The last common ancestor of all bilaterians is thought to have had four hox genes. most extant cnidarians have two hox genes, though some have three hox genes. on the basis of these observations, some have proposed that the ancestral cnidarian's were originally bilateral and, in stages, lost hox genes from their genomes. if true, this would mean that

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the relative atomic mass of an element is the mass of one atom of that element, relative to 1/12th of the mass of a  1212c atom. to convert to an absolute mass in kg, it is necessary to multiply by  1.661×10−271.661×10−27.

the relative atomic mass is roughly equal to the number of neutrons, plus the number of protons, in the nucleus of the atom.

for elements with more than one stable isotope (different types of nucleus with different numbers of neutrons but the same number of protons), the quoted mass is usually the average mass, weighted by the relative abundance of the different isotopes.