Chapter 14 The History of Life

Earth is 4.6 billion years old After the Bang, earth formed from hot gases and debris Atmosphere formed from gases spewing from volcanoes. No oxygen, nitrogren, water vapor, carbon dioxide, hydrogen sulfide, hydrogen cyanide – toxic to life as we know it. 4.4 billion years ago, earth began to cool and oceans formed from condensing water vapor causing millions of years of rain. Meteorites containing organic molecules were colliding with earth.

Life originated on earth between 3.9 and 3.4 billion years ago. The fossil record is indirect proof of the age of the earth. Fossils are the remains of past life on earth. Fossils are found in sedementary rock. Usually only hard body parts can make fossils, therefore there are many missing links in the fossil record.

How do fossils help us understand the History of the earth? By comparing the fossils of past life with today’s life we can make deductions about what the environment was like in the past. For example, if fossils look like plants or animals that are found today in tropical regions than that may tell a scientist that the climate in that was once tropical. Scientists who study the remains of life past are called paleontologists.

To determine the order of the appearance of life on earth, we must be able to date the fossils

Relative Dating. In undisturbed layers of sedimentary rock, the rock layers at the surface are younger than those that are deeper and therefore the fossils found in the top layers must also be younger than the fossil found in lower layers.

Relative dating cannot give an exact age; it can only arrange the fossils in order of appearance on earth.

Radiometric dating Radiometric dating is also called Absolute Dating, because it can give the actual date of rocks in which the fossils have been found and an actual age of the fossils. Radiometric dating uses the fact that radioactive isotopes found in rocks will decay at a set rate. This is called a half-life. In one half-life, half of the radioactive isotopes will have changed into a non-radioactive form . By looking at how much of the radioactive isotope is left, scientist can determine the age of the rock and/or fossil.

Some common isotopes used to date rocks and fossils. Carbon 14 will decay into nitrogen 14 with a half-life of 5730 years. Potassium 40 will decay into Argon 40 with a half-life of 1.3 billion years. Carbon 14 is used to date fossils less than 50,000 years old. Potassium 40 can be used to date the oldest rocks on earth.

Geologic Time The Geologic Time Scale is divided by the organisms that lived during the time interval. It is based on the evidence of fossils and the earth’s rocks. Geologic Time is divided into four long ERA’S. Precambrian, Paleozoic, Mesozoic, and Cenozoic. We are now in the Cenozoic Era. You are not responsible for the various periods of the geologic eras.

Mass Extinctions Mass extinction is the loss of 70% or more of the species on earth. Mass extinction has occurred five times in the history of the earth. Many scientists believe that we are in the midst of the sixth and greatest mass extinction, due to human activity.

The fossil record supports the Theory of Continental Drift. The Theory of Continental Drift suggests that the continents have moved during earth’s history and continue to move still at about the rate of six centimeters a year. Plate techtonics explains how this happens; plates float upon a layer of molten rock that allow them to move.

Origins of Life Spontaneous Generation Spontaneous Generation is the idea that life can be produced from non-life. This was commonly believed in the middle ages, when knowledge was based on observation.

Two scientists’ experiments disproved spontaneous generation Two scientists’ experiments disproved spontaneous generation. Francesco Redi and Louis Pasteur

Redi’s Experiment

Pasteur’s Experiment

Spontaneous Generation Disproved Redi disproved spontaneous generation for large organisms. Pasteur disproved spontaneous generation for microorganisms. Biogenesis – life can only come from other life now the accepted theory.

How did life on Earth begin? Hypothesis: small organic molecules formed and then became organized into more complex organic molecules. 1930’s OPARIN hypothesized that the conditions of early earth’s atmosphere and oceans, with energy from the sun and lightening, resulted in a primordial soup in which chemical reactions occurred that could have produced the molecules of life.

Miller and Urey tested Oparin’s Theory

Results of Miller and Urey’s experiment The chemicals found in the flask showed several kinds of amino acids, sugars, and other small organic molecules. Sydney Fox went on to show that if these molecules were heated without oxygen, the would link and form more complex molecules. It is believed that these molecules became trapped in bubbles that acted like membranes forming structures called Protocells.

First Organisms on Earth Fossils of the first organisms are about 3.8 billion years old and are similar to Archeabacteria. They were prokaryotes. They were heterotrophs, living on the primordial soup. They were anaerobic, because there was no oxygen in the atmosphere.

Autotrophic Bacteria change the planet. Early autotrophs were chemiosyntheic. They used hydrogen sulfide to provide energy to make food. Photosynthetic bacteria called cyanobacteria produced oxygen that changed the atmosphere. Oxygen allowed the development of the ozone layer that protected earth from ultraviolet radiation.

