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Since Darwin's day, science has made astounding advances in the methods by which we can study organisms. One of the most useful advances has been the understanding of genetics and how DNA and RNA play a role in inheritance. Since scientists now have the ability to sequence entire genomes, they can inspect the genetics of different organisms for similarities. Across many organisms, nucleotide sequences are conserved (the same or very similar). This is evidence that even organisms in very different taxa share a common ancestor that passed this genetic material on to future generations. Scientists can also look at slight changes that occur over time in these conserved molecules and calculate a molecular clock for how things changed. Ribosomal RNA has a very slow rate of substitution, so it is most commonly used in conjunction with fossil information to determine relationships between extremely ancient species.
