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:

1. organic molecules don’t form easily in the presence of N & CO2

2. UV radiation from the sun would harm DNA – no ozone (O3) layer for protection

3. no free oxygen4. frequent volcanic eruptions & violent

lightening storms5. 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 simulatingatmosphere of early Earth

Condensationchamber

Spark simulatinglightning storms

Watervapor

Liquid containing amino acids and other organiccompounds

Cold water cools chamber, causing droplets to form.

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.