Precambrian Era: Geologic Time

• Geologic time is the broad span of time encompassing Earth's geological history, serving as a calendar for events.
• The Geologic Time Scale subdivides all time into units of abstract time, in descending order of duration eons, eras, periods, epochs, and ages

Precambrian Era

• Extends from 4.6 billion years ago to the beginning of the Cambrian Period (541 million years ago).
• Encompasses the Archean and Proterozoic eons, and the informal Hadean Eon.
• Represents more than 80% of the total geologic record.
• Life forms in earlier rocks were grouped together in the Precambrian.
• Detailed mapping revealed primitive life forms existed approximately 3.5 billion years ago.
• Precambrian microfossils resemble:
• Algae
• Cysts of flagellates
• Tubes of filamentous organisms
• Stromatolites (sheetlike mats precipitated by microorganisms)
• Late Precambrian marked the evolution of the first multicellular organisms and the development of sexual division.
• Conditions at the end of the Precambrian set the stage for the explosion of life at the start of the Cambrian.
• Earth was already more than 600 million years old when life began.
• The planet had cooled down from its original molten state, forming a solid crust and oceans from water vapor.
• Primordial seas may have given rise to life, with volcanic vents catalyzing chemical reactions on water bubbles, leading to cell membranes.
• Other bubbles may formed self-replicating substances by attracting chemicals.
• Microscopic bacteria were the earliest living organisms, appearing in the fossil record as early as 3.4 billion years ago.
• Bacteria sought alternatives which led to photosynthesis that ultimately led to the advent of plants.
• The Earth's atmosphere was virtually devoid of oxygen three billion years ago.
• Oxygen was released as a byproduct of photosynthesis by cyanobacteria about 2.4 billion years ago.
• Oxygen reached about 1% around two billion years ago.
• Oxygen reached about 21% around 800 million years ago.
• The oxygen-rich ozone layer was established, shielding the surface from solar radiation.
• The first multicelled animals appeared in the fossil record almost 600 million years ago, known as the Ediacarans.
• They grew on the seabed and lacked heads, mouths, or digestive organs.
• The Ediacarans may be ancestors of later animals or may have been wiped out by extinction.
• Earliest multicelled animals that survived included sponges, cnidarians, and annelids.
• Sponges lacked organs or a nervous system, filtering food particles from water.
• Cnidarians (sea anemones, corals, jellyfish) had sac-like bodies and a simple digestive system.
• Annelids (segmented flatworms) had fluid-filled body cavities.

Cenozoic Era

• The most recent of the three major subdivisions of animal history
• Spans only about 65 million years and the end of the Cretaceous Period to the Present
• Also called the Age of Mammals.
• The Cenozoic (65.5 million years ago to the present) is divided into the Paleogene, Neogene, and Quaternary periods.
• The Paleogene is subdivided into the Paleocene, Eocene, and Oligocene epochs.
• The Neogene is subdivided into the Miocene and Pliocene epochs.

Neogene Period

• Spans 20.45 million years from the end of the Paleogene Period (23.03 Mya) to the beginning of the Quaternary Period (2.58 Mya).
• Mammals and birds evolved into modern forms.
• Early hominids, the ancestors of humans, appeared in Africa.
• The connection of North and South America at the Isthmus of Panama occurred late in the Pliocene.
• The world's climate cooled and continental glaciations resulted.
• Continents were close to their current positions at this time.
• The Isthmus of Panama was formed, connecting North and South America.
• The Indian subcontinent collided with Asia, forming the Himalayas.
• Sea levels fell, creating land bridges between Africa and Eurasia and between Eurasia and North America.
• Marine and continental flora and fauna have a modern appearance.
• Dominant terrestrial vertebrates were Mammals and birds.
• The cooler, seasonal climate led to deciduous plants replacing others.
• Grasslands replaced many forests.
• Herbivorous mammals evolved alongside grasses.

Paleogene Period

• A geologic period and system that spans 43 million years from the end of the Cretaceous Period (66 Mya) to the beginning of the Neogene Period (23.03 Mya).
• The global climate began a cooling and drying trend but it was disrupted by warm periods.
• The formation of the Antarctic Circumpolar Current significantly lowered oceanic water temperatures.
• Continents continued to drift closer to their current positions.
• India collided with Asia, forming the Himalayas.
• Africa moved north to meet with Europe, forming the Mediterranean Sea.
• Australia separated from Antarctica and drifted toward Southeast Asia.
• Mammals underwent rapid diversification.
• Dinosaurs died out.
• Primates evolved in the trees, the group to which humans belong.
• Cooling in the Oligocene led to a floral shift and the rise of modern plants.

