EEMB 3 Lecture 4

What makes a planet habitable?

Liquid water, energy sources, chemical building blocks, and environmental stability.

What is habitability?

Whether environmental conditions could support life.

Does habitability prove life exists on a planet?

No. It means conditions could support life, not that life is present.

What are the three major origin-of-life hypotheses covered?

Prebiotic chemistry, metabolism first, and organic molecules from space (panspermia).

What is the prebiotic chemistry hypothesis?

Simple chemical reactions on early Earth produced organic molecules needed for life.

What was the Miller-Urey experiment testing?

Whether simple organic molecules could form under simulated early Earth conditions.

What did the Miller-Urey experiment demonstrate?

Amino acids and other organic compounds can form abiotically under certain simulated conditions.

What does abiotically mean?

Occurring without living organisms.

What are amino acids?

Organic molecules that serve as building blocks of proteins.

Why is the Miller-Urey experiment important?

It showed that some building blocks of life can form through chemistry alone.

What does the Miller-Urey experiment NOT explain?

How the first living cells emerged.

What is the metabolism-first hypothesis?

Organized chemical reactions and energy flow developed before fully developed genetic systems.

According to metabolism-first, what may have come before genetic systems?

Organized chemical reactions, energy flow, and chemical cycling.

How could early chemical cycling lead to complexity?

Repeated reactions and energy flow may have produced increasingly complex chemical systems.

What does the metabolism-first hypothesis NOT explain?

Exactly how the first living cells emerged.

What is the panspermia hypothesis?

The idea that organic molecules, life's precursors, or possibly life came from space.

How could meteorites contribute to life's origins?

They can carry organic molecules that may have supplied building blocks to early Earth.

Does panspermia fully explain how life began?

No. It may explain where ingredients came from but not how living cells emerged.

What is the main limitation shared by the three origin-of-life hypotheses?

They do not fully explain how the first living cells emerged.

How do the three origin-of-life hypotheses differ?

Prebiotic chemistry focuses on chemical formation, metabolism-first on reactions, and panspermia on materials from space.

What is the faint young Sun paradox?

Early Earth had liquid water even though the young Sun was fainter and produced less energy.

Why is the faint young Sun paradox important?

It raises the question of how early Earth stayed warm enough for liquid water.

Why were microbes the first forms of life?

They were structurally simple and could survive early Earth's extreme, variable environments.

Why were microbes suited to early Earth?

They could use diverse energy sources and tolerate extreme environmental conditions.

Why were complex multicellular organisms unlikely to thrive on early Earth?

Early Earth lacked oxygen-rich conditions and the stable, complex ecosystems they often require.

What are chemical gradients?

Differences in chemical concentrations that can provide usable energy.

How did microbes obtain energy on early Earth?

Through diverse energy sources, chemical gradients, and in some cases sunlight.

How did early microbes affect Earth's elemental cycles?

They influenced carbon, sulfur, iron, and other elemental cycles.

How did some microbes capture energy from sunlight?

Through photosynthesis or other light-driven energy processes.

How did microbial activity change Earth?

It gradually altered the atmosphere and oceans, helping create conditions for later life.

How did microbial innovation support later life?

Microbial changes to Earth's environment made conditions more suitable for other organisms.

Why is the earliest life difficult to study?

Microscopic organisms rarely leave clear fossil evidence, and ancient rocks may be altered.

What is the Isua Supracrustal Belt?

An ancient rock formation in Greenland associated with possible evidence of early life.

How old is the possible early-life evidence from Isua?

Approximately 3.8 billion years old.

What structures at Isua were interpreted as possible evidence of life?

Layered, dome-shaped structures resembling microbial stromatolites.

Why is the Isua evidence for early life uncertain?

Metamorphism and deformation may have produced similar structures through non-biological processes.

What is the scientific conclusion about the Isua evidence?

It may represent early life, but the evidence is not definitive.

What does metamorphism mean?

The alteration of rocks by heat, pressure, or chemically active fluids.

What is a stromatolite?

A layered structure formed through microbial activity, often involving trapped sediments and mineral precipitation.

Where are the oldest and most convincing stromatolite-building microbial communities described in these notes?

The Dresser Formation in Western Australia.

How old are the stromatolites described in the Dresser Formation?

Approximately 3.4 billion years old.

What evidence supports microbial activity in the Dresser Formation?

Stromatolite textures plus supporting geochemical and sedimentological evidence.

What is a hydrothermal setting?

An environment where heated water interacts with rocks and minerals.

Why is the Dresser Formation evidence stronger than the Isua evidence?

It preserves stromatolite textures alongside geochemical and sedimentological evidence consistent with microbial activity.

Do stromatolites still exist today?

Yes. Modern stromatolites show that microbes can build layered structures.

What can ancient stromatolites tell us about early life?

Microbial communities existed and interacted with their environments billions of years ago.

What can evidence of early life suggest about metabolism?

Early organisms needed ways to obtain energy and process chemicals to survive.

What can evidence of early life suggest about ecology?

Microbes formed communities and interacted with their physical and chemical environments.

How do the Isua and Dresser records differ?

Isua is older but uncertain; Dresser is younger and has stronger evidence of microbial communities.

Why must scientists be cautious when identifying the earliest life?

Non-biological processes can mimic biological structures in ancient rocks.

What is the difference between organic molecules and living cells?

Organic molecules are chemical compounds; living cells are organized systems capable of life processes.

What is the key takeaway from the origin-of-life hypotheses?

Several ideas explain how life's building blocks or early chemical systems could form, but the emergence of cells remains unresolved.