All animals, including humans,
descend from a common ancestor.
Darwin knew the idea that man was
just another animal would go over like a lead balloon in Victorian society. But
in the Origin of Species, he was
unequivocal on this notion, writing that "I should infer from analogy
that probably all the organic beings which have ever lived on this earth have
descended from one primordial form, into which life was first breathed."
Evidence supporting this view has accumulated ever since, with arguably the
strongest support coming from evo devo. Scientists comparing the genomes of
members of the three domains of life—archaea, bacteria, and the eukaryotes
(animals, plants, fungi, and protists)—have discovered about 500 genes
that all living things share. These "immortal" genes have survived
essentially unchanged for over two billion years (including the portion of a
protein's sequence seen here, with shared amino acids indicated by shading). Says
Brown's Ken Miller: "Despite the extraordinary diversity of life,
all living organisms share a nearly identical set of essential genes,
reflecting their evolutionary development from a common ancestor."