A Brief Diversion: The Geological Scale

Now that we’re getting into time units of higher resolution, we should look at how the geological time Earth’s Oldest Life II scale works. The Rise of the Eukaryotes This will allow us to talk about time divisions by name instead of having to talk about time in longhand (by number of ) !

Structure of •Eon – the greatest expanse of time Geologic time scale • – subdivision of Eon • Eons •Period – subdivision of Era – • – subdivision of Period (“visible life”) – the most recent Oldest undisputed eukaryotes Eons Periods Epochs eon, began about Well-developed 545 million years redbeds ago BIF peaks here – – Archean Oldest stromatolites

– Hadean – the Oldest rocks oldest eon Origin of earth Smaller divisions of time

1 Period – subdivision of an Geologic time scale era

Names derived from:

Era – subdivision of an eon 1. “Type” localities (e.g. , named after Jura Mountains) Eras of the Phanerozoic eon 2. Rock characteristics (e.g. , -rich rocks in the (“recent life”) UK) (“middle life”) 3. From various whims (“ancient life”) (e.g. , named after Celtic tribe of )

-in other words, a big mess ! But it works.

A major innovation that accompanied the rise of oxygen in the Earth’s hydrosphere and atmosphere was the rise of the eukaryotes.

The origin of the first eukaryotic cell is obscure, but it is clear that a number of features had to be gained by And Onward to Eukaryotes… ancestor prokaryotes.

1. Larger size 2. Membrane-bounded nucleus 3. Organelles (especially mitochondria and chloroplasts).

2 Increase in cell size Acquisition of Nucleus

As a general rule, eukaryotic cells are about ten bigger than Making a nucleus is not a huge deal. It has been prokaryotic cells. speculated that the nucleus was produced by the invagination of the cell membrane, and engulfing of genetic Prokaryotic cells: 1 -10 microns Eukaryotic cells: 50-500 microns material within the cell. Thus the transition from prokaryote to eukaryote must have involved an increase in size. In this model, the membrane of the nucleus was derived from the cell membrane.

Genetic material in chromosome-like clump

Infolded Largest known bacterium: 200 microns in dia membrane Thiomargarita namibiensis surrounds genetic material A prokaryote of this size may have given rise to the first eukaryote

Acquisition of Organelles Acquisition of Organelles

The origin of mitochondria and chloroplasts demands a Serial Endosymbiosis Hypothesis (Lynn Margulis) more complex explanation for that of the nucleus.

cyanobacteria The current belief is that both mitochondria and chloroplasts originated as either invaders or slaves within a large prokaryotic host cell. S plant cell

Mitochondria could represent oxidizing bacteria that later S animal cell became integrated with a host cell.

Likewise, chloroplasts may well represent cyanobacteria purple bacteria that later became integrated with a host cell.

3 Giardia: Protist with no mitochondrion -Likes anaerobic environment -eats anaerobic bacteria

Evidence for Origin of Mitochondia and Chloroplasts Why the Association in the First Place ? As “Slave Bacteria”

Mitochondria and Chloroplasts are of similar size as bacteria One can imagine that mitochondria could process the (about 1-10 microns) waste of the host cell, and produce energy that would Mitochondria and Chloroplasts have complex membrane benefit the host cell (so the proto-mitochondrion would be systems, similar to bacteria. handy to hold on to). Mitochondria and Chloroplasts are somewhat self-contained, Similarly, a chloroplast would use the waste gas from as if they derived from functional cells. aerobic respiration (carbon dioxide) and make food for the Mitochondria and Chloroplasts divide by binary fission, host cell (so again, why not freeload on the guest). similar to bacteria. Plus…the mitochondrial and chloroplast DNA are different than the host cell !

4 Early eukaryotes were very simple structures SEX

Of course the other major innovation in the eukaryotes was sexual reproduction, which allowed the splitting and Whip-like recombination of genetic material. remains similar in This, of course, provided a whole lot more variation in appearance to eukaryote populations than their prokaryote ancestors. algae The incredible diversity and complexity of eukaryotes is no accident ! Oldest Known Eukaryote Fossils: Bottom line: Eukaryotes have more fun that prokaryotes, Grypania spiralis but also enjoy being different ! carbonized algal structures 2 mm wide and 10 cm long Found in rocks 2.1 Ga in Michigan and abundantly in rocks 1.4 Ga in China, Montana and Michigan

No pain, no gain “Resting bodies” of planktonic algae are also known (acritarchs)

…but the birth of metazoans (complex multicellular eukaryotes with differentiated tissues) was to come after a global-scale that some people argue should have wiped all life off the face of the Earth !

