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?