I intend for this to be one of a series of bite-sized OPs on the evidence for evolution.
Introduction to DNA
Genes are sequences of DNA made up of words (codons) each of which are three letters (bases) long. There are only four letters in the genetic alphabet (ACG&T) Each word or codon is the recipe for one amino acid. There are 20 different amino acids in living organisms. Amino acids are joined together to make protein molecules. We are made of protein molecules.
The important thing about a protein molecule is not the exact sequence of amino acids, it's simply whether or not it performs its function and that depends on its physical shape. There are many different possible ways to make the same protein molecule and it will still do the same job.
Some proteins are so vital to life that they are shared by every living thing. These are coded by bits of genetic code that are called ubiquitous genes. An example of such a gene is Cytochrome C which is essential to the function of cellular respiration.
The number of possible amino acid sequences that would result in a functional Cytochrome C protein molecule has been calculated to be a billion times larger than all the atoms in the known universe.
If all living things evolved from a common ancestor then we can make a prediction about the differences between the Cytochrome C protein in different species. We would expect them to differ from each other in a predictable way that mirrors their evolutionary history.
This is exactly what we find.
The more distant any two species are from each other in evolutionary history the more differences there are in their amino acid sequences. Humans and chimps have identical amino acid sequences in their Cytochrome C proteins. The yeast Candida krusei has 51 amino acid differences from the human sequence. In fact the entire tree of life can be reconstructed by comparing the amino acid sequences of this one protein. And yet any of these proteins can be switched and will work perfectly well.
This makes absolutely no sense unless every living thing from humans to yeast evolved from a common ancestor.