- Outline major events in the history of life that have had significant effects on the rock record.
- Outline important events in the history of major groups of animals and land plants chronologically.
- The origin of life
- The origin of photosynthesis
- The origin of sediment binding organisms
- The origin of hard parts
- The origin of land plants
- Atmospheric composition.
- Early atmosphere was result of volcanic outgassing of volatiles.
- Highly reducing atmosphere. Rich in CO2.
- Some free oxygen derived from photochemical dissociation by UV in upper atmosphere. 2 H2O + uv --> 2 H2 +O2This would have generated enough oxygen quickly to oxidize chemical building blocks of life near ocean surface.
- Oceanic conditions. Initially strongly acidic, precluding precipitation of carbonate rocks. This was because atmospheric CO2, dissolved in sea water, forming carbonic acid.
- Land conditions:
Here is one spot of optimism. By 3.8 g.a., we have continental conglomerates like those of Isua, Greenland (right), showing that streams of water were flowing on surface.
The origin of life:
Current thinking maintains that life probably originated in hydrothermal vents. These environments were:
- Sheltered from free oxygen, which is toxic
- Rich in thermal energy
- Interestingly, phylogenetic analyses suggest that among the most primitive organisms are thermophylic bacteria. Their special features:
- Live only at near-boiling temperatures
- Obtain energy from exotic reactions involving materials readily available in minerals, esp Sulfur.
- Find oxygen to be toxic.
- The environment in which these conditions are routinely found is near deep sea hydrothermal vents. Currently, these seem like the most likely locations for the origin of life.
- First recognizable microscopic fossil organisms: Archean (~3.5 g.a.)
We can't tell from looking at microscopic fossils which were photosynthesizers, but photosynthesis had momentous consequences for the rock record
Banded Iron formations (BIFs) :
Late Archean - Early Proterozoic (~3.0 - 1.8 g.a.) Cherts with alternation of gray and rust red bands of hematite (Fe2O3).
- BIFs first appear when photosynthesizers start cranking out free oxygen. This oxygen reacts with oxygen sinks like iron ions dissolved in sea water to form mineral oxides. NOTE: This first oxygen does not make it into the atmosphere. It is immediately removed from the oceans by being captured in oceanic mineral precipitates like BIFs.
- When all of the oxygen sinks (i.e. the dissolved iron and other ions waiting to be oxidized) are oxidized and removed from solution, BIFs no longer form. Their disappearance indicates the saturation of these sinks and the beginning of accumulation of high concentrations of oxygen in atmosphere. The deposition of BIFs and the buildup of oxygen in the atmosphere overlapped for a while. A geochemical method developed by UMD geologist James Farquhar indicates that oxygen first appeared in detectable quantities in the atmosphere about 2.4 g.a. (More info)
- Terrestrial red bed deposits:
Iron oxide cemented continental sediments begin to appear at this time. This tells us that free oxygen was now building up in the atmosphere and was available to oxidize terrestrial sediments.
- Ozone: As it accumulated, free oxygen in upper atmosphere recombined to form ozone layer (O3), allowing life to colonize surface waters.
- Oceanic acidity: Of course, by eating up atmospheric CO2, photosynthesizers caused the acidity of the oceans to diminish, allowing the direct precipitation of carbonate rocks for the first time. Once that was possible, CO2 concentrations fell very rapidly. as carbon became locked up in rock.
- Respiration: As oxygen was generated, some organisms evolved the ability to use it to release energy from sugars they had eaten. To such aerobic critters, oxygen became a necessity rather than a poison. Today anaerobic organisms are restricted to margial environments. The basic formula for respiration:C6H12O6+ 6 O2 ---> 6 CO2 + 6 H2O + energyLook familiar? It's just photosynthesis run backwards. In this case, the energy powers cell activities.
Stromatolites (The Age of Slime): Beginning about 3.0 g.a., we begin to see fossil stromatolites - laminated bacterial mats.
- These form when sediment falls onto a thin film of bacteria. The bacteria bind the sediment, and grow up through it. At any moment, only the top layer is alive.
- When there was nothing around to eat them, stromatolites were very common. Today, they only live in hypersaline environments that exclude other critters, like Shark Bay, Australia
- 1.5 g.a.: The first fossils of more complex (eukaryotic) cells are found. These are larger cells with nuclei and specialized organelles.
