I jumped on the twin theory because in order for language to provide an advantage, two people have to be able to use it since words and their meaning are a shared phenomena. One speaking hominid wouldn't have the same advantage as in earlier times, one animal did by mutating an opposable thumb or color vision. Not only would speaking twins have an advantage, but the mutation would spread more quickly as a result of having two original versions.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13