We already know that the original already filmed ending of Brad Pitt's apocalyptic zombie thriller, "World War Z," was scrapped. But which scenes in the final version, which hits theaters this weekend, were rewritten by Drew Goddard, Damon Lindelof and Chris McQuarrie?

A source close to the production of "World War Z" revealed the answers to The Huffington Post -- and yes, some of them are surprising. (Mild spoilers ahead.)

In the June issue of Vanity Fair, writer Laura M. Holson describes how the film originally climaxed with a large-scale zombie battle in Russia, which followed the large-scale zombie battle in Jerusalem that can be seen in recent trailers. This ending was eventually cut from the film -- Drew Goddard and Damon Lindelof were then hired to write a new ending.

In the article, Lindelof is quoted as saying, "everything changes after Brad leaves Israel." Our source gets more specific, explaining that the dividing line between the original version and the final cut comes when Pitt's character, Gerry Lane, boards the plane on the Jerusalem tarmac. Everything starting with Lane entering the aircraft, including every incident on the flight, was written by Goddard and Lindelof. Our source adds that, while Goddard and Lindelof wrote the ending, Chris McQuarrie (who directed Paramount's Jack Reacher) was brought in to "sharpen" it.

That's not so surprising, though. We already knew that the ending changed. What we didn't know is that the beginning of the movie changed, too. "World War Z" opens with Lane and his wife, Karen (Mireille Enos), being awakened by their daughters, Rachel and Constance. The next scene finds the family in the kitchen eating breakfast as reports of the first "rabies" (i.e. zombie) outbreak is announced on the television. Our source confirms that these two scenes were part of the rewrite, as well, and did not appear in the original film.

Two other, later scenes were added as well. In one, Lane, who is driving the family from Philadelphia to Newark, New Jersey, in a commandeered RV, has to pull over to treat Rachel's asthma attack. In the other, Lane phones Karen during a plane ride from South Korea to Jerusalem.

It's hard to imagine "World War Z" without the above scenes. Each one -- especially the extended new ending -- grounds the characters and gives viewers a needed respite from all that over-the-top zombie carnage.

Mike Ryan is senior writer for Huffington Post Entertainment. You can contact him directly on Twitter.

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  • This publicity image released by Paramount Pictures shows, from left, Mireille Enos as Karin Lane, Sterling Jerins as Constance Lane, Abigail Hargrove as Rachel Lane, and Brad Pitt as Gerry Lane in a scene from "World War Z." (AP Photo/Paramount Pictures, Jaap Buitendijk)

  • This publicity image released by Paramount Pictures shows Brad Pitt in a scene from "World War Z." (AP Photo/Paramount Pictures, Jaap Buitendijk)

  • This publicity image released by Paramount Pictures shows a scene from "World War Z." (AP Photo/Paramount Pictures)

  • This publicity image released by Paramount Pictures shows, from center left, Brad Pitt as Gerry Lane, Abigail Hargrove as Rachel Lane, and Mireille Enos as Karin Lanein a scene from "World War Z." (AP Photo/Paramount Pictures, Jaap Buitendijk)

  • Brad Pitt

    Actor Brad Pitt attends the premiere of "World War Z" in Times Square on Monday, June 17, 2013, in New York. (Photo by Evan Agostini/Invision/AP)

  • Brad PItt

    Brad Pitt attends the "World War Z" premiere on Monday, June 17, 2013, in New York. (Photo by Charles Sykes/Invision/AP)

  • Brad PItt

    Brad Pitt attends the "World War Z" premiere on Monday, June 17, 2013, in New York. (Photo by Charles Sykes/Invision/AP)

  • Brad PItt

    Brad Pitt attends the "World War Z" premiere on Monday, June 17, 2013, in New York. (Photo by Charles Sykes/Invision/AP)

  • Brad Pitt

    Brad Pitt attends the "World War Z" premiere on Monday, June 17, 2013, in New York. (Photo by Charles Sykes/Invision/AP)

  • Brad Pitt

    Actor Brad Pitt attends the premiere of "World War Z" in Times Square on Monday, June 17, 2013, in New York. (Photo by Evan Agostini/Invision/AP)

  • Brad Pitt

    Actor Brad Pitt attends the premiere of "World War Z" in Times Square on Monday, June 17, 2013, in New York. (Photo by Evan Agostini/Invision/AP)

  • Brad Pitt

    Actor Brad Pitt taking a photo with a fan at the premiere of "World War Z" in Times Square on Monday, June 17, 2013, in New York. (Photo by Evan Agostini/Invision/AP)

  • Brad Pitt

    Actor Brad Pitt attends the premiere of "World War Z" in Times Square on Monday, June 17, 2013 in New York. (Photo by Evan Agostini/Invision/AP)

  • Brad Pitt

    Actor Brad Pitt attends the premiere of "World War Z" in Times Square on Monday, June 17, 2013, in New York. (Photo by Evan Agostini/Invision/AP)

  • Brad Pitt

    Actor Brad Pitt attends the premiere of "World War Z" in Times Square on Monday, June 17, 2013 in New York. (Photo by Evan Agostini/Invision/AP)

  • Brad Pitt, Alicia Quarles

    Actor Brad Pitt, right, being interviewed by E! entertainment reporter Alicia Quarles at the premiere of "World War Z" in Times Square on Monday, June 17, 2013, in New York. (Photo by Evan Agostini/Invision/AP)

  • James Badge Dale

    Actor James Badge Dale attends the premiere of "World War Z" in Times Square on Monday, June 17, 2013, in New York. (Photo by Evan Agostini/Invision/AP)

  • Marc Forster

    Director Marc Forster attends the premiere of "World War Z" in Times Square on Monday, June 17, 2013, in New York. (Photo by Evan Agostini/Invision/AP)

  • Mireille Enos

    Actress Mireille Enos attends the premiere of "World War Z" in Times Square on Monday, June 17, 2013, in New York. (Photo by Evan Agostini/Invision/AP)

  • David Andrews

    Actor David Andrews attends the premiere of "World War Z" in Times Square on Monday, June 17, 2013, in New York. (Photo by Evan Agostini/Invision/AP)

  • Mireille Enos

    Actress Mireille Enos attends the premiere of "World War Z" in Times Square on Monday, June 17, 2013, in New York. (Photo by Evan Agostini/Invision/AP)

  • Max Brooks

    Author Max Brooks attends the "World War Z" premiere on Monday, June 17, 2013, in New York. (Photo by Charles Sykes/Invision/AP)

  • Marc Forster, Sterling Jerins

    Director Marc Forster, left, and actress Sterling Jerins attend the premiere of "World War Z" in Times Square on Monday, June 17, 2013, in New York. (Photo by Evan Agostini/Invision/AP)

  • Mireille Enos, Alan Ruck

    Actress Mireille Enos, right, and husband Alan Ruck attend the premiere of "World War Z" in Times Square on Monday, June 17, 2013, in New York. (Photo by Evan Agostini/Invision/AP)

  • Abigail Hargrove

    Actress Abigail Hargrove attends the premiere of "World War Z" in Times Square on Monday, June 17, 2013, in New York. (Photo by Evan Agostini/Invision/AP)

  • Mireille Enos, Alan Ruck

    Actress Mireille Enos, right, and husband Alan Ruck attend the premiere of "World War Z" in Times Square on Monday, June 17, 2013, in New York. (Photo by Evan Agostini/Invision/AP)