The evolution of different organisms has a lot in common. They look different to our eyes because evolution results from a lot random events.
Two major forces are worth mentioning: mutation and selection. Mutation is the random error that happens then DNA replicates. Hmm, this may sound ridiculous, but those mistakes make the gene pool more diverse, and is actually good in the long run. Selection, on the other hand, decide which type of gene is more fit in the current environment.