First Eukaryotes First eukaryotes appear around 2.1 billion years ago. They are thought to have developed from the merging of prokaryotes seeking protection from oxygen. The nucleus developed from the merging of their DNA. Other organelles developed through symbiotic relationships with other ancient prokaryotes.

Endosymbiotic Theory

Proofs of Endosymbiosis Mitochondria and Chloroplasts Have separate DNA that is like prokaryote DNA Can reproduce independent of the cell and do so in the manner that prokaryotes. They have ribosomes similar to prokaryotic ribosomes. They are the same size as prokaryotes. They have two membranes: inner like prokaryotes, outer like eukaryotes.

Chapter 15 The theory of evolution

Charles Darwin and the Theory of Evolution by Natural Selection 1831 Charles Darwin took a job as a naturalist on a five year trip around the world on the ship the Beagle. On his trip Darwin collected and studied specimens from around the world. His trip to the Galapagos Islands led him to consider that life has changed over time. He began to form his theory.

Darwin needed more evidence. Lyell – geologist who showed that the earth was very old; this was needed to provide time for evolution to occur. Malthus - economist who studied the human population and proposed that the human population was kept under control by war, famine, and disease. Darwin used this idea and the basis for competition and selection. Artificial selection – Darwin raised pigeons and realized if he could select the best traits to breed than nature could also – natural selection.

How Natural Selection Works Organisms produce more offspring than can survive. Amongst the offspring, variations exist. Competition for resources occurs. Those with the most useful variations survive and reproduce, passing their traits to the next generation. These traits become more common in the population.

Evidence for Evolution Adaptations – Organisms seem to have variations that match their environment. Camouflage: Organisms blend in with their environment. Mimicry: Harmless species looks like another harmful species.

Direct Evidence: Physiological Resistance Direct Evidence can be observed. Antibiotic resistance in bacteria. Herbicide and pesticide resistance.

Fossil Evidence

Anatomical Evidence Homologous Structures: Structures that share a common ancestor, but they may look different because the organisms evolved in different environments. Analogous Structures: Structures found in organisms that do not have common ancestors, but may look similar because evolved in similar environments. Vestigial Structures: Structure that has no function in present day organisms, but was probably useful in the past.

Vestigial Structures

Embryological Evidence

Biochemical Evidence All life shares DNA as its hereditary material. The closer related organisms are, the more similar the amino acid sequences in their proteins. Cytochrome c is a protein that has been studied to show evolutionary relationships.

Mechanisms of evolution

Definitions: Population – all members of a species within a given area. Species - organisms that look similar and can breed together and produce fertile offspring. Gene pool - all the alleles within a population Allelic frequency - percentage of any specific allele within a population.

Gene Pool

Genetic Equilibrium In genetic equilibrium the frequency of alleles remains unchanged over generations. When a population is in genetic equilibrium, no evolution is taking place.

Mechanisms for Genetic Change Mutation - may be good or bad; mutations that are lethal will decrease in the population; mutations that increase fitness will remain. Genetic Drift - the random removal of alleles from the population. Gene Flow - individual members of the population leave or new individuals enter, changing the allelic frequency. All of these can affect small populations significantly.

Genetic Drift Gene Flow

Natural Selection Three types of natural selection that act on variation: Stabilizing - favors the average individuals in a population, reduces variation. Directional - favors one of the extreme variations of a trait in a population. Disruptive - favors both extremes and tends to eliminate the intermediates.

Speciation: the evolution of new species Speciation results when members of similar populations can no longer interbreed to produce fertile offspring within their natural environments. A population may be broken into smaller ones by physical barriers. This is called Geographic Isolation. The new groups adapt to new environments and become reproductively isolated.

Reproductive Isolation can result from: Different mating seasons Different mating displays Genetic material has changed so much, it no longer matches. Polyploidy - in plants, errors in meiosis produce multiple sets of chromosomes, preventing fertilization.

Rates of Speciation Gradualism - change occurs through small changes over long periods of time. Punctuated Equilibrium - change occurs in rapid bursts when the environment changes and is followed by periods of no change. Both can be supported by the fossil record.

Patterns of Evolution Adaptive Radiation - new species develop from a common ancestor as they adapt to new niches in a new environment. Divergent Evolution - Species that were once similar adapt to new environments and change as they adapt. Associated with Homologous structures. Convergent Evolution - Unrelated species evolve similar traits in similar environments; associated with Analogous structures.

Divergent Evolution

Convergent Evolution

Adaptive Radiation