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 – Gyr 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 ( 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  H 2 O and CH 4, NH 3  Add electric spark  Pass condensates back to water flask.  Amino acids and many organics.  But, what was 1 st 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.