Chapter 18

» We’ve identified over 300 characteristics that a planet needs to support life. Lose one, and the planet is barren

» The earth formed approximately 4.6 billion years ago (bya). ˃ The growing gravitational energy from our sun pulled space debris in until it

began to collect in the orbit of the future planet.

˃ As debris piled up, it personally increased its gravitational pull, attracting the rest of the debris

˃ Intense heat at the core from this gravity caused the stratification of layers and “sphere-ing”

˃ Volcanic magma became trapped, super-heated, erupted, and slowly piled up and hardened on top of these layers, forming the crust

˃ Eventually the gravity grew strong enough that gasses could no longer fully escape. They bonded together to form an atmosphere

˃ This atmosphere contained basic inorganic chemicals like water vapor, nitrogen, and carbon dioxide

» The elements on our planet all originated from the sun or other stars ˃ Only stars have the necessary heat and pressure to fuse protons, neutrons and

electrons together

» These elements still had to come together, but how?

» Stanley Miller’s famous experiment in 1953 recreated conditions on the primitive earth and proved that, in this environment, monomers could form from atoms

» Since 1953, however, our understanding of the primitive earth conditions have changed, challenging Miller’s results ˃ Miller’s experiment was based on the assumption that early Earth contained

mostly water vapor (H2O), hydrogen gas (H2), methane (CH4) and ammonia (NH3)

˃ Now we’re not sure whether the carbon source was CO2 and whether the nitrogen source was N2 or NO3.

» Early-earth stories are all hypotheses. Only small amounts of evidence exist about the earth 1-4.5 bya.

» Whatever the mechanism, we know the following must have occurred: ˃ Inorganic molecules and elements must have grouped together

to form polymers

˃ Lipids, the simplest biomolecule, would’ve been first to form because they are both structural and metabolic

+ Of course, there’s the chicken and the egg argument…

˃ Individual protocells, all contributing one or two functions, must have merged to form the first cell

˃ Cells were able to merge molecules into one of four categories: structure, metabolism, energy, and information

» Macroevolution is large-scale changes of populations over long spans of time

» The best way to view the spans of time are in strata, or layers of sedimentary rock that have been exposed.

» The strata give us clues to what organisms lived, in what time period, what natural disasters occurred and when, and how long time periods went without any significant change in the environment

» Fossils are bones, footprints, burrows, casts, or any other preservation that gives details of the past

» Besides giving us anatomical and behavioral clues about the organisms of the past, they also allow us to chemically estimate age of various organisms

» Fossils of different animals found in the same layer of strata most likely lived on the planet at the same time

» We can determine the age of a fossil based on half-life’s. ˃ A half-life is the amount of time isotopes of elements take to decay (lose

electrons) from one isotope to the other

» Carbon has a half-life of ~5715 years. It was the first element to be used for dating properties.

» A sample of a fossil containing C14 can be measured to see how it’s C14 content compares to other isotopes ˃ If 50% is C14 and 50% is other isotopes, it’s assumed that the fossil has

undergone one half-life.

» Carbon dating is only accurate to around 50,000 years old.

» Anything older, we use nitrogen or potassium dating, which is capable of fairly accurate dating periods of up to 1.3 bya

» The Precambrian is the term for the period of time between earth’s formation 4.5 bya and 600 mya (around 87% of the geologic timescale)

» During this time, the first prokaryotic organisms emerged (probably archaea).

» No continents existed during this time most likely, and shallow beaches were covered in boulders called stromalites

» Stromalites are boulders that contain cyanobacteria on their outer surface

» Cyanobacteria are photosynthetic and produce oxygen.

» With no respiring organisms, the oxygen saturated the oceans ˃ Glycolysis and chemosynthesis were the likely energy systems

» Over hundreds of millions of years, the oxygen would have secreted out of the ocean and into the atmosphere

» In the upper atmosphere, the collected oxygen would have formed molecules of O3, or ozone

» The ozone would help filter out UV light from the sun, allowing for the eventual formation of land organisms

» This would give rise to the first eukaryotic cells

» The endosymbiotic theory says that a nucleated cell engulfed a group of prokaryotes which then became organelles.

