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NSCI Final Study Guide

Chapter 18: Life’s Origin and Early Evolution

18-1 Looking for Life
-All metabolic reactions involve interactions among molecules in aqueous solution (dissolved in water)
-We assume that the same physical and chemical laws operate throughout the universe, so liquid water is considered an essential requirement for our life.
-Finding life on Mars would support thehypothesis that life on Earth arose as a consequence of physical and chemical processes that occur throughout the universe.
-The more places life exists, the more likely that intelligent life evolved on other planets in the same manner that it did on Earth
18-2 Earth’s Origin and Early Conditions
Big Bang Theory-universe began in a single instant. In that instant all existing matter and energysuddenly appeared and exploded outward from a single point.
Over millions of years, gravity drew the gases together and they condensed to form giant stars
-Explosions of these stars scattered the heavier elements from which today’s galaxies formed.
-Sun was oribited by a cloud of dust and asteroids.
The asteroids collided and merged into bigger asteroids
-The heavier the pre-planetaryrocks became, the more gravitational pull they exerted and the more material they gathered, forming planets.
B. Conditions on the Early Earth
-Gases released by volcanoes and meteorite impacts were the main components of the early atmosphere.
-Studies of volcanic eruptions, meteorites, ancient rocks and other planets suggest that the air contained water vapor, carbon dioxide, hydrogen andnitrogen.
-Little or no oxygen-oldest existing rocks show no evidence of iron oxidation (rusting)
If oxygen had been present, it would have caused rust to form
-Oxygen would have interfered with the assembly of organic compounds necessary for life
Oxygen would have reacted with and destroyed the compounds as fast as they formed
-Any water falling on Earth’s surface evaporated immediately
-Assurface cooled, rocks formed
-Rains washed mineral salts out of these rocks and the salty runoff pooled in early seas. That’s how sea life began.
Take Home Message:
-Meteor impacts were common
-Atmosphere had little or no oxygen
-Seas contained mineral salts leached from rocks.

18-3 Source of Life’s Building Blocks
-All living things are made from:
Simple sugars, amino acids, fatty acidsand nucleotides.
Where did these organic subunits come from?
A. Lightning-Fueled Atmospheric Reactions
-Earth’s early atmosphere could have produced building blocks for the first life
Miller-Urey experiment: placed methane, ammonia, hydrogen gas and water into a reaction chamber.
As the mix circulated, sparks from electrodes simulated lightning. Within a week a variety of amino acidsand other small molecules formed.
-Demonstrated the first step on the road to life.
-Then, idea that Earth’s early atmosphere consisted mainly of carbon dioxide and nitrogen dioxide gained favor. When he redid the experiment, unable to detect any amino acid formation
-Amino acids actually do form in a simulated carbon dioxide and nitrogen atmosphere
-Failed when he used these gases because hisexperiment also formed compounds that break down amino acids.
-On early Earth, rains would have washed amino acids formed by atmospheric reactions into the sea, where breakdown reactions might not occur.
C. Delivery From Space
-The presence of amino acids, sugars and nucleotide bases in meteorites that fell to Earth suggest another possible origin for life’s building blocks.
Molecules mayhave been formed in interstellar clouds of ice, dust and gases being delivered to Earth by meteorites.
-During Earth’s early years, meteorites fell to Earth thousands of times more frequently than they do today.
Take Home Message
-Stimulation experiments support the hypothesis that simple organic compounds could have formed by chemical reactions in Earth’s early atmosphere or in the sea near a...