The first Earthling in space was not a human.
Soviet Cosmonaut Yuri Gagarin was the first human sent into orbit. He was launched on April 12, 1961 onboard the Vostok 1 and spent one hour and 48 minutes in space.
However the distinction of being the first Earthling in space goes to Laika (Лайка, "Barker"). She was launched on Sputnik 2 on November 3, 1957.
Unlike Gagarin, Laika was sent on a suicide mission. She died during the flight from stress and overheating. At a media conference in Moscow in 1998, Soviet scientist Oleg Gazenko who worked on the project said, "The more time passes, the more I'm sorry about it. We did not learn enough from the mission to justify the death of the dog."
On July 2, 1959, a dog named Otvazhnaya (Brave One) was sent into space with her fellow passengers, another dog named Snezhinka (Snowflake) and a rabbit named Marfusha (Little Martha).
So by the time the first American, Alan Shepard, flew into space, he had already been preceded by quite a few dogs, some mice and a rabbit.