There wasn't one. Man doesn't even have a first name, let alone a last name. Adam simply means man, male, or husband in Hebrew. Eve was given a name by the man based on her subjugated role (part of the Falling) as source of man's family in contrast to her God-given identity as man's helpmate, an equal or possibly even superior role (as God is Israel's helpmate).
to my knowledge they didnt have a lastname, no one really did until later on when last names where instituted to rember who your ancester was so for example your name is John and your fathers name is Paul, your name would be John Paul, then your sons last name would be John and so on.