When you stroll through the park and see a mutt running through the grass, it's not all that hard to identify parts of its heritage. You might say that its short black hair shows a lab heritage and its long, thin snout shows it has some collie in it. You make these evaluations without thinking too much about it, because you know the dog's characteristics come from its parents. That's the same for all creatures. Characteristics are transmitted from generation to generation; so the fact that the genetic code among all organisms is essentially the same implies that the code originated from one distant ancestor and was passed down through the ages.
Life from Life
Some 3.5 billion years ago, from a sea of raw chemical materials, self-sustaining, replicating chemical reactions began to take place on Earth. That was the beginning of life on the planet. The conditions that stimulated that development are long gone. Now every living organism comes from one or two living parents. The parent or parents provide the child organism with long molecules of deoxyribonucleic acid, more commonly known as DNA. DNA contains all the information necessary to build the organism -- including the information the child needs to pass the DNA to its own children.
DNA and Evolution
Information in DNA is used to build proteins. Proteins are responsible for most of the structures and functions of the body, from digesting food to building skin. When DNA specifies the proteins and functional RNA in an organism, it also specifies the organism's appearance and function. Unlike RNA, proteins cannot be simply copied from the DNA to form a functional unit; they require a special system of encoding, known as the genetic code.
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The Genetic Code
DNA is built from a long string of components called nuclear bases. Those bases are adenine, thymine, cytosine and guanine, which are usually abbreviated A, T, C and G. The protein-building information in DNA is contained in three-base sequences. Each three-base stretch contains a "code" for an amino acid. Proteins are built from chains of amino acids, so a stretch of three-base codes in DNA will direct the formation of an entire protein. The three-base codes are called "codons." Each codon specifies only one amino acid, although some amino acids are specified by more than one codon. The correspondence between codons and amino acids is called the genetic code, and it is essentially the same for every organism on Earth.
You could conceivably look at all winged organisms on Earth and argue that they all must have come from a single common organism. You could do the same for fish and mammals, because you'd look at their common characteristics and see that they could have resulted from slight modifications over millions of years. But when you look more closely -- beyond the macroscopic characteristics of an organism -- you see a different picture.
Every organism shares the most fundamental chemical process of all: the chemistry of DNA. Most organisms have the same genetic code. (One notable exception is within our own cells: mitochondrial DNA uses a slightly different genetic code from nuclear DNA. This is because mitochondria are descended from bacteria that were once independent organisms.) All organisms have highly similar genetic codes, and that means that all organisms descended from a single parent, one alive billions of years ago.