The genetic code is said to be universal because nearly all living organisms on Earth use the same four-letter code to direct their metabolic functions and build bodies. A single sequence of nucleotides is able to synthesize a single protein, regardless of the species it is in.
Every one of the millions of species currently living on Earth has a body built according to instructions found in its genetic code. This code, which consists of repeating sequences of four nucleotides along a double-stranded molecule called DNA, operates in essentially identical ways regardless of the bodies it builds. Any gene found in more than one species can, when activated, produce the same protein, which then performs much the same function in the creature's body. This is why some genes can be transferred from one species to another to achieve a desired effect, such as resistance to frost in domesticated tomatoes or pest resistance in corn.
The universality of the genetic code is a result of all organisms' shared descent from a common ancestor. The most recent ancestor of all living things had a body, most likely a single cell, built according to instructions coded in its DNA. All subsequent descendants, including humans, inherited the same basic mechanism for transmitting heredity.