Mitch Sogin analyses the genes of a sponge in his search for the origin of animals. In order to sequence the genes, he extracts them from the cells. We see the threads of DNA in a test tube. He compares a similar gene found in all the sequences in the animals he tested. Over time small mutations occur in all genes. The more closely related the animals are, the more similar is the code in a shared gene. He created a family tree by comparing the number of changes in the code. The sponge was at the bottom because its gene had the fewest changes.