The Microbial World: Evidence of Earth’s Earliest Life

Oldest Known Rock on Earth Acasta Gneiss: Northwest Territories, Canada (3.96 Ga) (note: Ga = billion years, Ma = million years) It’s ours, eh !

Second Oldest Known Rocks on Earth Isua Group, West Greenland (3.85 Ga)

Oldest Known Zircons on Earth Oldest zircon crystals 4.4 Ga In coarse clastic sedimentary rocks y billion years old Jack Hills, Western Australia Indicates that some solid, granitic, crust existed prior to oldest known rocks (but probably not much)

Life became established relatively early in Earth’s history ! Oldest crust: 4.4 Ga Oldest known life: 3.5 Ga

Stromatolite (structure constructed by cyanobacteria) Warrawoona Series, North Pole, Western Australia 3.5 Ga Oldest known evidence of life

Clues to formation of stromatolites: Modern stromatolites in Shark Bay, Western Australia Hypersaline Intertidal Conditions

Sediment Trapping by Bacterial Mats (forms the fine laminated structure observed in stromatolites) Growth of filaments Trapping of sediment Growth of filaments Trapping of sediment Growth of filaments Trapping of sediment

Single-celled bacteria-like microfossils Fig Tree Chert, Swaziland, South Africa recently estimated at 3.4 Ga

Filamentous Prokaryote Microfossils (Probably Cyanobacteria) Apex Chert, Marble Bar, Western Australia ~ Ga

So… there is evidence that prokaryotic life existed on Earth by at least 3.5 billion years ago. Carbon isotopic evidence from 3.8 Ga rocks in Greenland suggests that microbes may have existed even earlier. Very low δ 13 C signatures in these rocks suggests some carbon fractionation by methanogens or photosynthesizers Such organisms preferentially take up 12 C over 13 C, so extreme enrichment in light carbon ( 12 C) might suggest primitive life… …but this evidence is tenuous at present (perhaps 13 C could be depleted by inorganic means).

Low-oxygen conditions on early Earth Note: up to about Ga, hydrosphere contains minimal free oxygen: -uranium occurs in solid particles (uranium dissolves in presence of oxygen) -fluvial (river) sediments contain reduced iron (so not red) -iron oxide is deposited in ocean in “banded iron formations” (BIF) soils are iron-deficient BIF production peaks here

2.5- ~2.0 Ga A time characterized by widespread Banded Iron Formation (BIF) deposition -interbedded chert (SiO 2 ) and magnetite (Fe 3 O 4 )/hematite (Fe 2 O 3 ) Iron oxide formation may have precipitated inorganically (via simple oxidation of iron by free oxygen in water) …or assisted by metabolic activities of bacteria

Ga First Definite Appearance of Redbeds (soils and river sediments containing red iron oxides) indicates enough free oxygen in atmosphere to oxidize iron in river water before it reaches the sea (note BIF production shuts off soon afterward)

The time of about Ga is significant because by this time, significant amounts of oxygen occurred in the hydrosphere and atmosphere, thus fundamentally changing the dynamics of the Earth’s biosphere. For this reason this time is considered to mark the beginning of the “Oxygen Revolution.”

BIF production really drops off Red soils By Ga: Redbeds well-established -Soils and fluvial (river) sediments are enriched in oxidized iron By about 1.8 Ga, BIF production really drops off (indicating that transport of dissolved iron from land has been effectively shut off due to oxidation on land) Red river sediments Earth’s “Modern” Atmosphere Takes Shape

But if photosynthesizers were largely responsible for producing most of the free oxygen on Earth, why did it take them so long ? Possible reasons: 1.Oxygen was used by organisms (for metabolic processes) as soon as it was produced. 2.Oxygen-bearing organic molecules were buried (and therefore was not readily released into the atmosphere in significant quantities) 3.Oxygen was used up through oxidation of dissolved iron (thus forming the huge deposits of BIF) 4.Nutrients such as phosphorus were in short supply (phosphorus tends to be adsorbed onto iron oxide particles), so may have limited the growth of cyanobacteria (and therefore oxygen production).

Stromatolites in Gunflint Chert, Near Schreiber Ontario 1.9 Ga

Even after the initial buildup of oxygen to significant levels in the atmosphere, things were pretty boring in terms of the variety of living things. Stromatolites (and associated fossil bacteria) continue to dominate the Earth’s biosphere…

Prokaryote Microfossils (probably cyanobacteria) Belcher Islands, Arctic Canada 2 Ga

Cyanobacterial Filaments Bitter Springs Chert, Northern Australia 1.5 Ga

Spherical CyanobacteriaFilamentous Cyanobacteria Modern Form Bitter Springs Chert, Northern Australia, 1.5 Ga Modern Form Note remarkable similarity between ancient and modern forms

But there are some hints of progress…

END OF LECTURE