One reason of how they was NOT is : Africans crossed into South America when Africa and South America were one continent.
heliocentric theory, the theory of evolution, relativity theory
A theory unites and explains a range of observations.
The rapid decay theory explains the decrease in the strength of the earth's magnetic field better then the dynamo theory. Although the dynamo theory explains the explains the reversals where the field pointed in the opposite directions better. The rapid decay theory allows for it, but only if there is an event of cataclysmic volcanic and geological activity. The rapid decay theory also explains the magnetic fields on the other planets correctly.
If by "why" you mean "what," a well tested framework that explains a wide range of observations in science is called a theory.
Theory
it is a Theory! :)
heliocentric theory, the theory of evolution, relativity theory
The theory is Tectonics.
It is the wave theory of light that best explains interference. The particle theory has problems when applied to this observable phenomenon.
A theory unites and explains a range of observations.
opponent-process theory
Opponent-processing theory
That is Quantum Theory.
dynamo theory
Contenintal Drift theory
That is Quantum Theory.
compare and contrast Expectancy Theory and Equity Theory