A popular public speaking joke was cited in print in March 1949 and credited to Dublin Opinion:
 
“The human brain is a wonderful thing. It starts working the moment you are born, and never stops until you stand up to speak in public.”
 
Bronx-born actor George Jessel (1898-1981), often called the “Toastmaster General of the United States” because of his public speaking, became associated with the quote in 1949, but Jessel probably did not originate it. Jessel was famous and the earliest citations would have credited him, had he said it first.
   
“The human brain is a wonderful organ. It starts working the moment we get up in the morning, and doesn’t stop until we get to the office” has been cited in print since 1928 and is a similar saying.
       
 
Wikipedia: George Jessel (actor)
George Albert Jessel (April 3, 1898 – May 23, 1981), sometimes called “Georgie” Jessel, was an American illustrated song “model,” actor, singer, songwriter, and Academy Award-winning movie producer. He was famous in his lifetime as a multitalented comedic entertainer, achieving a level of recognition that transcended his limited roles in movies. He was widely known by his nickname, the “Toastmaster General of the United States,” for his frequent role as the master of ceremonies at political and entertainment gatherings. Jessel originated the title role in the stage production of The Jazz Singer.
 
24 December 1937, The Evening Democrat (Fort Madison, IA), “Madisonia,” pg. 4, col. 3:
The local woman who sent this article in for this column, says she thinks this is the truth.
 
The human brain is a wonderful thing, they say.
Beginning its work when you start your day,
And it never lets down the whole day through,
Unless the boss shoots a question at you.
 
14 March 1949, Rockford (IL) Register-Republic, pg. 22, col. 8:
MEMO TO SPEAKERS
The human brain is a wonderful thing. It starts working the moment you are born, and never stops until you stand up to speak in public.—Dublin Opinion.
 
10 May 1949, Boston (MA) Traveler, “Take It From Me” by Neal O’Hara, pg. 63, cols. 2-3:
From the Webster Times: “The human brain is a wonderful thing…It starts working the moment you are born and never stops until you start to speak in public.”
   
Google Books
Saturday Review of Literature
Volume 32
1949
Pg. 4:
Irving Hoffman quotes George Jessel, America’s Toastmaster General, as saying, “The human brain is a wonderful thing. It starts working the moment you are born and never stops until you start to speak in public.”
 
Google Books
How to Speak Like a Pro
By Leon Fletcher
New York, NY: Ballantine Books
1983
Pg. ?:
The human brain is a wonderful thing. It operates from the moment you’re born until the first time you get up to make a speech.
HOWARD GOSHORN
 
Twitter
Jewish Comedians
‏@JewishComedians
George Jessel: The human brain starts working the moment you are born and never stops until you stand up to speak in public.
11:53 PM - 27 Sep 2015