Chapter 12, Lesson 1-2 How did life begin? Age of the earth: 4.5 bya From a ball of fiery, molten rock, the Earth cooled over 500 million years Water vapor in the early atmosphere condensed and fell as rain Shallow oceans formed on Earth

Measuring Earth’s Age Radiometric Dating Estimating the age of an object by measuring its content of radioactive isotopes Isotope-form of an element with an atomic mass that is different from other atoms of the element Radioisotopes are unstable isotopes that break down and give off energy called radiation

Measuring Method As radioisotopes break down, radioactive decay results in other isotopes that are stable (e. g. potassium-40 becomes argon-40 and calcium-40) The time it takes for one-half of a given amount of radioisotope to decay is called the isotope’s half-life Scientists measure proportions of radioisotopes and their products of decay

Formation of Basic Life Chemicals Molecules of nonliving matter probably reacted chemically during the first billion years of Earth’s history ( sun, volcanic heat) These chemical reactions produced many different, simple organic molecules (molecules that contain carbon) The hypothesis that organic molecules necessary for life can be made from nonliving matter has been tested and supported by recent laboratory experiments

Competing Models Primordial Soup 1920s Oparin, Haldane Earth’s oceans filled with organic molecules Gases in atmosphere (ammonia, hydrogen, methane, water vapor) reacted chemically and fell into oceans This hypothesis tested by Miller and Urey in the 1950’s Bubble Model 1986 Lerman Undersea volcanos produced gases (ammonia, methane) that became trapped in bubbles underwater Gases became concentrated and reacted Bubbles burst at surface to release organic molecules into the atmosphere

Problems… Reductant molecules could not have been in abundance/Early Earth---no ozone UV radiation would have destroyed ammonia and methane So, if not present in the atmosphere…where did these chemicals come from? Ocean vents? ???

Forming Cells The biochemistry of cells is complex How did simple organic molecules link to form these highly complex molecules? Scientists have not been able to make proteins or DNA form spontaneously RNA, a second type of nucleic acid found in all cells and some viruses today, can form spontaneously in water

Role of RNA Certain RNA molecules can act as enzymes and catalyze chemical reactions Cech and Altman (1980s) hypothesized that RNA was the first self-replicating information storage molecule and it directed the formation of proteins Since RNA is self-replicating, it could have evolved and changed over generations

Other Chemical Findings Lipids (fats, oils) form tiny droplets whose surface resembles cell membranes Chains of amino acids can gather in these small droplets = microspheres Linked amino acids and sugars can gather in another type of droplet = coacervates First step toward cellular organization?

Evolution of Cellular Life, Section 2 Prokaryotes Unicellular Microscopically tiny No nucleus No membrane-bound organelles Eubacteria=true bacteria Archaebacteria=old bacteria Eukaryotes Most are multicellular Protists are only unicellular eukaryotes Large and specialized Nucleus Many membrane-bound organelles Protists, Fungi, Plants, Animals

Prokaryotes Oldest fossilized cells (2.5 bya) Cyanobacteria= photosynthetic bacteria Eubacteria are bacterial cells that cause disease and decay Archaebacteria are bacteria that live in extreme earth environments (similar to those on prehistoric Earth) anaerobic forms, extreme temperatures, volcanic vents, sulfur and mineral springs

Eukaryotes Later in fossil record, 1.5 bya All eukaryotic cells have mitochondria Protists and plants have cholorplasts Mitochondria and chloroplasts are about the size of a prokaryote and they contain their own DNA Endosymbiosis (Margulis) Theory that mitochondria and chloroplasts are descendants of symbiotic eubacteria

Evidence for Endosymbiosis Mitochondria are about the same size as most eubacteria 2 membranes Circular DNA similar to bacterial cells Contain genes that are different from the DNA in the nucleus Reproduce by simple fission Chloroplasts are similar in size to cyanobacteria Surrounded by 2 membranes Have thylakoids Have circular DNA Have genes that are different from DNA in nucleus Reproduce by simple fission

Multicellularity Protists---oldest eukaryotes, 700 mya Multicellular life has advantages—specialized cells for food capture, movement, protection Three protistan ancestors evolved to produce very successful groups of multicellular organisms: Fungi, Plants, Animals

Origin of Modern Life Most animal groups alive today probably originated during a brief period of time lasting 10 to 100 million years = Cambrian Explosion Cambrian period is a time of great evolutionary expansion due to changing geological and atmospheric conditions New habitats = new opportunities Burgess Shale (1909) is a rich collection of Cambrian fossils found in Canada that includes strange animals unlike anything alive today

Mass Extinctions First Mass Extinction Second Mass Extinction Third Mass Extinction Fourth Mass Extinction Fifth Mass Extinction 440 mya 360 mya 245 mya 210 mya 65 mya