Mechanics Historically How it Evolves --- “Perks”.

We cannot talk about “Skill Tree” without mentioning “Perks”.

According to Shamoon (2007):

"Perks" are special bonuses that video game players can add to their characters to give special abilities. The term refers to the general usage of “Perks” as an abbreviation of “prerequisite”.

As a variation of the power-up mechanics, “Perks” mechanics displaces the temporary ability boost concept and develops into an advanced concept, where power-ups are progressively unlocked and permanent through experience points.

The history of “Perks” can be dated back to 1980s.
The concept of “Perks” were firstly shown in two of the NES action RPGs:

Deadly Towers (1986)

Rygar (1987)

According to Wikipedia (2014), these two games were particularly notable for their permanent power-up mechanics, which blurred the line between the power-ups used in action-adventures and the experience points used in role-playing video games.

Even so, the term “Perks” was officially first used to describe this mechanics in 1997 role-playing video game Fallout:

Now, "Perks" has extensive application in modern video games.

For example:

• Call of Duty 4: Modern Warfare (2007)
• Metal Gear Online (2008)
• Call of Duty: Modern Warfare (2009)
• Killing Floor (2009).

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Shamoon, Evan (08/10/2007). "Call of Duty 4: Modern Warfare (XBOX 360): Impressions of guns, perks and multiplayer mode". 1UP.com. Retrieved 29 Jan 2015.