What does it mean when scientists say that living organisms share a universal genetic code?

When a scientist say's organisms have the same universal genetic code it mainly means that the "organisms use the same four nucleotide bases." Now organic molecules that serve as the monomer units for forming the nucleic acid polymers. 

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It means that all organisms have a common ancestor, an organism from which all actual organisms derived from.

Our microscopic ancestor lived billions of years ago. Scientists named it LUCA which is short for Last Universal Common Ancestor.

Scientists believe that is was a unicellular organism, composed by a simple cell, that gave way to the cells we know today, by forming new organelles across the years.

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