Chapter 19/20 Section 19-3: Earth’s Early History
The Mysteries of Life’s Origins Earth formed as pieces of cosmic debris collided Young planet struck by one or more huge objects and melted Elements redistributed by density Millions of years of violent volcanic activity, comets/asteroids hitting surface About 4.2 bya surface cooled enough for solid rocks to form, water to condense, permanent oceans form
The Mysteries of Life’s Origins Early atmosphere had little to no oxygen Mostly carbon dioxide, water vapor, nitrogen Smaller amounts of carbon monoxide, hydrogen sulfide, hydrogen cyanide Sky pinkish-orange Oceans brown with dissolved iron
The First Organic Molecules In 1953, chemists Stanley Miller and Harold Urey tried recreating conditions on early Earth to see if organic molecules could be assembled under these conditions
The First Organic Molecules After a week they had produced 21 amino acids Showed how mixtures of organic compounds necessary for life could have arisen Idea of atmospheric composition incorrect
Formation of Microspheres Geological evidence shows that about mya after Earth cooled enough to carry liquid water cells similar to bacteria were common Large organic molecules form bubbles called proteinoid microspheres under certain condition They are not cells, but have some living characteristics – selectively permeable membranes, means of storing/releasing energy Thought to have acquired characteristics of living cells about 3.8 bya
Evolution of RNA and DNA Central dogma The “RNA World” hypothesis about the origin of life suggests RNA evolved before DNA Simple RNA-based system underwent several changes to DNA-directed protein synthesis Experiments show how small RNA sequences could have formed from simpler molecules Under certain conditions, RNA sequences help DNA replicate, process mRNA after transcription, catalyze chemical reactions Some can even grow/replicate on their own
Production of Free Oxygen Microfossils,of prokaryotes that resemble bacteria have been found in rocks more than 3.5 billion years old Evolved in the absence of oxygen Photosynthetic bacteria became common and producing oxygen by 2.2 bya Oxygen combined with iron in the oceans, producing iron oxide which sank to ocean floor and formed great bands of iron that are the source of most iron ore mined today Oceans changed blue-green
Production of Free Oxygen Next oxygen started accumulating in the atmosphere Ozone layer formed, skies turned blue Early atmosphere thought to be similar to volcanic gases
Production of Free Oxygen First cells evolved in absence of oxygen Deadly poison, many cells went extinct Some evolved metabolic pathways to use the oxygen (cellular respiration) or ways to protect themselves from it
Evolution of Eukaryotic Cells It is believed that about 2 bya some ancient prokaryotes began evolving internal membranes – ancestors of eukaryotes According to endosymbiotic theory, prokaryotic cells entered and began living inside those ancestral eukaryotes Over time, they developed symbiotic relationships
Evolution of Eukaryotic Cells Microscopists saw that the membranes of mitochondria and chloroplasts resembled the cell membranes of free- living prokaryotes Two related hypotheses: Mitochondria evolved from endosymbiotic prokaryotes that were able to use oxygen to generate energy-rich ATP molecules (now could use oxygen) Chloroplasts evolved from endosymbiotic prokaryotes that had the ability to photosynthesize
Modern Evidence During the 1960s, Lynn Margulis of Boston University noted that mitochondria and chloroplasts contain DNA similar to bacterial DNA Also have ribosomes that resemble those of bacteria Mitochondria and chloroplasts, like bacteria, reproduce by binary fission
Significance of Sexual Reproduction During asexual reproduction (prokaryotes), genetic variation is restricted to mutations in DNA When eukaryotes reproduce sexually, offspring receive genetic material from two parents Meiosis and fertilization shuffle genes, generating genetic diversity. Offspring of sexually reproducing organisms are never identical to parents or siblings Increases the likelihood of a population’s adapting to new or changing environmental conditions
Multicellularity Multicellular organisms evolved a few hundred million years after the evolution of sexual reproduction Even greater diversity