Note: The origins for most phrases and sayings cannot be said with a certainty. What's provided are theories that may be plausible to how a phrase originated, but not necessarily so. 

In addition, quotes that contain a particular phrase may be taken from old newspapers, poems, or books that were written centuries ago, but this by no means confirms that the phrase originates from said newspapers, poems, or books. In all likelihood, if an expression is being used in a newspaper, it's probably already a well known saying and is from an older time.
* My wife was trying to put together a bike for our child, but she was having difficulty. I told her that it wasn't brain surgery and we finished building it together. It wasn't as easy as I thought.
Know Your Phrase - The Origins And Meanings For Idioms And Phrases  |  I-Letter Sayings
KnowYourPhrase.com - The Meanings and Origins of Common Idioms

A list of phrases starting with the letter I.
Phrases and Idioms
A task that's easy to accomplish; something that's lacking complexity.
The meaning for the phrase 'it's not brain surgery.'
Origins:
Examples
A man named Will is assembling a chair that he had bought from the store, but he's
having a hard time. So a friend of Will's tells him that putting together a chair "isn't exactly 
brain surgery," or perhaps the friend uses a similar phrase and says that "it's not rocket science." 
Why do people say these things? Well, professions like brain surgeons, rocket scientists, among other occupations, are thought of as being very complicated jobs. Hence, when these phrases are typically used, the person is implying that, in this case, what someone like Bill is doing—building a chair—is simple in comparison to what's done in these other, more complex professions.

So when did this saying originate? Well, it doesn't look to be that old. It seems likely that it originated sometime in the 20th century. I say this because the earliest I could find of this expression in writing is from the 1970s. For example, this saying is used in the Ames Daily Tribune newspaper from 1971. In an article from it, there's a part that reads:

    "He determined that his book, which is as yet untitled and will be published by Prentice-Hall late next 
     year, would dispel the mystery and get women over the 'original hurdle--the psycological block.' 'After 
     all,' he added, 'it's not like brain surgery.'"