Simple single-celled organisms called prokaryotes, such as bacteria are the earliest forms of life on earth, and still abundant today. Much later evolved the more complex, but still single celled organisms called eukaryotes. From those humble beginnings came the multi-cellular life forms called metazoans. All animal cells, including humans, are eukaryotic cells. Since they share a common origin, they bear a resemblance to each other. Many molecular mechanisms (genes, enzymes, etc.) and biochemical pathways are conserved throughout the evolution towards more complex organisms.