Astronomy190 - Topics in Astronomy

Astronomy and Astrobiology

Lecture 13 : Life’s History

Ty Robinson

Questions of the Day

• What was early life like?

• How has life evolved with the Earth’s environment?

• How did life change Earth’s environment?

Geologic Time

Origin of life (prokarya, archaea)

Rise of atmospheric O2 (Ice age)

First ‘shelly’ fossils (Cambrian explosion)‘Snowball Earth’ ice ages

Ice age (?)

First fossil evidence for eukarya

PhanerozoicTime

First shelly fossils

Age of fish

First vascular plants on landIce age

Ice age

First dinosaurs

Dinosaurs goextinct

Ice age (Pleistocene)

Earth’s Prebiotic Atmosphere

• dominantly N2 and CO2

0.3 bars CO2 + ? to offset the faint young Sun

• abiotic net source of O2: photolysis of H2O followed by escape of H to space

• possible high-altitude O2 source: photolysis of CO2 followed by

O + O + M O2 + MJ. F. Kasting, Science (1993)

Questions?Why is it difficult to find evidence for the existence of O2 in Earth’s pre-biotic atmosphere?

Humans destroyed this O2 through respiration

reactions with CO2 destroyed any atmospheric O2

the O2 was in the upper atmosphere, never reacting with the surface

the evidence has been destroyed

Earth’s Earliest Life

Evidence for the earliest life on the Earth comes from three different sources.

• Stromatolites (3.5 Gya)

• Microfossils (3.5 Gya)

• 12C/13C Isotopic Evidence (3.85 Gya)

3.5-GaStromatolites(Warrawoona)

Prokaryotic organisms in colonies

From: Earth’s EarliestBiosphere, J. W. Schopf,ed. (1993)

Old and New

Archean

Modern

Modern StromatolitesShark Bay, Western Australia

• earliest organisms were probably chemoautotrophs, i.e., they derived their energy from chemical reactions and took their carbon from the environment

Examples• Sulfur reducers: S + H2 H2S (stinky!)

sulfur hydrogen hydrogen sulfide

• Methanogens: CO2 + 4 H2 CH4 + 2 H2O

What effect would life have had on the earlyatmosphere?

• photosynthesis may well have been invented early, but since there was no surface oxygen, it originally used H2 (or H2S) rather than H2O

Oxygenic photosynthesis (prevalent today)

CO2 + H2O (+ h) CH2O + O2

Anoxygenic photosynthesis (early Archean)

CO2 + 2 H2 (+ h) CH2O + H2O

CO2 + 2 H2S (+ h) CH2O + H2O + 2 S

no O2 production!

Questions?Why is it difficult to determine the time when forms of non-oxygenic photosynthesis evolved?

the microbes were too small

the products of these reactions are also created abiotically

the products of these reactions are not as reactive as O2

the evidence has been destroyed

CO2 + 2 H2 (+ h) CH2O + H2O

CO2 + 2 H2S (+ h) CH2O + H2O + 2 S

How do we learn about ancient organisms?

1. from the fossil record (but this isn’t very good prior to the Cambrian)2. from molecular phylogeny, that is, the sequencing of DNA and RNA

The “Universal” Tree of Life

• rRNA—ribosomal RNA

• ribosomes: organelles within cells that are responsible for making proteins

• ribosomal RNA mutates slowly trees constructed from this molecule show deep divergences in evolution

methanogenicarchaea

Courtesy ofNorm Pace

“Universal”(rRNA) Tree

Of life

hyperthermophiles root?

Cyanobacteria and The Rise of Oxygen

• atmospheric O2 rose 2.3 Gya

• land plants weren’t around until about 425 Mya

• Cyanobacteria (prokaryotes) were responsible

cyanobacteria

Implications• oxygenic photosynthesis was only invented once!• cyanobacteria invented it, and then some eukaryote

‘imported’ a cyanobacterium (endosymbiosis)• higher plants and algae descended from this primitive

eukaryote

Geological O2 Indicators

H. D. Holland, in Early Life on Earth, S. Bengtsson, ed. (1994)

Banded Iron Formations• (>1.8 Ga)

• can only form when large amounts of iron are dissolved in the oceans

• this is only possible if oxygen is not present

• to form the layering, periods of no oxygen have to be alternated with a condition that oxidizes iron• cyanobacterial bloom?• iron oxidizing bacteria?

Timing the rise of oxygen• ratios of different sulfur isotopes in rocks suggest

that O2 started to rise 2.35 Gya

The Story So Far

• bacteria, archaea and eukaryotes are all well established by 2.1 Gya (although they were all probably around much earlier than that)

• oxygen rises at 2.3-2.4 Gya • …then nothing much happens until….• oxygen rises again near 545Mya to nearer

present-day levels (~60% of present day) • at 545 Mya we get the ‘Cambrian Explosion’

Phyla

• biologists classify animals according to “body plan”

• mammals and reptiles are “phylum Chordata”• insects, crabs, spiders are “phylum Arthropoda”

• there are about 30 different phyla today

• Almost all these phyla first appeared between 545-505 Mya!

The Cambrian Explosion

• Why did this occur so suddenly? Why hasn’t it happened since?

The Timing of the Cambrian Explosion

• oxygen rose to the present-day level just prior to the Cambrian explosion– respiration needed for complex organisms?

• organisms may have developed a critical amount of complexity so that large diversity could evolve rapidly

• Earth was just coming out of a Snowball Earth phase

• efficient predators hadn’t evolved yet– maybe why it hasn’t happened since?

Crawling from the Primeval Slime…

• early microbial life protected itself from UV in water

• for land-based organisms, it is tougher– need UV shield

• plants arrived on land 475 Mya– probably evolved from algae– no land animals yet to eat them!

• by about 400 Mya amphibians and insects were eating plants

• dinosaurs and mammals arrived 245 Mya

Not a Bed of Roses!

Questions of the Day

• What was early life like?

• How has life evolved with the Earth’s environment?

• How did life change altered Earth’s environment?

Quiz

3 - What is one thing you did not understand from today’s lecture?

2 - What are two possible explanations for the “Cambrian Explosion”?

1 - Describe one possible location for the origin of life. What are the pros/cons for this location?