» Some evidence for this includes ˃ Organelles with plasma membranes

˃ Organelles with DNA and ribosomes (mitochondria, chloroplast)

˃ Organelles found on their own outside of cells in certain environments

» At the end of the Precambrian, the first multicellular protists and soft-bodied invertebrates emerged.

» The Paleozoic era lasted 300 million years, from 550 mya to 245 mya

» Three major mass extinctions occurred during the Paleozoic, separating the era into six periods

» A mass extinction is the disappearance of a large number of species (usually 50% or more) or taxonomic group in a relatively small interval (a few million years)

» Modern animal’s ancestors may have existed prior to the Cambrian, but the fossil record doesn’t support this fact yet.

» The Cambrian is the first sign of skeletal components. The major explanation for the development of skeletons are the need to protect from predation

» Dominant organisms of the period include trilobites, a group of arthropods with thick skeletons along their entire bodies

» Skeletons make it hard to extract oxygen from water, which may have played a role in the movement of organisms from water to land

» The movement to land most likely began with algae migrating from salt-water beds to fresh water.

» The freshwater would seep onto the land and carry algae with it, allowing algae to deposit nutrients

» Aquatic plants and fungi would then have the necessary nutrients to begin the process of growth and extracting water in soil

» The first migrators: seedless vascular plants, mosses, horsetails and ferns

» While skeletons would’ve allowed arthropods and invertebrates to move on land, they couldn’t have done it without the nutrients from these plants

» Insects are the first in the animal kingdom to show any fossil history on land

» This would be followed by the evolution of fish climbing onto the surface of land and practicing breathing through direct intake of oxygen rather than extraction

» In the oceans, meanwhile, the evolution of fishes included development of fleshy fins, sharks, lobed fins, and even amphibians

https://www.youtube.com/watch?v=NdpDNx2p67E

» Nonflowering seed plants called gymnosperms survived the mass extinction at the end of the Paleozoic. ˃ Cycad plants, with large palms and cone seeds mainly

» According to records in rocks, this time period would’ve been unusually hot and dry compared to other time periods on earth.

» This would’ve allowed the oxygen levels to be around 30% instead of the 14% it is now

» The extra oxygen could support much larger plant and animal life

» The Mesozoic era—particularly the Jurassic period, was the age of flying pterosaurs, giant apatosaurs, and theropods.

» Birds most likely began here as well with the emergence of archaeopteryx

» In the third period, the cretaceous, dinosaurs began to group in herds.

» The triceratops, tyrannosaurus rex, and the first mammals arrive on the scene

» What set mammals apart from other organisms was the presence of hair and warm bodies.

» They also produced milk, which meant mothers—temporarily—provided food for their children from their own reserves

» The appearance of hooves allowed animals to migrate greater distances at greater speeds

» Angiosperms, or flowering plants, emerged as well.

» The first primates, organisms adapted to living in flowering trees, appeared

» The climate also began to drastically cool near the end of the Cenozoic.

» The final epoch of the period, called the pleistocene, is more commonly known as the ice age.

» 1/3 of the land on Earth was covered in glaciation, snow and/or ice.

» The first mammoths, wolves, bison, wooly rhinos, mastodons, sloths, and humans all emerged

» Humans have no natural advantage to fighting other organisms or extremes in weather. Yet they survived

» Humans must have been some of the most resourceful organisms in history ˃ Almost as soon as human fossils are found, the discovery of hides, tools,

wheels, weapons, and other items show up too

» While life was evolving, the earth may also have been evolving too

» Continental Drift is the theory that the earths crust shifts and moves over time

» What once was one large continent called Pangaea is now the continents we know today

» Every portion of the earth, land and water, sits on top of a plate.

» These plates are constantly drifting into each other

» Where tectonic plates separate and divide, the result is high levels of volcanic activity or black smokers

» Where tectonic plates press against each other, the result is high levels of earthquakes

» Where tectonic plates have crashed into each other over the eons, the impact created large mountain ranges

» At least five mass extinctions have occurred throughout history. ˃ The biggest being at the end of the Permian period, where 90% of all species

disappeared

» Theories include meteorites, extreme results of continental drift, stagnation of currents, and chain reactions of extinctions when lower trophic levels die out

» Most of the details in this chapter do have evidence to support them. This is the subject of biology that we have the least information and basis on though.

» It will probably forever be simply a guessing game.