|An activation energy diagram, showing the energy barrier, separating reactants from products.|
Reactants → Products
During this process the energy of the system increases to a maximum, then decreases to the energy of the products. The “activation energy” is the difference between the maximum energy and the energy of the reactants; i.e. it is the energy barrier that has to be overcome for the reaction to proceed.  The activation energy is said to be the heightened measure of energy during the transition state of the chemical reaction.
In 1889, building on the previous 1884 work of Dutch physical chemist Jacobus van't Hoff, Swedish physical chemist Svante Arrhenius introduced term “activation energy”, by suggesting that a catalyst forms an intermediate complex with the reactants, which acts to lower the activation energy or barrier to reaction or product formation, the logic of which is embodied in the Arrhenius equation.  In particular, Arrhenius found that the plot of the logarithm of the rate constant (ln k) against the inverse of the absolute temperature (1/T) is a straight line: 
where the intercept is denoted ln A and the slope is denoted –EA/R, where R is the ideal gas constant, A is the pre-exponential factor, and EA is the activation energy:
German physical chemist Wilhelm Ostwald, according to Keith Laidler (1993), the first to realize that a catalyst acts without altering the energy relations of the reaction, and that it usually speeds up a reaction by lowering the activation energy. 
In 1912, Austrian-born American physical chemist and mathematician Alfred Lotka was using the term "trigger action" to explain the way in which the sight of a predator by the eyes of a prey acts to start the flee reaction, which is a crude way of explaining activation energy in biological terms. 
Russian bio-geologist Vladimir Vernadsky was one of the first to state that radiation from the sun operates as a factor of activation energy: 
“The radiations that pour upon the earth cause the biosphere to take on properties unknown to lifeless planetary surfaces, and thus transform the face of the earth. Activated by radiation, the matter of the biosphere collects and redistributes solar energy, and converts it ultimately into free energy capable of doing work on earth.”
It is difficult to know exactly what Vernadsky means by ‘activated’ here as this is a somewhat modern term for 1926 as the theory of activation energy (1884) was a relatively new concept and the electron-photon model of activation energy (Bohr, 1913) in terms of thermodynamics (Lewis, 1916) was even more modern.
See main: Social activation energyThe study of activation energy in human chemistry is a complex subject, being that a number of potential theories can be used to explain the activation energy aspects of human chemical reactions, such as molecular orbital orientation, entropy of activation, heat (e.g. summer vs winter), trajectory collisions, catalysts, human catalysts, substrate factors (e.g. renting vs owning), among other aspects. 
A simple example, Lotka's model that light "triggers" flee reaction, it is know that, via the process of love at first sight, "light" triggers the male female reaction, and it is statistically known that about 2/3 of people believe in love at first sight and 20% fall in love at first sight and marry that person. 
In simple terms, a useful generalization supported by the Arrhenius equation is that, for many common chemical reactions at room temperature, the reaction rate doubles for every 10 degree Celsius increase in temperature. Thus, two human molecules in proximity during the winter months may not have sufficient activation energy to react, say, into a bonded relationship, but the same two molecules, in the summer months, may successfully collide and react, owing to a warmer reaction system or in other words owing to a lower activation energy.
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2. Laidler, Keith J. (1993). The World of Physical Chemistry (pg. 212). Oxford University Press.
3. (a) Thims, Libb. (2007). Human Chemistry (Volume One) (activation energy, pgs. 94-98, 100, 279-84). Morrisville, NC: LuLu.
(b) Thims, Libb. (2007). Human Chemistry (Volume Two) (activation energy, pgs. 448, 458, 498, 549, 695). Morrisville, NC: LuLu.
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(b) Anon. (1980). Academic American Encyclopedia (pg. 213). Aret Pub. Co.
5. Atkins, Peter and Jones, Loretta. (2007). Chemical Principles: the Quest for Insight (pg. 556). MacMillan.
6. (a) Lotka, Alfred. (1912). “Evolution in Discontinuous Systems” (pg. 71), Journal of the Washington Academy of Sciences, Vol. 2.
(b) Guilleminot, Hyacinthe. (1919). Matter and Life (La Matiere et la Vie) (pg. 115). Flammarion.
(c) Johnstone, John. (1921). The Mechanism of Life (pg. 49). Liverpool.
7. Vernadsky, Vladimir I. (1926). The Biosphere (pg. 44). Copernicus.
8. Fisher, Helen. (1992). Anatomy of Love - a Natural History of Mating, Marriage, and Why we Stray, (section: "Love at First Sight", pgs.49-50). New York: Fawcett Columbine.
● Activation energy – Wikipedia.
● Activation energy – IoHT Glossary.