ASTR-3040:Astrobiology

Day 12

The Origin & Evolution of Life on Earth

Chapter 6

Homework

Due Tue. March 1

Chapter 6: 1, 4, 8, 13, 23, 28, 34, 35, 42, 46, 49, 52, 53, 56

Exam 1 – Tuesday March 1 Chapters 1 - 6

Searching for Life's Origins Geologic record details much of life history.

Evolution theory tells us how life has changed.

But, how did life arise?

Three lines of fossil evidence.

Stromatolites – date to 3.5 Gyr. Photosynthesis

Microfossils – date to 3.5 Gyr.

Isotopic evidence

Stromatolites

Photosynthetic at least in top microbes.

Modern ones resemble old fossils.

Date to 3.5 Gyr

Microfossils

Biological?

photosynthetic?

Australia – 3.5 Gyr

Africa – 3.2-3.5 Gyr

2.7-3.0 Gyr - conclusive

Isotopic Evidence Carbon-13 evidence of 3.85 Gyr life

But, no microfossils in the rocks

Sedimentary – so fossils might be destroyed.

Implications?

The carbon dating – if it stands – puts life at 3.85 Gyr ago – at least. Rocks this old are scarce.

Life itself must be older than this.

Arose and colonized Earth in ~100 Myr?

Probably after the LHB period (4.2-3.9 Gyr)

Suggests life will arise and spread quickly.

What did early life look like?

Evolutionary relationships. Track changes through DNA sequence.

Large difference in genome between two life forms indicates a longer time since they shared common ancestor.

Extremophiles (hyperthermophiles) are probably the closest to first life. Chemoheterotrophes?

Where? Deep-sea hot water vents – most likely

Origin of Life

Experiments try to re-create chemical conditions on Earth indicate life may have started through natural, chemical processes.

Panspermia – could life have originated elsewhere and been transported to Earth?

How Did Life Begin?

Miller-Urey Experiment 1950s

H2O and CH

4, NH

3

Add electric spark

Pass condensates back to water flask.

Amino acids and many organics.

But, what was 1st atmo?

Other sources of organics

Chemical reactions near deep-sea vents

Material from space – meteorites, comets Organics can form in space?

Protoplanet & solar nebula

When was chemical ==> biological transition? DNA is a complex molecule.

RNA

Single strand rather than double

Easier to manufacture

Recent (early 1980s) work show RNA can self-catalyze using rybozymes

Experiments show “clay” can facilitate self-assembly of complex, organic molecules.

Abundant on Earth and in oceans

Laboratory experiments

Then what?

Assuming self-replicating RNA is formed Rapid modification –

natural selection

Mutations

Then what?

Pre-cells Keep molecules

concentrated – increase reaction rates

Protect from the outside world

Primitive structures form naturally and easily.

Pre-cells

Amino acids will form spherical structures when cooled. Grow by adding chains

Split to form daughters

Lipids in water form membrane-like structures

Put it together 1. Some combination of atmo. chemistry, deep-

sea chemistry, molecules from space.

2. More complex molecules -RNA- grew form the building blocks. Some become self-replicating.

3. Membranes form spontaneously.

4. Natural selection among RNA molecules . Eventually these become true living organisms.

5. Natural selection – diversity. DNA becomes favored hereditary molecule.

Migration of Life to Earth

We've seen some organisms survive in space.

Could life arise on Venus or Mars first?

Possibility of migration 20,000 meteorites cataloged

~36 come from Mars.

1. Large impacts.

2. Survival during transit.

3. Atmo. entry.

ALH8400

Transit

Endoliths could survive both blast and entry.

Transit survival depends on time in space. Most rocks millions or billions of years

A few ten years or less.

Probably no interstellar meteorites (none known).

Why migration? Does life form easily on early Earth?

Does life form too easily on any planet?

Implications of Transit

Of the early solar system planets Mercury and Moon are probably not favorable.

Early Venus and Mars might have been hospitable.

Migration from Earth?

Why migration?

Evolution of Life

Major events. Early microbes – anaerobic (primitive atmosphere).

Chemoautotrophes – underwater probably

Photosynthesis – multiple steps to arise ~3.5 Gyr ago (stromatolites)

Oxygen crisis ~2.4 Gyr ago?

Evolution of Eukarya – cell complexity Symbiosis?

Mitochondria & Chloroplasts

Cambrian Explosion

Life started slowly (?)

Multi-cell organisms ~1.2 Gyr ago Microbes had 2+ Gyr by themselves

Animals – little change from 1.2 – 0.7 Gyr ago

Then a huge diversification 30 body plans 40 Myr for all this to occur.

Why Cambrian Explosion

Oxygen level reached a critical value Survival of large, energy-intensive life forms

Genetic diversity of eukaryotes

Climate change – coming out of snowball

No efficient predators May explain why no similar explosion since.

Colonization of Land

Oxygen level reached a critical value Ozone could form UV protective layer.

Need to evolve a method to obtain oxygen and nutrients.

Plants first ~475 Myr ago Probably evolved from alga.

Specialization in larger plants (leaves, roots)

Amphibians and insects within 75 Myr

Carboniferous Period By 360 Myr ago – vast forests, insects

Flooded land masses – so little decay These deposits formed coal.

Rise of Oxygen Critical to animal life

Molecular Oxygen – reactive gas. Disappears quickly if not replenished

Early – oxidation reactions (rust, iron-oxides...)

Now – use by animals

Cyanobacteria

Timing Fossil and rock studies

2-3 Gyrs – banded iron formations

< 1% of present level

Sulfur isotope studies ~2.35 Gyrs for oxygen.

Cyanobacteria started ~2.7 Gyrs (350 Myr gap) Removal by non-biologicals – oxidation

Slow build-up – no “explosion”

200 Myr ago – first charcoal

Implications If Earth is typical – probably few planets with

complex, oxygen using life (rqr ~4 Gyr to form)

If Earth was delayed – complex life might be flourishing elsewhere.