Chapter 8 The History of Life on Earth

Radioactive Half-Life The radioactive half-life for a given radioisotope is the time for half the radioactive nuclei in any sample to undergo radioactive decay. After two half-lives, there will be one fourth the original sample, after three half-lives one eight the original sample, and so forth. radioactive

Half-Life The amount of time for an unstable substance to reduce its mass by one half

Graph of Radioactive Decay The radioactive half-life gives a pattern of reduction to half in any successive half-life period. half-life

See Board for Graph A.The unstable atoms in a sample of volcanic rock containing a trilobite fossil has a half-life of 1.3 billion years. The sample contained 4 mg. of unstable atoms when the rock formed. B.After 1.3 billion years, or one half-life, only 2 mg. of the unstable will be left in the rock, 2 mg. of its stable decay product have formed. C.After another 1.3 billion years (2 half-lives), there will only be 1 mg. of the unstable atoms and 3 mg of the decay atoms. D.How many milligrams of unstable atoms are left after 3 half-lives?

Geologic Time Scale Paleontologists have divided Earth’s time scale into Eras. –Eras are characterized by the dominant animal of the time Precambrian – First unicellualr and multicellular orgs. Paleozoic – early plants and animals Mesozoic - dinosaurs Cenozoic - now

Mass Extinctions There have been several periods in Earth’s history when a large number of species died out at the same time called mass extinctions. Some scientists believe the mass extinction of the dinosaurs occurred when a meteorite struck the Earth and caused catastrophic climate changes

"The impact was the equivalent of so many nuclear bombs it’s incomprehensible." The crater, now buried under more than a half a mile (one kilometer) of limestone deposits, was blasted out by the impact of either a meteorite or comet perhaps 10 miles (16 kilometers) across. Scientists think its size and timing make it the smoking gun that would explain the extinction of the dinosaurs - as well as 70 percent of all species living at the end of the cretaceous period. "The impact was the equivalent of so many nuclear bombs it’s incomprehensible," said Virgil "Buck" Sharpton, one of two lead scientists on the project.

The Changing Earth

Chapter 8 continued

The Changing Earth Dinosaur fossils have been found in every continent, including Antarctica. Pangaea (“All Earth”) – a supercontinent which existed in prehistoric times consisting of all the continents pieced together like a puzzle.

Continental Drift The continents move 1-10 cm per year. Plate Tectonics – the continents and oceans ride on top of huge pieces of the Earth’s crust.

Precambrian Time Started with formation of the Earth (4.6 billion years ago) and ended 540 million years ago. During this era life began. Early Earth –Atmosphere lacked oxygen –Frequent meteor hits –Constant volcanic eruptions –Intense radiation from the sun

How did Life Begin? Hypothesized that under these conditions life developed from non-living matter. –The chemicals that combined included water, clay, dissolved minerals, and atmospheric gases –These new molecules floated in the oceans for millions of years, slowly joining together to form larger molecules. Hydrothermal vents

Eventually prokaryotic organisms developed –They were anaerobic – they did not require oxygen (there was no free oxygen available in the atmosphere –Early cyanobacteria used photosynthesis a byproduct of which is oxygen.

Radiation Shield As atmosphere filled with oxygen a protective ozone layer formed. Ozone absorbs UV radiation –Thus allowing life to exit the oceans

Paleozoic Era 540 million years to 240million years ago. Sponges, corals, snails, clams, squids, trilobites, fishes, sharks, etc. Greening of Earth – plants, fungi, and air-breathing animals developed The First Animals on land appear to be crawling insects. At the end of era – reptiles, winged insects, cockroaches, and dragonflies Largest Mass Extinction at the end of this era – 90% of all marine species died out.

Mesozoic Era 248 million years to 65 million years ago. The Age of Reptiles Burst of evolution with surviving reptiles Dinosaurs, first birds, large forests, flowering plants Ends with another mass extinction

Cenozoic Era 65 million years ago through now Mammals Mastodons, saber-toothed tigers, cats, camels, sloths, small horses, humans

Human Evolution Theorized that humans evolved over millions of years from a distant ancestor that is common to apes and monkeys. Human beings are classified as primates Humans are also hominids – we walk upright on 2 legs = bipedalism All hominids are now extinct except for humans

Human skeleton vs. Gorilla skeleton