Merritt Ruhlen suggests that the geographical distribution of Eurasiatic shows that it and the Dené–Caucasian family are the result of separate migrations. Dené–Caucasian is the older of the two groups, with the emergence of Eurasiatic being more recent. The Eurasiatic expansion overwhelmed Dené–Caucasian, leaving speakers of the latter restricted mainly to isolated pockets (the Basques in the Pyrenees Mountains, Caucasian peoples in the Caucasus Mountains, and the Burushaski in the Hindu Kush Mountains) surrounded by Eurasiatic speakers. Dené–Caucasian survived in these areas because they were difficult to access and therefore easy to defend; the reasons for its survival elsewhere are unclear. Ruhlen argues that Eurasiatic is supported by stronger and clearer evidence than Dené–Caucasian, and that this also indicates that the spread of Dené–Caucasian occurred before that of Eurasiatic.