Lesson 6- Earth's Early History Flashcards
Formation of Earth
(Blank) about Earth’s early history are based on a relatively small amount of evidence.
(Blank) and (blank) make it likely that scientific ideas about the origin of life will change.
Hypotheses, Gaps and uncertainties
Evidence shows that Earth was not “(blank)” in a single event.
Pieces of (blank) (blank) were probably attracted to one another over the course of 100 million years.
While Earth was young, it was struck by one or more objects, producing enough heat to melt the entire globe.
born, cosmic debris
Once Earth melted, its elements rearranged themselves according to (blank).
The most dense elements formed the planet’s (blank).
Moderately dense elements floated to the (blank), cooled, and formed a solid crust.
The least dense elements formed the (blank) (blank).
density, core, surface, first atmosphere
Earth’s early atmosphere probably contained:
hydrogen cyanide, carbon dioxide, carbon monoxide, nitrogen, hydrogen sulfide, and water.
Scientists infer that about four billion years ago, Earth cooled and solid rocks formed on its surface.
Millions of years later, (blank) (blank) shook Earth’s crust.
About (blank) years ago, Earth’s surface cooled enough for water to remain a liquid, and oceans covered much of the surface.
volcanic activity, 3.8 billion years ago
The First Organic Molecules
Could organic molecules have evolved under conditions on early Earth?
In the (blank), Stanley (blank) and Harold (blank) tried to answer that question by simulating conditions on the early Earth in a laboratory setting.
1950s, Miller, Urey
Miller and Urey’s experiments suggested what
How mixtures of the organic compounds necessary for life could have arisen from simpler compounds present on a primitive Earth.
The Puzzle of Life’s Origin
Evidence suggests that 200-300 million years after Earth had (blank) (blank), (blank) similar to modern bacteria were common.
liquid water, cells
Formation of Microspheres
In certain conditions, large organic molecules form tiny bubbles called (blank) (blank)
Microspheres are not cells, but they have (blank) (blank) (blank) and can store and release energy.
proteinoid microspheres, selectively permeable membranes
Hypotheses suggest that structures similar to (blank) might have acquired more characteristics of living cells.
microspheres
Evolution of RNA and DNA
How could DNA and RNA have evolved? Several hypotheses suggest:
• Some RNA sequences can help DNA replicate under the right conditions.
• Some RNA molecules can even grow and duplicate themselves suggesting RNA might have existed before DNA.
Free Oxygen
(Blank) (blank), or (blank), of unicellular prokaryotic organisms resembling modern bacteria have been found in rocks over 3.5 billion years old.
These first life-forms evolved without (blank).
Microscopic fossils, oxygen
About 2.2 billion years ago, (blank) (blank) began to pump oxygen into the oceans.
Next, oxygen gas accumulated in the atmosphere.
photosynthetic bacteria
What occurred when oxygen was added to Earth’s atmosphere?
The rise of oxygen in the atmosphere drove some life forms to (blank), while other life forms evolved new, more efficient metabolic pathways that used oxygen for (blank).
extinction, respiration
What hypothesis explains the origin of eukaryotic cells?
The (blank) (blank) proposes that eukaryotic cells arose from living communities formed by prokaryotic organisms.
endosymbiotic theory
