemical reactions.

What is the importance of universal agreement on the definition of “billion”?

It avoids confusion in international scientific communication by ensuring consistent numerical understanding: 1 billion = 1,000,000,000.

How does the Nature of Science apply to theories on the origin of life?

Scientific claims must be testable, but origin theories are hard to test due to lack of fossil evidence and inability to replicate early Earth conditions exactly.

Is there a definitive answer to where life started?

No, the exact location and conditions of life's origin are still debated, though LUCA is considered the origin of all modern life forms.

What is LUCA?

The Last Universal Common Ancestor; a hypothetical organism that gave rise to all current life on Earth.

What does LUCA stand for?

Last Universal Common Ancestor.

What kind of organism was LUCA believed to be?

A simple, single-celled, autotrophic microbe likely with an RNA (or possibly DNA) genome.

When did LUCA exist?

Approximately 2.5 to 3.5 billion years ago.

Why is LUCA important?

LUCA is the common ancestor of all modern organisms and provides a key to understanding life’s origin.

Why do scientists believe LUCA was the only life form to persist?

LUCA or its descendants likely outcompeted or caused the extinction of other early life forms.

What is the strongest evidence for LUCA?

The universal genetic code shared across all organisms and viruses.

Why is the genetic code considered universal?

All organisms use the same codons to code for amino acids (e.g., AUG for methionine).

Why is the universality of the genetic code important?

It supports the theory of common ancestry and is essential for stable genetic information transmission.

What role does the genetic code play in studying evolution?

Changes in the genetic code help scientists trace evolutionary paths and divergence.

What was the old view of LUCA’s descendants?

That three domains (Bacteria, Archaea, Eukaryotes) evolved directly from LUCA.

What is the current view of LUCA’s descendants?

Bacteria and Archaea evolved from LUCA; Eukaryotes arose later through endosymbiosis.

How did scientists refine their search for LUCA's genes?

By focusing on conserved genes in Bacteria and Archaea that were not affected by horizontal gene transfer.

How many genes are now believed to have existed in LUCA?

355 ancient genes shared by Bacteria and Archaea.

What is horizontal gene transfer?

The transfer of genes between species or domains, not through traditional reproduction.

Why does horizontal gene transfer complicate evolutionary studies?

It makes it difficult to identify which genes were truly present in LUCA.

What fossil evidence supports the study of early life?

Stromatolites, which are layered fossils formed by microorganisms.

What are stromatolites?

Fossilised layered microbial structures found in rocks, evidence of early life.

Where have stromatolites been found?

In places like Shark Bay, Australia.

Where is LUCA believed to have lived?

In deep ocean alkaline hydrothermal vents.

What conditions were present in hydrothermal vents?

High temperatures, rich in hydrogen, sulfur, methane, and iron—ideal for early biochemical reactions.

Why are hydrothermal vents considered ideal for life’s origin?

They offer chemical energy, protection, and the right conditions for complex organic molecule formation.

Was LUCA aerobic or anaerobic?

Anaerobic, consistent with the lack of oxygen on early Earth.

Was LUCA autotrophic or heterotrophic?

Autotrophic—used inorganic carbon and hydrogen to produce molecules like carbon dioxide and formic acid.

What methods do scientists use to study LUCA?

Molecular phylogenetics (comparing genetic sequences) and fossil analysis.

What is molecular phylogenetics?

The study of evolutionary relationships through genetic sequence comparison.