Earth History Using relative dating and radiometric dating, geologists have pieced together A history of planet Earth.the calendar is the Geologic Time Scale. The Geologic Time Scale has irregular episodes (not like the modern calendar of days, weeks and months). 1. Eons are the longest spans of time covering half a billion years or more. There are only two eons, the Precambrian (characterized by only single-celled organisms, or the total absence of life) and the Phanerozoic, meaning visible life indicating that fossils became larger and easier to see. 2. Eras are shorter, covering hundreds of millions of years. The eras consist of the Paleozoic or old life Era, the Mesozoic or middle life Era, and the Cenozoic or new life Era. 3. Periods are the most common division of time, usually lasting several tens of millions of years in duration. 4. Epochs are the shortest t division, i i covering several million to thousands of years in length. http://soest.hawaii.edu/coasts/cgg_main.html Phanerozoic The Geologic Time Scale 1
Placing a boundary - The Cretaceous-Tertiary Boundary 65 Ma Rocks record the event Rocks of the Cretaceous Period (144 to 65 million years ago) are found beneath rocks from the Tertiary Period (65 to 1.8 million years ago). At many places in the world where these rocks are found, the boundary of the two rock units is marked by clay. Known to geologists as the K/T boundary, this time is characterized by the extinction of the giant lizards, the dinosaurs. The clay often contains four special features: small glass spherules, high concentrations of the element Iridium, and minute crystals of shocked quartz, the total extinction of numerous land and marine-based animals. An asteroid measuring 6 to 12 miles in diameter struck the Yucatan Peninsula. The K/T impact resulted in 100 million megatons of energy released in dust, water vapor, fire, shock waves, sound, and tsunami as well as excavating a crater over 200 kilometers across. 93% of marine reptiles and 56% of land-based reptiles became extinct, however, non-avian dinosaurs and pterosaurs had 100% extinction. On land, nothing bigger than 25 kilograms survived. Survivors included; snakes, small Lizards, crocodiles, turtles, salamanders, frogs, mammals and birds. 2
(thank god!) "The Shiva Hypothesis" describes a 26 million year cycle of mass extinctions over the past 540 million years. One hypothesis is that this corresponds the solar system oscillating through the galactic plane as it orbits the Milky Way. Rampino notes that t the last crossing of the galactic plane occurred a few million years ago and it has been suggested that this led to a disturbance of comets in the Oort Cloud, some of which could now be approaching the inner solar system. Another theory holds that our Sun has a companion star that returns on a regular cycle and disrupts objects in the Oort Cloud or the asteroid belt and leads to impacts. Nemesis Hypothesis http://jan.ucc.nau.edu/~rcb7/globaltext.html 3
The fossil record Nicholas Steno (Danish) 1638-1686 Was the first to suggest that figured stones looked like living organisms and had in fact once been living. James Hutton (Scottish) 1726-1797 Strong proponent that geologic processes alter Earths surface. Dominant view at the time was that Earth was unchanging all rocks had been formed by precipitation or sedimentation from a great ocean called Neptunism. Hutton argued that geologic time had been indefinitely long and that Earth was like a self-renewing machine, as mountains eroded away, new ones were uplifted, as the sea covered some lands it receded from others. Perhaps Hutton s most lasting contribution to the science of geology is the concept of Uniformitarianism which states that the past history of Earth is best explained by our observation of modern processes. That is, geologic principles have been uniform over time. Sir Archibald Geike (1835-1924) famously summed up uniformitarianism as the present is the key to the past. He is famous for his quote that Earth has no vestige of a beginning no prospect of an end. 4
Charles Darwin (1809-1882) provided the general theory that accounted for changes seen in the fossil record, the Theory of Evolution. Fossils are the remains of animals and plants, or the record of their presence, preserved in the rocks of Earth. Fossilization is the process that turns a once living thing into a fossil. There are lots of fossils to be found, but only a tiny number of all the animals and plants that ever lived have been fossilized. His Theory of Evolution, described in the Origin of Species (1859) and The Descent of Man (1871), stated that all living things developed from very few simple forms through the process of natural selection. Natural selection is the tendency for certain populations of the same species to have the best chance of surviving and transmitting their genes to the next generation because they possess favorable variations. Skin of Dinosaur embryo - 1989 Take a look at your body. Which parts are most likely to become fossilized? Teeth, hard bones, and nails are the obvious candidates. The same is true of other animals. The occasional discovery of fossils with soft parts is rare and exciting but in most cases it is hard parts that are preserved. Darwin observed that all living things reproduced at high rates and yet no one group of organisms had been able to overwhelm Earth s surface (other than cockroaches!). In fact, the actual size of any population tends to remain fairly constant over time. This led Darwin to conclude that not all individuals in a generation will survive hence nature must select those with favorable variations. Natural selection was the mechanism he proposed by which evolution occurred. Selective pressure Environmental changes from forests to grasslands across Europe and North America -teeth -hoof -speed and strength -brain size -lengthened jaws Phylogeny ancestral lineage Evidence of evolution Similarity in bone structure possessed by diverse organisms as birds, swimming mammals, four legged animals, humans and insect-eating reptiles. Homologous structures suggests these animals have evolved from a common ancestor and that survival pressure (natural selection) has preferentially selected for the specific functions served by each limb. In four-limbed vertebrates, limb bones may vary in size and shape but they are reproductions of one another in terms of the number and position of specific bones. How would totally t different families of organisms, under specific and fundamentally unique selective pressures, tend to converge on a single basic blueprint for limb design unless a common ancestor was involved? 5
Vestigial structures- the result of evolution under selective pressure. Vestigial structures are the vestiges of body parts that had been used by ancestral forms but are now useless. Whales possess a vestigial pelvis and femur originally designed for walking. Proof that modern whales have evolved from walking ancestors was found in 1994 in the form of a fossil whale with front legs designed as flippers and long hind limbs with elongate toes for webbed feet. The boa constrictor also displays vestiges of legs. How many vestigial structures do you have? Embryology- The early stage of development of embryos of various fish, birds and mammals display strikingly similar characteristics. Human embryos have tails, and gill slits as do other live forms in their early stages. It is thought that all these animals inherited basic sets of genes from distant common ancestors that control early embryologic development. Later, as development progresses, other genes assume control and lead to individual species. A virus can evolve faster than the medical community is able to design medicines to fight it. In scientific parlance, the virus "escapes" drug therapy. Aids evolves AIDS is caused by a virus called HIV, Human Immunodeficiency virus. A virus is a living microscopic organism that lives in a cell of another living thing. Being alive, can evolve or change, and antibodies designed to attack one form of the virus find themselves useless against the new form. This is why vaccines have not succeeded in eliminating the common flu or AIDS for example. Effective vaccines against particular strains of flu virus constantly need to be updated for this reason. 6