In order to survive, humans require a minimum of one-third of the oxygen content at sea level.Using the figure below, what was the most recent point in Earth's history at which a human could (barely) survive unaided on the surface? 
Definitions:
Takings Clause
A provision in the Fifth Amendment of the U.S. Constitution that prohibits the government from taking private property for public use without just compensation.
Public Use
The utilization of land or property by the general public or for a public purpose, often justified for projects involving eminent domain.
Just Compensation
The fair market value that must be paid for property taken by the government through eminent domain.
Commercial Speech
Speech that promotes a commercial transaction, or advertisement, which is granted protection under the First Amendment but is subject to regulation.
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