Nails have nothing much in common with bone. They're made of keratin, which is the same material that hair consists of. With some mammals this keratin also forms into horns (eg. the rhino) and even beaks (the platypus and echidnas).
Our flattened nails are characteristics shared with other apes. They protect the ends of the fingers from cuts, scratches and that sort of damage. More common among mammals are claws rather than nails, but those are also formed from keratin.Mamals lack exoskeleton then why nails grow?
nails grow because keratocytes keep on producing keratin, the same reason why our hair keeps getting longer too. a diet rich in protein encourages keratin production.
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