- 0.85 to 0.6 g.a.: The Snowball Earth episode
An series of ice ages which seem to have frozen most of the ocean surface at times. Life, perhaps, took refuge in hydrothermal environments.
- 0.6 g.a.: The first multicellular organisms. It's tempting to think that the Snowball Earth episode somehow catalyzed this evolutionary jump, but we don't really know. Maybe a critical level of oxygen concentration had been reached.
Note: plants, fungi, animals, and several groups of algae achieved multicellularity independently.
The earliest good fossil fauna is the Ediacaran fauna from the Ediacara Hills of Australia, about 0.6 g.a. Contains critters that mostly can't readily be classified into any familiar groups. One familiar morph: the jellyfish, is present.
But note: These creatures were entirely soft bodied. We only know them from the impressions they made in the sediment.
- Chemical change: A threshold concentration in sea water of calcium and carbonate ions had been reached that enabled organisms to start building skeletons out of it.
- Predation: Somewhere, somehow, some animal had evolved the ability to eat other animals and for the first time there was a reason to evolve armor.
During the Early Cambrian, the diversity of readily fossilizable animals with large hard parts explodes. By the end of the Cambrian, almost all of the familiar major modern animal phyla with hard parts are represented and predators like Anomalocaris were most definitely present. At this point, living things start contributing large volumes of calcite, opal, silicate, and phosphatic material to the formation of biogenic sedimentary rocks.
Result: A totally new depositional environment - the carbonate shelf.
Reef builders: Some organisms modify their environment even more radically, forming reefs: fixed upright calcareous structures that trap other sediments and create a complex environment for other organisms. Throughout the Phanerozoic Eon, reef builders have been at work, although the exact critter acting as the dominant reef former changed over time.
The effect: Life remodeled its environment to its own benefit.
One final issue. During the Proterozoic and early Paleozoic, the physical topography of the Earth seems to have been different from what we know today in two related ways:
- Exposed land surfaces tended to be eroded very flat very quickly.
- Marine depositional environments often contain layers of sediment that are very thick and extensive compared with those of later times.
Arguably, these were effects of the total absence of any kind of vegetation on land to bind sediment to the ground and limit erosion. The appearance of land plants, therefore, had a profound effect on rates of erosion, transport, and sedimentation.
The first evidence of vascular plants - plants that don't have to be in a permanently moist environment - is from the Silurian. These were small plants that would have grown in environments like heaths or peat bogs.
- As soon as plants were established, animals emerged from the oceans to dine on them. The first land animals were arthropods, including ancient scorpions, centipedes, and millipedes.
- Tree-sized vascular plants appear at the end of the Devonian. They create a more complex land biota - the first forests. At this time, fish-like vertebrates with fingers and toes - the first suggestion of land vertebrates - appear.
- Since the Devonian, plants have pursued more and more refined strategies for living in drier and drier environments, with the result that the Earth has become progressively greener during the Phanerozoic.
Again and as always, life altered its environment to its own advantage. (Of course, if it didn't, we wouldn't be having this conversation.)
Key concepts and vocabulary:
- Archaean environments and Origin of life
- Reducing CO2 atmosphere
- Trace amounts of O2
- Flowing water on continents preserved at Isua
- Hydrothermal vent environments
- First performed by cyanobacteria roughly 3 Ga.
- Releases O2 into oceans
- O2 binds to iron in solution to form banded iron formations (3.0 - 1.2 Ga)
- Reduces CO2
- This raises oceanic pH (less acidic) making precipitation of calcite possible
- In turn much CO2 removed from atmosphere
- Terrestrial red beds appear as BIFs diminish because oceanic "oxygen sinks" like free iron are filled and oxygen accumulates in atmosphere.
- Atmospheric oxygen forms ozone, which shields Earth's sruface from UV radiation.
- Organisms evolve that can use free oxygen in respiration
- Snowball Earth episode preceeds multicellular organisms
- Ediacaran fauna (latest Proterozoic) of soft-bodied animals
- First animals with hard parts near Proterozoic - Phanerozoic boundary
- Cambrian explosion
- Proliferation of cretters with CO2 skeletons results in new environment - carbonate shelf
- Paleozoic plants colonize land, permanently altering patterns of weathering and erosion.
- Big message: Organisms radically change Earth environments and typically modify them to their benefit.