Earth’s Early History (Ch 19.3) Unit 5: Evolution

The Mysteries of Life’s origins Early Earth Evidence proves cosmic debris collided with one another to form the earth The earth was struck by huge objects (comets and asteroids) Volcanic activity shook the earth Atmosphere Little to no oxygen Major gases: Carbon dioxide, water vapor, nitrogen Minor gases: carbon monoxide, hydrogen sulfide, hydrogen cyanide 4.2 BILLION YEARS AGO – Earth cooled and allowed water to condense (STABILITY)

The first organic molecules Miller and Urey Can organic molecules assemble in early earth? They tested the theory by combining elements found on earth with water and stimulating lightning – then they let the water condense and cool Results: 21 amino acids were formed! (building blocks of life) Their results suggested how organic compounds necessary for life arose on a primitive earth Formation of microspheres Organic molecules for tiny bubbles called proteinoid microspheres Microspheres are not cells but have some characteristics of living systems They appeared on earth 3.8 billion years ago

Miller and urey Experiment

Evolution of RNA and DNA Scientists have NOT found the answer yet, but have some hypotheses Rna formed from simpler molecules RNA existed before dna – DNA directed protein synthesis would result from the prescence of rna Production of Free Oxygen bacteria have been found that lived in the presence of little to no oxygen (3.5 billion years ago) Photosynthetic bacteria became common 2.2 billion years ago Oxygen combined with iron – iron oxide (rust) No iron = blue green oceans Oxygen accumulated in the atmosphere = formation of ozone Some organisms went extinct because of the oxygen while others prospered or adapted

Origin of eukaryotic cells Endosymbiotic theory Ancient prokaryotes began evolving internal cell membranes (ancestor of eukaryotic organisms) Prokaryotes entered the ancient eukaryotes, and instead of digesting the prokaryotes, the prokaryotes lived in the eukaryotes Symbiotic relationships evolved over time between primitive eukaryotic cells and the prokaryotic cells within them Two Hypotheses: Mitochondria evolved from endosymbiotic prokaryotes that were able to use oxygen to generate ATP (used in respiration) Chloroplasts evolved from endosymbiotic prokaryotes that had the ability to photosynthesize (used in photosynthesis)

Endosymbiotic theory

Modern evidence for endosymbiotic theory Marguilis gathered evidence to support endosymbiotic theory Mitochondria and chloroplasts contain DNA similar to bacterial DNA They have ribosomes that resemble those of bacteria They reproduce by binary fission like bacteria when cells divide by mitosis Sexual Reproduction and Multicellularity After eukaryotic cells arose they reproduced sexually to speed up evolutionary change and produce genetic variation