Mesozoic Era

• Mesozoic Era is also known as the Age of Reptiles and the Age of Conifers.
• Lasted almost 180 million years, it includes the Triassic, Jurassic, and Cretaceous periods.
• Mass extinction occurred at the beginning and end of it.
• The transition from the Paleozoic Era to the Mesozoic Era was the greatest extinction, wiping out about 95% of all marine life and 70% of land life.
• Dinosaurs became the lords of the earth.
• The earths continent's, Pangaea was were transformed to the continents as we know them.
• Pangaea became Laurasia and Gondwana.
• The Atlantic Ocean began to grow.
• Laurasia split into North America and Eurasia.
• Gondwana became South America, Africa, Australia, Antarctica, and the India subcontinent.
• The climate was unusually warm.
• The dominant land plant species were gymnosperms that are similar modern day angiosperms.

Triassic Period

• The Triassic Period marked the beginning of changes throughout the Mesozoic Era for:
• Distribution of continents
• Evolution of life
• Geographic distribution of living things
• All major landmasses were combined into the supercontinent Pangea.
• Terrestrial climates were warm and dry, though monsoons occurred over large areas.
• At the end of the Triassic, plate tectonic activity picked up, and continental rifting began.
• Shallow seas dwindled but became had become more extensive as sea levels rose, thus colonizing the waters of continental shelves.
• The Triassic event followed the heels of the largest mass extinction, when 85% to 95% of marine invertebrate species and 70% of terrestrial vertebrate genera went extinct.
• During recovery in the Triassic, the importance of land animals grew. Reptiles grew, and the first dinosaurs appeared.
• The end of the Triassic saw the appearance of mammals.
• Another episode of mass extinction occurred.

Jurassic Period

• Ranges from 200 million to 145 million years ago.
• It features three major epochs: the Early Jurassic, the Middle Jurassic, and the Late Jurassic.
• The Early Jurassic spanned from 200 to 175 million years ago.
• The climate was tropical, much more humid than the Triassic.
• Plesiosaurs, ichthyosaurs, and ammonites were abundant in the oceans.
• Dinosaurs and other archosaurs became the dominant race on land, with theropods.
• True crocodiles evolved, and the first true mammals evolved, remaining relatively small.
• The Middle Jurassic spans from 175 to 163 million years ago, during which dinosaurs flourished.
• The Late Jurassic spans from 163 to 145 million years ago.
• The first avialans, like Archaeopteryx, evolved from small coelurosaurian dinosaurs.
• The increase in sea levels opened up the Atlantic seaway.

Cretaceous Period

• The last and longest segment of the Mesozoic Era.
• It lasted approximately 79 million years, from about 145.5 million years ago to about 65.5 million years ago.
• The development and radiation of flowering plants was one of the hallmarks.
• The oldest angiosperm fossil is Archaefructus liaoningensis, which used to be thought that the same time pollinating insects evolved.
• The Cretaceous is the Age of Dinosaurs".
• The breakup of the world continent Pangea, which began to disperse during the Jurassic, continued.
• The end brought the end of many previously successful and diverse groups of organisms.
• Dinosaurs moved through forests of ferns, cycads, and conifers.
• Saw the first appearance of flowering plants.

Paleozoic Life

• It is dominated by sea life.
• Fossil records provide no evidence that simple fungi and freshwater environments existed.
• Fossils of trilobites are dominated in Cambodian rock forms, whereas brachiopods predominate the strata during the Ordovician through the Permian Period.
• Leafless vascular plants (psilophytes) and invertebrate animals (centipede-like arthropods) were established on land in the Silurian time.
• Vertebrates animals made transitions via the evolution of amphibians.
• Conquest of the land during the Carboniferous was possible, in which waterbourne spores in plants with seeds replaced amniote eggs with protective shells of reptilian origins.
• The flight was achieved during the Carboniferous Period as insects evolved wings.
• The Permian extinction eliminated major invertebrate species as 95% perished, yet the extinction rates were lower.