lecture 16 (3 29-2016) slides

Lecture 16: Microbiology of Early Earth

Review for exam IIIWednesday April 6, 2-3:30pm

LSL N210I will bring all my slides and notes.

You will bring your questions!

Objectives

• Describe the major events in the evolution of life, and when they happened.

• Describe the origin of life on earth, and approximate age of origins.

• What do we know about LUCA? • What is the evidence for early life?• How is it possible to know what early life

forms were, and how old they were?

The Early days of Earth … (first few million years)

1. Accretion. Impacting bodies bombard the Earth and convert their energy of motion (kinetic energy) into heat. In recent years we also learned that an early collision with a very large object was responsible for the "extraction" of the Moon from Earth.

2. Self-compression. As the Earth gets bigger, the extra gravity forces the mass to contract into a smaller volume, producing heat (just like a bicycle pump gets hot on compression).

3. Differentiation. Conversion of gravitational potential energy to heat during core formation

4. Short-lived radiogenic isotopes. The surrounding material absorbs the energy released in radioactivity, heating up.

The Early days … (first few million years)

The Early days … (first few million years)

• Lots of bolide impacts, volcanic activity • Much warmer average global temperature than

today (80°C ?) • Mildly reducing conditions in the atmosphere

(CO,CO2, N2,H2O,CH4) • Oceans formed > 3.8 bya (condensation from

atmosphere as Earth cooled) • No free oxygen (O2) !!!

What are some possible requirements for the origin of life?

• Generation of simple organic molecules• Production of complex organic molecules and

metabolic networks• Origin of self-replication - genotype• Compartmentalization – cells• Linking genotype to phenotype• Origin of genetic code• Takeover of replication system by DNA

Miller-Urey experiments

RNA the first biological molecule

RNA as an autocatalytic network

Layers of clay as catalysts for RNA polymerization

The primordial sandwich hypothesis

• Surfaces can concentrate organics by adsorption

• Polymerization is favored on surfaces• Surface chemistry is stereospecific• Activated precursors are not required

Compartmentalization isolates a replicating system, increasing self-organization and allowing for systems

to become more complex.

Protocells & assisted reproduction

From protocells to bacteria…

From protocells to bacteria

1. First protocell is a lipid sack of RNA

2. RNA catalysis3. Metabolism begins4. Proteins appear5. Proteins take over

function6. DNA is created7. Bacterial / archaeal world

Hypotheses on the origin of viruses

• The progressive, or escape, hypothesis states that viruses arose from genetic elements that gained the ability to move between cells– HIV act like retrotransposons

• The regressive, or reduction, hypothesis asserts that viruses are ‘escaped’ portions, or remnants, from cellular organisms– Mimivirus

• The virus-first hypothesis states that viruses predate or coevolved with their current cellular hosts– ssRNA viruses

Great Oxygenation Event

Diversity increased exponentially

THE EVIDENCE for Early Life

1. Isotopic record 2. Rocks and Microfossils 3. Organic Geochemical Record, or “molecular

fossils”4. Molecular Evolution

Isotopic fractionation

• Bonds involving “light” isotopes break more readily than those involving “heavy” isotopes.

• Rate determining step which includes breaking of bond dictates isotopic fractionation of entire process

• Typical of processes which are unidirectional and irreversible

R is the ISOTOPIC RATIO e.g., (13C/12C) sampleR = HEAVY ISOTOPE/ LIGHT ISOTOPER = RARE ISOTOPE / ABUNDANT ISOTOPE

Isotopic signatures are based on stable carbon isotopes

“delta 13 Carbon”

Stable isotopes as C tracer: The Standard is Pee Dee Belemnite (PDB)

Belemnitella americanaIs an ancient squid

Belemnitellaamericana

is an ancient squid

Stable isotopes as C tracer: The Standard is Pee Dee Belemnite (PDB)

The Peedee Formation, North Carolina

Isotopic evidence for early life• The 3,416-Myr-old Buck Reef Chert, South Africa, is marine

sediment, not hydrothermal• Laminated mat-like structures with stable carbon isotope

signature suggests CO2 fixation, – δ 13C = -35 o/oo– “...isotopic composition of BRC carbonaceous matter is consistent with

fixation by autotrophs employing the Calvin cycle.”• “Taken together, the carbon isotopic composition of BRC

carbonaceous matter, the presence of siderite and lack of primary ferric oxides, and the restriction of microbial mats to shallow water indicate that photosynthetic, probably anoxygenic, microbes were active in the 3,416-Myr-old ocean.” – Tice & Lowe, Nature 431, 549 - 552 (2004)

Isotopic records of microfossils

Microfossils of the Early Archean Apex Chert: New Evidence of the Antiquity of LifeJ. William Schopf Science, New Series, Vol. 260, No. 5108. (Apr. 30, 1993), pp. 640-646

“Molecular fossils”organic geochemical markers

• Archaean Molecular Fossils and the Early Rise of Eukaryotes Jochen J. Brocks, Graham A. Logan, Roger Buick, Roger E. Summons– Science, Vol 285, Issue 5430, 1033-1036 , 13 August 1999

• Organic biomarkers from 2.7 billion year old shalesA. Steranes (cholestane) = eukaryotesB. 2-methyl hopanes = cyanobacteria

• Lots of care has to be taken to ensure organics are derived from the rock was buried – and not contaminating material.

• Hard stuff to do !!!

Banded Iron Formations• Very large bodies of sedimentary

rock laid down some 2.5 – 1.8 billion years ago (Precambrian)– Fe2+ very soluble – Fe3+ insoluble, precipitates from

solution• BIF formation seems to require

anoxic deep waters for formation. Thus, if deep-sea O2 became abundant, it could inhibit BIF formation

• BUT, alternative theory suggests Fe-sulfides can also highly insoluble. Was it oxygen or sulfide, that ended BIF deposition ?

Banded Iron Formations

Stromatolites

Molecular evolution: Reconstructing past events

David & Alm, Rapid evolutionary innovation during an Archaean genetic expansion. Nature 2011; Colours indicate abundance normalized to present-day values. Values in parentheses give the overall number of gene families in each group.

Objectives

• Describe the major events in the evolution of life, and when they happened.

• Describe the origin of life on earth, and approximate age of origins.

• What do we know about LUCA? • What is the evidence for early life?• How is it possible to know what early life

forms were, and how old they were?