17-2 Earth’s Early History

Formation of Earth Evidence shows that Earth was not “born” in a single event. The whole universe was concentrated into one superdense mass that exploded = “big bang” Pieces of cosmic debris were probably attracted to one another over the course of 100 million years.

Once Earth formed (~4.6 billion years ago = bya), its elements rearranged themselves according to density. Core = most dense Crust = moderately dense Atmosphere = least dense ~ 3.8 billion years ago: Earth’s surface cooled enough for water to remain a liquid, and oceans covered much of the surface. atmosphere probably contained hydrogen cyanide, carbon dioxide, carbon monoxide, nitrogen, hydrogen sulfide, and water. notice the 4 elements present: C, O, H, N

Early earth was hostile to modern life: organic molecules don’t form easily in the presence of N & CO2 UV radiation from the sun would harm DNA – no ozone (O3) layer for protection no free oxygen frequent volcanic eruptions & violent lightening storms extreme temperature variations (no greenhouse effect)

Creating Life in a Lab Miller and Urey (1950’s) recreated the conditions of the early earth including volcanic gases, energy from heat & electricity, & water vapor as rain formed simple amino acids in only 1 week more complex compounds formed with more time.

Miller and Urey’s Experiment Mixture of gases simulating atmosphere of early Earth Spark simulating lightning storms Condensation chamber Water vapor Cold water cools chamber, causing droplets to form. Miller and Urey produced amino acids, which are needed to make proteins, by passing sparks through a mixture of hydrogen, methane, ammonia, and water. This and other experiments suggested how simple compounds found on the early Earth could have combined to form the organic compounds needed for life. Liquid containing amino acids and other organic compounds

Could organic molecules have evolved under conditions on early Earth? evidence from meteorites shows that organic compounds form in space organic compounds form at volcanic vents deep in the ocean

The Puzzle of Life's Origin Evidence suggests that 200–300 million years after Earth had liquid water, cells similar to modern bacteria were common. Organic compounds accumulated in the ocean = primordial soup Life evolved from further chemical reactions & transformations The 1st life forms were heterotrophs that fed on organic compounds in the primordial soup

Free Oxygen Microscopic fossils (=microfossils) of unicellular prokaryotic organisms resembling modern bacteria have been found in rocks over 3.5 billion years old. These first life-forms evolved without oxygen. About 2.2 billion years ago, photosynthetic bacteria began to pump oxygen into the oceans. Next, oxygen gas accumulated in the atmosphere, but it was not abundant until 600-800 million years ago.

The rise of oxygen in the atmosphere drove some life-forms to extinction, while other life-forms evolved new, more efficient metabolic pathways that used oxygen for respiration.

Origin of Eukaryotic Cells The endosymbiotic theory proposes that eukaryotic cells arose from living communities of prokaryotic organisms. Occurred about 2 billion years ago the hypothesis was proposed over a century ago, but did not receive support from the scientific community until Lynn Margulis of BU unveiled key evidence in the 1960s

Endosymbiotic Theory Eukaryotic cells developed from a symbiotic relationship between several kinds of prokaryotes (bacteria) – each had its own “specialty” and together – formed a great “unit”

Evidence for Endosymbiotic Theory Membranes of mitochondria and chloroplasts resemble the plasma membranes of free-living cells. Mitochondria and chloroplasts contain DNA Mitochondria and chloroplasts have ribosomes similar in size and structure to those of bacteria. Mitochondria and chloroplasts reproduce by binary fission, as bacteria do.