Lecture chapter 25 the history of life on Earth 1 17 12 Lost Worlds Past organisms were very different from now Fossil record shows macroevolutionary changes over large time scales including The emergence of terrestrial vertebrates The origin of photosynthesis Long term impacts of mass extinctions Conditions on early Earth made the origin of life possible Chemical and physical processes on early Earth may have produced very simple cells through a sequence of stages Abiotic synthesis of small organic molecules Joining of small molecules into macromolecules carbohydrates proteins lipids nucleic acids Packaging of molecules into protobionts Origin of self replicating molecules DNA RNA Synthesis of organic compounds Earth was formed about 4 6 billion years ago with the rest of the solar system Earth s early atmosphere contained water vapor and chemicals released by volcanic eruptions Nitrogen Nitrogen Oxides CO2 Methane Ammonia Hydrogen Hydrogen Sulfide A I Oparin and J B S Haldane hypothesized that the early atmosphere was a reducing environment Stanley Miller and Harold Urey conduced lab experiments that showed that the abiotic synthesis of organic molecules in a reducing atmosphere is possible 1953 Instead of forming in the atmosphere the 1st compounds may have been synthesis near submerged volcanoes and deep sea vents Amino acids have also been found in meteorites Lecture Chapter 25 11 19 12 Abiotic synthesis of macromolecules and protobionts Small organic molecules polymerize when they are concentrated on hot sand clay or rock Replication and metabolism are key properties of life Protobions are aggregates of abiotically produced molecules surrounded by a membrane or membrane like structure Protobions exhibit simple reproduction and maintain and internal chemical environment Experiments demonstrate that protobionts could have formed spontaneously from abiotically produced organic compounds For example small membrane bounded droplets called liposomes can form when lipids or other molecules are added to water Self replicating RNA and the Dawn of Natural selection The first genetic material was probably RNA not DNA RNA molecules called ribozymes have been found to catalyze many different reactions For example ribozymes can make complementary copies of short stretches of their own sequence or other short pieces of RNA Early protobions with self replicating catalytic RNA would have been more effective at using resources and would have increased in number through natural selection The early genetic material might have formed and RNA world The fossil record documents the history of life The fossil record reveals changes in the history of life on earth Sedimentary rocks are deposited into layers called strata and are richest source of fossils Few individuals have fossilized and even fewer have been discovered The fossil record is biased in favor of species that Existed for a long time Were abundant and widespread Had hard parts How rocks and fossils are dated Sedimentary strata reveal the relative ages of fossils The absolute ages of fossils can be determined by radiometric dating A parent isotope decays to a daughter isotope at a constant rate Each isotope has a known half life the time required for half the parent isotope to decay Carbon 14 2 more neutrons 5 000 6 000 year half life max 75 000 yrs old For older fossils some isotopes can be used to date sedimentary rock layers above and below the fossil The magnetism of rocks can provide dating information Reversals of the magnetic poles leave their record on rocks throughout the world Key events in life s history include the origins of single celled and multi celled organisms and the colonization of land The geologic record is divided into the Archaean the Proterozoic and the Phanerozoic eons The Phanerozoic is divided into 3 eras the Paleozoic Mesozoic and Cenozoic Major boundaries between geological divisions correspond to extinction events in the fossil record The first single celled organisms The oldest known fossils are stromatolites rock like structures composed of many layers o bacteria and sediment Stromatolites date back 3 5 billion yrs ago Prokaryotes were earth s sole inhabitants from 3 5 to about 2 1 billion yrs ago Photosynthesis and the oxygen revolution Most atmospheric oxygen O2 is of biological origin O2 produced by oxygenic photosynthesis reacted with dissolved iron and precipitated out to form banded iron formations The source of O2 was likely bacteria similar to modern cyanobacteria By about 2 7 billion yrs ago O2 began accumulating in the atmosphere and rusting iron rich terrestrial rocks This oxygen revolution from 2 7 to 2 2 billion yrs ago Posed a challenge for life Provided opportunity to gain energy from light Allowed organisms to exploit new ecosystems The first eukaryotes The oldest fossils of eukaryotic cells date back to 2 1 billion yrs ago The hypothesis on endosymbiosis proposes that mitochondria and plastids chloroplasts and related organelles were formerly small prokaryotes living within larger host cells An endosymbiont is a cell that lives within a host cell The prokaryotic ancestors of mitochondria and plastids probably gained entry to the host cell as undigested prey or internal parasites Serial endosymbiosis supposes that mitochondria evolved before plastids through a sequence of endosymbiotic events Key evidence supporting an endosymbiotic origin of mitochondria and plastids Similarities in inner membrane structures and functions Division is similar in these organelles and some prokaryotes These organelles transcribe and translate their own DNA Their ribosomes are more similar to prokaryotic then eukaryotic ribosomes The origin of muiltcellularity The evolution of eukaryotic cells allowed for a greater range of unicellular forms A second wave of diversification occurred when muiltcellularity evolved and gave rise to algae plants fungi and animals Comparisons of DNA sequences date the common ancestor of multicellular The Cambrian explosion The Cambrian explosion refers to the sudden appearance of fossils resembling modern phyla in the Cambrian period 535 to 525 million yrs ago The Cambrian explosion provides the first evidence of predator prey interactions DNA analyses suggest that many animal phyla diverged before the Cambrian explosion perhaps as early as 700 million to 1 billion yrs ago Fossils in China provide evidence of modern animal phyla tens of millions of years before the Cambrian