Bio 1B Lecture Outline (please print and bring along) Fall, 2007B.D. Mishler, Dept. of Integrative Biology 2-6810, [email protected] lecture #7 -- Early earth, continental drift, early life -- Nov. 19th, 2007(ch.25: 492-493; ch. 26: 510-533)• Summary of topics• Early Earth• Origin of life• Fossils -- their kinds & how they are made• Division of eras into periods and epochs• Continental drift (plate tectonics)• Early EarthSolar System: Our sun formed from the collapse of a cloud of gas and dust in the outskirts of the Milky Way Galaxy about 4.6 BYA. This cloud began to rotate, and lumps in it began to stick together, eventually forming planets (which continued collecting debris)Impacts: were a major feature of early earth; they became less frequent about 4.0 - 3.8 BYA as the extra matter in the solar system was mostly collected into the planets.Appearance of life: definitely present 3.5 BYA; perhaps as early as 3.9 BYA. Definitive evidence is presence of fossil cells of bacteria. Suggestive evidence of a slightly earlier origin in the isotopic signal of life in the rocks (carbon-12 enrichment, more later).Early Earth had no atmosphere, the first real atmosphere developed along with the first ocean about 4.0 - 3.8 BYA and was composed of CO2 and many other compounds such as hydrogen, nitrogen, methane, hydrogen sulfide, and ammonia (some serious greenhouse gases -- temperatureat the surface was around 850C! ).• Origin of lifeStanley Miller: studied chemical reactions simulating those of early earth (1950's).early atmosphere of earth: as described above, plenty of organic compounds and water vapor, but no free oxygen was present. Much volcanism, lightening, and UV radiation. Evolution #7, pg. 1Miller's experimental results: Within a few hours the system contained numerous simple organic compounds. In water these were rapidly converted into amino acids, simple acids and other compounds, and because these molecules are relatively stable, they quickly accumulate in solution.Comparable results are obtained, including production of DNA and RNA, under a variety of conditions, provided that free oxygen is absent. Thus, once earth cooled enough for water to condense and form oceans, molecules of many kinds formed spontaneously, and they probably accumulated until they reached relatively high concentrations.Life may have emerged from non-life in such an anaerobic "primordial soup", and non-living materials became ordered into molecular aggregates that were eventually capable of replication andmetabolism. Because of the presence of oxygen in our atmosphere now life cannot spontaneously occur. And even if it did, something now living would probably eat it!Life may also have evolved around deep-sea vents, or it may have resulted from extraterrestrial sources.• Fossilsfossil: any remains, impressions or trace of a living thing of a former geologic age, as a skeleton, footprint etc. (Fig. 25.4 (7th) (Fig. 25.1 6th)).Most species that have ever lived left no fossils.Most fossils that have formed have been destroyed.Only a minute fraction of existing fossils have been discovered.Most fossils are formed from the hard parts of animals and plants, such as shells, bones, teeth, or wood.Fossilization is a very chancy process and happens rarely.The burial process is usually by sand or mud washed down by water, or in a desert sandstorm. Most fossils occur in sedimentary rocks – sandstone, clays, shales, chalk.Animals and plants have also been preserved in peat and coal (swampy plants), oil (tiny plankton plants), tar, ice, and amber (the resin of ancient trees), and may still have DNA.Evolution #7, pg. 2Fossils may be virtually unchanged from the originals (rare), or they may be mineral replacements, casts or molds (impressions), "mummies", or impressions of the skin, feathers, and some soft tissues may be preserved.Eggs, footprints, and burrows can be fossilized.Some animals are more likely to be fossilized than others, e.g., those with shells.Four types of FossilsIntact - The pollen was preserved intact because no decomposition occurred.Compression - Sediments accumulated on top of the leaf and compressed it into a thin carbon-rich film.Cast - The branch decomposed after it was buried. This left a hole that filled with dissolved minerals, faithfully creating a cast of the original.Permineralized - The wood decayed very slowly, allowing dissolved minerals to gradually infiltrate the cells and then harden into stone.What do fossils tell us?bones: size and shape of animal, muscle attachment sites and size;defense mechanisms: horns, claws, etc.;teeth: diet;footprints: movement of animals, speed, whether or not they lived in herds, etc.;eggs: fossil eggs can reveal evidence of nesting and parental behavior in animals;skin: type of skin, armor plates in dinosaurs, etc.Principle of Superposition, Steno's Law: in an undisturbed sequence of rocks, the oldest layer is atthe bottom and the youngest is at the top (see Figs. 25.3 and 22.3 (7th) (Figs. 22.3 and 22.4 (6th))).index fossils: the rock strata at one location can often be correlated with strata at another location by the presence of similar fossils.radioactive dating: a method of determining the age of fossils and rocks using half-lives of radioactive isotopes (Fig. 26.7 (7th) (Fig. 25.2 (6th))).When molten rock cools, forming what are called igneous rocks, radioactive atoms are trapped inside. Afterwards, they decay at a predictable rate. By measuring the quantity of unstable atoms left in a rock and comparing it to the quantity of stable atoms in the rock, scientists can estimate the amount of time that has passed since that rock formed.Evolution #7, pg. 3bracketing the fossils: fossils are generally found in sedimentary rock, not igneous rock. Sedimentary rocks can be dated using radioactive carbon, but because carbon decays relatively quickly, this only works for rocks younger than about 50 thousand years.In order to date most older fossils, scientists look for layers of igneous rock or volcanic ash above and below the fossil. Scientists date igneous rock using elements slower to decay, such as uraniumand potassium. By dating these surrounding layers, they can figure out the youngest and oldest that the fossil might be; this is known as “bracketing” the age of the sedimentary layer in which the fossils occur. • Division of eras into periods and epochsEras: See Table 26.1 (7th) (Table 25.1
View Full Document
Unlocking...