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 O 2 (Ice age) First ‘shelly’ fossils (Cambrian explosion) ‘Snowball Earth’ ice ages Ice age (?) First fossil evidence for eukarya

Phanerozoic Time First shelly fossils Age of fish First vascular plants on land Ice age First dinosaurs Dinosaurs go extinct Ice age (Pleistocene)

Earth’s Prebiotic Atmosphere dominantly N 2 and CO bars CO 2 + ? to offset the faint young Sun abiotic net source of O 2 : photolysis of H 2 O followed by escape of H to space possible high-altitude O 2 source: photolysis of CO 2 followed by O + O + M  O 2 + M J. F. Kasting, Science (1993)

Questions? Why is it difficult to find evidence for the existence of O 2 in Earth’s pre-biotic atmosphere? Humans destroyed this O 2 through respiration reactions with CO 2 destroyed any atmospheric O 2 the O 2 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) 12 C/ 13 C Isotopic Evidence (3.85 Gya)

3.5-Ga Stromatolites (Warrawoona) Prokaryotic organisms in colonies From: Earth’s Earliest Biosphere, J. W. Schopf, ed. (1993)

Old and New Archean Modern

Modern Stromatolites Shark 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 + H 2  H 2 S (stinky!) sulfur hydrogen hydrogen sulfide Methanogens: CO H 2  CH H 2 O What effect would life have had on the early atmosphere?

photosynthesis may well have been invented early, but since there was no surface oxygen, it originally used H 2 (or H 2 S) rather than H 2 O Oxygenic photosynthesis (prevalent today) CO 2 + H 2 O (+ h )  CH 2 O + O 2 Anoxygenic photosynthesis (early Archean) CO H 2 (+ h )  CH 2 O + H 2 O CO H 2 S (+ h )  CH 2 O + H 2 O + 2 S  no O 2 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 O 2 the evidence has been destroyed CO H 2 (+ h )  CH 2 O + H 2 O CO H 2 S (+ h )  CH 2 O + H 2 O + 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

methanogenic archaea Courtesy of Norm Pace “Universal” (rRNA) Tree Of life hyperthermophiles root?

Cyanobacteria and The Rise of Oxygen atmospheric O 2 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 O 2 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 O 2 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 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 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?