CHAPTER 25 THE HISTORY OF LIFE ON EARTH Lost Worlds Some Antarctic explorers discovered fossil evidence that showed life thrived in Antarctica the extreme opposite as it is today Fossil Evidence also showed that ancient organisms are completely different from the organisms that live today Fossil Evidence helps illustrate MACROEVOLUTION the broad pattern of evolution above the species level An example of macroevolution involves the emergence of terrestrial vertebrates through speciation 25 1 CONDITIONS ON EARTH MADE THE ORIGIN OF LIFE POSSIBLE Many scientists ask how the first living cells appear Scientists hypothesize that living cells came through this four step process This scenario although speculative allows us to make predictions using man made environments in the laboratory Synthesis of Organic Compounds on Early Earth Earth formed 4 6 billion years ago condensing from a cloud of dust and rocks which was surrounded by the young sun Earth was bombarded by rock and ice for the next 100 million years with the heat produced vaporizing all of the water on the sun The end of the bombardment ended 4 billion years ago and set the stage for life The atmosphere was initially oxygen poor and which with water vapor and other essential compounds and elements C N H As the earth cooled the water vapor condensed into oceans and much of the H2 escaped into space Scientists Oparin and Haldane believed that Earth s atmosphere was a reducing atmosphere which would have allowed organic compounds to form from simple molecules given energy from the sun Haldane also suggested the Oceans were key to the formation of organic molecules Miller and Urey tested the O H hypothesis and found that the ancient environment could have produced organic molecules A 2008 study found that numerous amino acids formed under conditions that simulated a volcanic eruption Another hypothesis states that organic molecules were produced in deep sea hydrothermal vents which releases water and minerals at temperatures of 300 400 C Other vents called alkaline vents release water at warm temperatures and at a pH of 9 11 Overall alkaline vents may have been more suitable for the origin of life The synthesis of organic molecules is possible under certain environmental conditions Meteorites could also have led to the origin of life because they contain more than 80 amino acids The Murchison meteorite also contained other key organic molecules including lipids sugars and nitrogenous bases such as uracil Abiotic Synthesis of Macromolecules Small organic molecules such as amino acids and nitrogenous bases is not sufficient for the emergence of life as we know it Every cell has many macromolecules which include enzymes proteins and nucleic acids A study showed that the abiotic synthesis could occur spontaneously from simple precursor molecules without the help of enzymes or ribosomes Researchers have also been able to produce polymers of RNA nucleotides and amino acids by dripping them on to sand rock and clay Protocells All organisms must be able to carry out both reproduction and energy processing metabolism DNA molecules carry genetic information and the instructions for accurate replication Because of the necessity of enzymes and building blocks provided by the cell s metabolism this suggests that replicating molecules and a metabolism like source of building blocks may have appeared together Vesicles may have been able to provide these environmental conditions because they can metabolize and reproduce simply Also Vesicles can reproduce when lipids and other organic molecules are added to water Vesicles can also form a selectively permeable bilayer just like the one in our cells and can perform metabolic reactions using external reagents Self Replicating RNA The first genetic material was most likely RNA and not DNA because they can function as enzyme catalysts ribozymes Some ribozymes can make complementary copies using nucleotide building blocks Natural Selection has produced ribozymes capable of self replication and the ribozymes that could reproduce left the most descendent molecules They carried the genetic information about the vesicles that carried them In addition RNA could have provided a template for DNA which is more stable and could be replicated less accurately DNA thus provided an advantage as genomes grew larger through gene duplication and other processes and as more protecells became coded The descent of DNA from RNA ultimately allowed for the diversification of life 25 2 THE FOSSIL RECORD DOCUMENTS THE HISTORY OF LIFE The fossil record opens a window into the world of long ago and provides glimpses of the evolution of life over billions of years THE FOSSIL RECORD Sedimentary rocks are the richest source of fossils As a result the fossil record is based primarily on the sequence of rocks that have accumulated on sedimentary rock The fossil record shows there has been a great change in the kinds of organisms on earth They also show how new groups of organisms arose from previous ones However the fossil record is incomplete as many organisms that did die did not die in the right place to preserve as a fossil Furthermore geological fossils could have destroyed fossils Therefore the fossil record is biased in favor of species that existed for a long time and had hard shells skeletons or other parts that helped with fossilization HOW ROCKS AND FOSSILS ARE DATED Fossils can tell us about relative aging by determining the sequence they were laid down but they do not give us an absolute time One of the most common techniques used to find the absolute age of a fossil is radiometric dating which is based on the decay of radioactive isotopes The rate of decay is expressed by the half life Fossils contain isotopes of elements that accumulated in organisms when they were alive Organisms have carbon 12 and radioactive carbon 14 By measuring the ratio of C 12 to C 14 the exact age can be determined for organisms that lived up to 75 000 years ago THE ORIGIN OF NEW GROUPS OF ORGANISMS Fossils can help illustrate new features that arise and how long it takes for such changes to occur Fossils illustrate how mammalian jaws and teeth evolved 25 3 KEY EVENTS IN LIFE S HISTORY INCLUDE THE ORIGINS OF UNICELLULAR AND MULTICELLULAR ORGANISMS AND THE COLONIZATION OF LAND The study of fossils helped geologists establish a geologic record which divides Earth s history into four eons and further subdivisions The first 3 eons are the