Bio 1B Lecture Outline please print and bring along Fall 2007 B D Mishler Dept of Integrative Biology 2 6810 bmishler berkeley edu Evolution 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 Earth Solar 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 temperature at the surface was around 850C Origin of life Stanley 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 1 Miller 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 and metabolism 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 Fossils fossil 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 2 Fossils 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 Fossils Intact The pollen was preserved intact because no decomposition occurred Compression Sediments accumulated on top of the leaf and compressed it into a thin carbonrich 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 at the 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 3 bracketing 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 uranium and 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 epochs Eras See Table 26 1 7th Table 25 1 6th Precambrian 4 6 billion to 542 Myr Paleozoic 542 to 251 Myr Mesozoic 251 to 65 Myr Cenozoic 65 Myr to present These eras are not evenly spaced time intervals but are based on major changes in the fossil record the beginning of an abundant fossil record at the start of
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