Chapter 94: LUCY, Really?!

However, during the Proterozoic, a large evolutionary step occurred with the appearance of eukaryotes. 

Evolving around 2.1-1.6 billion years ago, eukaryotic cells are more complex with cell organelles and a nucleus with more complex DNA replication and regulation, mitochondria for additional energy, and chloroplasts to perform photosynthesis and produce energy.

Certain organelles even have their own DNA, like mitochondria. Eukaryotes are the branch of the tree of life that gave rise to fungi, plants, and animals.

About 1.2 billion years ago, another important event in Earth's biological history occurred when some eukaryotes invented reproducion.

By sharing genetic material between reproducing individuals (male and female), evolutionary change was enhanced by increasing genetic variability. This allowed more complexity among individual organisms, and eventually, ecosystems.

It is important to realize that the Proterozoic land surfaces were barren, at least of plants like grasses, trees, and animals.

Geologic processes were active just like today, but the application of the Uniformity Principle requires the realization of differences in the environments in which the processes operate.

For example, rain and rivers were present but erosion on barren land surfaces would have operated at different rates than on modern land surfaces protected by plants.

The Ediacaran fauna (635.5-541 million years ago) offers the first glimpse at these evolving ecosystems toward the end of the Proterozoic.

These organisms were among the first multicellular life forms and may have been similar to soft jellyfish or worm-like organisms. 

Since the Ediacaran fauna did not have hard parts like shells, they are not well preserved in Proterozoic rocks. However, studies suggest that they were widespread around the earth. 

Scientists still debate how many of these are extinct evolutionary dead-ends or the ancestors to modern biological groups.

The transition of life from the soft-bodied Ediacaran forms to the explosion of forms with hard parts at the end of the Proterozoic and beginning of the Phanerozoic made a dramatic difference in our ability to understand earth history and the history of life.

After hearing upto the Proterozoic Eon Jia felt as if she was listening the narration of the movie named LUCY which she had watched some years back.

After the Protreozoic it was time for the last Eon, The Phanerozoic Eon. The span of geologic time extending about 541 million years from the end of the Proterozoic Eon (which began about 2.5 billion years ago) to the present.

The Phanerozoic, the eon of visible life, is divided into three major spans of time largely on the basis of characteristic assemblages of life-forms: the Paleozoic (541 million to 252 million years ago), Mesozoic (252 million to 66 million years ago), and Cenozoic (66 million years ago to the present) eras.

Although life clearly originated at some time, probably quite early, in the Archean Eon (which lasted from 4 billion to 2.5 billion years ago), not until the Phanerozoic did a rapid expansion and evolution of forms occur and fill the various ecological niches available. 

The key to that great Phanerozoic expansion appears to lie in the development of plants able to carry out the photosynthetic process and thus release free oxygen into the atmosphere.

Before that time, Earth's atmosphere contained negligible amounts of free oxygen, and animals, in which energy transfers involving the process of respiration are critical, were unable to develop. 

During the Phanerozoic, Earth gradually assumed its present configuration and physical features through such processes as continental drift, mountain building, and continental glaciation.

Thus, although the Phanerozoic Eon represents only about the last one-eighth of time since Earth's crust formed, its importance far exceeds its relatively short duration.

The Cambrian Period begins the Phanerozoic Eon, the last 542 million years during which fossils with hard parts have existed. 

It is the first division of the Paleozoic Era (542Ma -251Ma). Marine animals with mineralized skeletons make their first appearance in the shallow seas of the Cambrian, though only "small shelly fossils" (tiny shells, spines and scales from early metazoans) and trace fossils are preserved for the first ten million years or so.

In the "Cambrian explosion" of metazoan diversity most animal groups appear over the short span of the following ten million years. All of the invertebrate phyla as well as the chordates are apparently established by the end of the Period.

A major early Cambrian event was the transformation of the seabed. Early Cambrian sea floors, like late Proterozoic (Ediacaran) seafloors, were covered in microbial mats with an oxygen-free, sulfide-rich, hard layer of mud just below the surface.

The animals (metazoans) of the Cambrian Explosion were organized into a unique marine Cambrian fauna, one of three recognized marine fauna of the Phanerozoic. This faunal ecosystem was mostly deposit feeders with nearly all animals living near the surface of the sea bottom. Most of these metazoans are living on, attached to, or making shallow borrows in the sea bottom. Even suspension feeders, which were uncommon, such as brachiopods, echinoderms and the reef-building archeocyathids, make their livings near the seafloor. Trilobites dominate from the Cambrian explosion to the endof the Cambrian, comprising 80-90% of the skeletonized remains. Most benthic1 trilobites were apparently epifaunal2 deposit feeders.

The Cambrian is unique in the fossil record in the number of Lagerstätten, deposits where soft-body parts and soft-bodied organisms are preserved.