Oldest known Acritarchs (algal cysts) up to 150 microns (0.15 mm) in diameter 1.6 to 1.4 Ga

5 Formation and Breakup of

Rodinia breaks up at about 750 million years ago Supercontinent Rodinia forms -continents clustered near at about 1100 million years ago equator 750-600 Ma Widespread Glaciation Believed to have triggered the “Snowball Earth” event (glacial deposits found on all continents) (when Earth froze over)

Glacial Deposits (Tillites) overlain by Carbonates The Snowball Earth

Snowball Earth Model: Runaway Icehouse Effect Paul Hoffman and Daniel Schrag, 1998 carbonates In most extreme scenario, almost entire Earth frozen over

Up to 4 major snowball events glacial deposits between 750 and 580 Ma

Skeleton Coast, Namibia

6 Prologue: Before the Snowball (about 770 Ma) Into the Ice House

Average global temperatures plummet to -50 oC shortly after the runaway Breakup of a single landmass (Rodinia) leaves small continents freeze begins. scattered near the equator. Oceans freeze ice to average depth of more than 1 km, limited only by heat Formerly landlocked areas are now closer to oceanic sources of slowly released from Earth's interior. moisture. Increased rainfall scrubs heat-trapping carbon dioxide out Most microscopic marine organisms die, but a few cling to life around volcanic of the air, eroding continental rocks more quickly. hot springs. Global temperatures fall, and large ice packs form in the polar With no rainfall, carbon dioxide emitted from volcanoes is not removed from the oceans. White ice reflects more solar energy than darker seawater, atmosphere. driving temperatures even lower. As carbon dioxide accumulates, Earth warms and sea ice slowly thins. Feedback cycle triggers strong cooling effect.

Snowball to Slushball From Freeze to Fry

Concentrations of carbon dioxide in the atmosphere increase 1,000x due to 10 million years of normal volcanic activity As tropical oceans thaw, seawater evaporates and works along with carbon Ongoing greenhouse warming effect pushes temperatures to the melting point dioxide to produce even more intense greenhouse conditions. at the equator Surface temperatures soar to more than 50 oC driving an intense cycle of As Earth heats up, moisture from sea ice sublimating near the equator evaporation and rainfall. refreezes at higher elevations and feeds the growth of land glaciers. Torrents of carbonic acid rain erode the rock debris left in the wake of the Open water in the tropics absorbs more solar energy and initiates a faster rise retreating glaciers. in global temperatures Swollen rivers wash bicarbonate and other ions into the oceans, where they form carbonate sediment.

7 Oldest evidence of animals Snowball or Slushball ? Possible animal embryos found in Doushantuo phosphorite (Weng'an region of South China): ~ 600 Ma - clusters of round cells inside a membrane A source of argument about the Snowball Earth model concerns the severity of the event.

Why wouldn’t the “freeze” and “fry” episodes completely exterminate life ?

This question is particularly relevant to the origin of metazoans, which appeared shortly after Snowball Earth “embryo” at “embryos” in earlier time. approximately the 256- stages of division cell division (about 0.5 millimetres in diameter) But the identity of these organisms is unclear

World’s Oldest Metazoan Preservation of Fauna at Mistaken Point, Newfoundland World’s Oldest Known Uranium-Lead Dates Metazoan: From zircons in ash: 565±3 Ma Charnia wardi Preservation Portugal Cove, of soft-bodied Newfoundland remains due 575 Ma to rapid Up to 2 metres long ! deposition of volcanic 10 Ma earlier than ash traditionally accepted date of origin of metazoa

Guy Narbonne and Jim Gehling An ideal situation: Exceptional preservation PLUS can be dated !

8 Oldest Known Complex Multicellular Organisms Evolution of Complex Life and Snowball Earth: At Portugal Cove, Newfoundland Cause-and-Effect or Coincidence ?

Mistaken Point biota: 565 Ma Portugal Cove biota: 575 Ma

Species of Charnia fronds Snowball Earth Modern Pennatulacean 575 Ma now known ! 750 to 600 Ma (Sea Pen a possible Portugal Cove South modern relative ?)

Mistaken Point

Possibilities/questions posed by timing of earliest complex metazoan and Snowball Earth

1. Complex metazoans evolved from unicellular eukaryotes REALLY fast (?620 to 600 million years provides only 20 million years for development of first known metazoa, and 620-575 million years provides only ~45 million years for complex metazoa to develop).

2. Metazoans evolved before or during Snowball Earth (perhaps just no record up until 575 Ma) ? If so, how did they survive on ice-covered Earth ?

3. Perhaps Snowball Earth wasn’t as severe as originally asserted by Hoffman and Schrag (to permit survival of life) ? Latest Proterozoic Perhaps a fair amount of open ocean at equator ? (~620 to ~545 Million Years Ago) Oldest Complex Metazoa (complex multicellular organisms) Result: Snowball Fight ! well established Many in favour of Snowball Earth concept “Ediacaran” or “Vendian” fauna BUT many against !

9 So the “Ediacaran fauna” record the rise of multicellular organisms The Ediacaran organisms are interesting in that Also the basis of a new geological period they show the first stages of complex Recognition of these latest organization. Proterozoic fossils have resulted in the construction of a new period

The Ediacaran Period was established in March, 2004 - the first period to be defined in since 1891 (when American Henry Williams divided the Carboniferous into two.

Type section of boundary: The “Rangeomorph” element is common to Enorama Creek, Flinders the many Ediacaran life forms. Range, Southern Australia

Fossil fractals Ediacaran Organisms: A Beginning or an End ?

Guy Narbonne (Queen’s University) believes that Ediacaran organisms may represent stem groups of the Phanerozoic metazoans.

Adolph Seilacher (Yale University) maintains that the Ediacaran organisms represent a completely unique phylum that went extinct at the end of the Proterozoic.

Current evidence is in Narbonne’s favour !

Note the “frond” pattern at both small-scale and large-scale levels (the pattern in self-repeating).

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