Bio 1B, Spring, 2008, Evolution section 1 of 3 Updated 4/3/08 8:47 AM Lecture 13 13 Precambrian and Paleozoic. Reading: 7th edition 516-526, 628-629; 6th edition 484-492, 642-644. Fossils are preserved remains or traces of living organisms. Fossils are found in sedimentary rocks. • Fossilization is not random. Hard parts — teeth, bones, shells — are most likely to be fossilized. Only under very rare conditions are soft body parts fossilized. Taphonomy is discussed in the laboratory on macroevolution. Trace fossils preserve some behaviors. • Footprints, tracks, burrows, and underground nests occasionally provide information about behavior, e. g. stride length, gait, etc. Some recent fossils can yield DNA for analysis • DNA, primarily mitochondrial DNA, has been extracted from many fossils. The current upper limit is roughly 100,000 years. Whether DNA can be extracted from a fossil depends on the conditions of preservation and storage. Heat and humidity degrade DNA rapidly. • Claims of sequencing DNA extracted from older specimens, including ancient bacteria, leaves, dinosaurs, and insects in amber, are not valid. Relative and absolute dates can be estimated Relative dates. • Relative ages are indicated by relative position. The principle of superposition, that older strata lie under younger strata, was recognized in the 18th century by mining engineers. • A stratum was and is recognized by finding index fossils, which are fossil species or groups of fossil species that together uniquely identify it. For example, trilobites are fossils that indicate the beginning of the Paleozoic era. • The names of the eras and periods were established by early in the 19th century. You have to learn the 4 eras and the approximate times of the boundaries) (Precambrian 542, Paleozoic 251, Mesozoic 65.5, Cenozoic 0, from Fig. 26.1 in 7th edition; dates in the 6th edition are slightly different but close enough). Absolute dates • Radioactive elements, such as carbon-14 and uranium-238 decay at rates that are measurable today. Using the uniformitarian assumption, you can convert the ratio of “parent” to “daughter” isotopes into an estimate of time, as illustrated in Fig. 26.7. • Different elements are used to estimate ages of fossils of very different age. C14 has a half-life of 5730 years, which makes it suitable for dating organic remains found at archaeological sites. N14 is the daughter isotope, which is otherwise very rare. U238 has a half-life of 4.5 billion years. The daughter isotope is Pb206.Bio 1B, Spring, 2008, Evolution section 2 of 3 Updated 4/3/08 8:47 AM Lecture 13 The broad features the fossil record are well understood. The first evidence of life appeared relatively early in the earth’s history. • Life evolved relatively soon after the earth cooled sufficiently. Fossils similar to modern stromatolites are found in rocks up to 3500 mya. • From 3500 mya to 2200 mya only prokaryotes were present. Fossilized impressions of cells indicate that they were the same size as modern prokaryotes (Bacteria and Archaea) and much smaller than any modern eukaryote. • The early atmosphere of the earth did not contain free oxygen (O2). Free oxygen in the earth’s atmosphere was created by photosynthesis in prokaryotes and, later, algae and plants. Photosynthetic bacteria had to have evolved by 2700 mya, because by then oxidized iron (rust) is found in the banded iron formations of that age. The first eukaryotic cells. • Cells of the same size as modern eukaryotic cells found in strata dated at 2200 mya. Multicellular organism • Multicellular organisms were present by 1200 mya. They appear to have been similar to simple modern algae. • More complex multicellular organisms were not present before 600 mya. • The true diversity of Precambrian soft-bodied organisms is difficult to assess both because of the difficulty in finding rock strata of the right age and because of the lack of material that could be fossilized. An important set of fossils known as the Ediacaran fauna (Fig. 32.5) found in several parts of the world shows that by 570 mya, soft-bodied multicellular animals had become relatively large and diverse. Some are similar to modern invertebrates such as sea pens and jellyfish but their relationships to modern species is still uncertain. Cambrian explosion. • In the early Cambrian, the earliest period in the Paleozoic era, diverse forms of animals with hard body parts first appeared. Representatives of all major modern phyla except the Bryozoa (ectoprocts, see Fig. 33.14a) were present by 540 mya. The Burgess Shale (Fig. 32.6) contains well-preserved early Cambrian fossils. • During the Cambrian, fossils of organisms that cannot be classified as belonging to any modern phylum appeared but did not leave descendents. Hallucegenia and Opabinia, both found in the Burgess Shale, are two of several such examples. • Three kinds of causes of the Cambrian explosion have been proposed: ecological (e. g. predation), geophysical (e. g. atmospheric oxygen), and genetic (e. g. evolution of the Hox gene complex). Many modern groups of plants and animals first appeared in the Paleozoic • By the mid-Paleozoic, bony fishes, amphibians and insects were present. By the late Paleozoic, reptiles, many modern orders of insects, and gymnosperms had appeared. Permian extinction • The largest mass extinction seen in the fossil record occurred at the end of the last period of the Paleozoic, the Permian. Approximately 90% of marine genera and more than 1/3 of the orders of insects went extinct. • The cause of the Permian extinction is not well understood. Several unusual events occurred at the end of the Permian but it is difficult to say which one of them is the cause: continents coalesced to form a single land mass, Pangaea; there were massive volcanic eruptions in China; and there are chemical signatures of meteorite impactsBio 1B, Spring, 2008, Evolution section 3 of 3 Updated 4/3/08 8:47 AM Lecture 13 (shocked quartz, which is formed when quartz crystals undergo a sudden pulse of great pressure, and high concentrations of iridium and other noble elements found in high concentration only in meteorites). Sample questions Which is the correct assignment of approximate dates to the ends of the eras listed? a. Precambrian
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