These deposits provide a unique view of the extraordinary diversity of the Cambrian fauna, as only 5-10% of the organisms preserved in them would have been fossilized under normal conditions. Cambrian lagerstätten are preserved in part due to the unique chemical conditions resulting from the minimal levels of burrowing during ths Period.

Fossil deposits including soft-bodied organisms, such as the Lower Cambrian Chengjiang deposits in China and the Middle Cambrian Burgess Shale in Canada, are still dominated by trilobites.

Most of the other Chengjiang and Burgess Shale organisms were also deposit feeders, though a few soft-bodied predators were preserved. Overall, Cambrian animals are skewed towards epifauna or infauna with even suspension feeders clustered close to the sea bottom.

The Cambrian Period may be divided into three divisions: Lower (Early), Middle, and Furongian (Late).

As noted above, trilobites are the most common fossil types, and these dominant animals of the Cambrian seas characterize this Period. Each division of the Cambrian is identified with particular trilobite genera. Nearly 75% of trilobites and other animals, including the reef-building Archeocyathids, vanished in a great mid-period extinction event when shallow seas withdrew.

When the shallow seas returned an even greater diversity of Cambrian animal life resulted, again filling the oceans with a wide variety of exotic organisms. After nearly 54 million years, The Cambrian ends with another major extinction event. Nearly 75% of trilobite families and 50% of sponge families disappeared at this time. 

The unique Cambrian evolutionary fauna continues through the Paleozoic, but the Paleozoic fauna quickly come to dominate. The few remaining organisms of the Cambrian fauna are finally lost in the great Permian extinction events.

Globally, the Cambrian was a time of warm climate, while exhibiting strong provincialism among its fauna. Tectonically the Cambrian saw the opening of the Iapetus Ocean and the separation of  the Launtentia, Baltica and Siberia plates.

The Ordovician lasted about 45 million years and saw the transition from very primitive to relatively modern life-forms in the seas. The "Ordovician radiation" which followed the late Cambrian extinctions, lead to a tripling of marine diversity, the greatest increase in the history of life, and giving the highest levels of diversity seen during the Paleozoic Era.

As a result, all of the common invertebrate fossil groups and a few vertebrates were present by the end of this period. Starfish, brittle stars, crinoids, and echinoids, all of which have living representatives, first appeared in the Ordovician. In the evolutionary history of animal life this radiation was second only to the "Cambrian explosion" in importance. 

The new Paleozoic fauna created by the "Ordovician radiation" dominated the seas for the next 230 million years. Pandemic species of planktonic graptolites and conodontes appear in the fossil record during this Period. 

Their world-wide distribution and evolution during the Ordovician make them key species for correlating fossil deposits. Unlike in the Cambrian, most animal evolution in the Ordovician involved refining existing body plans rather than developing new ones.

Bryozoans, the last animal phyla to appear in the fossil record, have the only new body plan, and they may have evolved in the Cambrian, but only became mineralized, and thus left fossils, in the Ordovician.

The Ordovician saw the replacement of the Cambrian marine fauna composed largely of deposit feeders, such as trilobites, with the Paleozoic fauna dominated by filter feeders arranged in tiers.

Tiered suspension feeder communities attached to the sea floor dominate the new Paleozoic ecosystems: brachiopods filter bottom waters, corals and branched bryozoans filter water just above, and crinoids filter water at the highest level.

The Paleozoic fauna was also characterized by a move away from the interface of the ocean and ocean floor—animals both learned to bore deeper into the substrate and to invade the pelagic (in the water column ,not on shore or the bottom) regions of the oceans. 

Grazing gastropods increased in importance while efficient predators like sea stars and cephalopods evolved. Remnant Cambrian fauna, such as deposit feeding worms and trilobites, continued with diminishing importance throughout the Paleozoic.

Tectonically, the Ordovician saw a rapid reorganization of plates around the Iapetus Ocean. Today's southern continents were fused into the supercontinent of Gondwana, while others remained independent. Plate movement resulted in a migration of Gondwana from an equatorial position toward the South pole, as a result the South Pole moved from North Africa to central Africa.

The movement of Gondwana to the South Pole resulted in global cooling and the most severe ice age of the Phanerozoic Eon. Sea levels rose through much of the Ordovician to the highest sea levels of the Paleozoic before receding. The period ended with a strong lowering of sea levels (a strong negative eustatic event) as high latitude glaciation occurred.

There were significant fluctuations in climate with prolonged periods of warm "hothouse" conditions with periodic cold, "icehouse" intervals and turnovers of the oceans. High levels of CO2 early in the Period lead to very high temperatures—oceans may have reached 113°F (45°C). These high temperatures may have delayed animal diversification in the early Ordovician. Later, ocean temperatures decreased gradually to those seen in modern equatorial waters.

The Ordovician ended with the second greatest extinction event of the Phanerozoic Eon. Two massive reductions, a million years apart, created this event—resulting in the extinction of more than 60% of invertebrates. This event is associated with sea level changes and glacier formation. Only the great Permian extinction event was more devastating to Earth's biosphere.