Tlingit) and appears in position −3 in Haida. In Sino-Tibetan, Classical Tibetan has a "directive" prefix /d/-, and Nung has a causative prefix /d/- (positions do not apply because Sino-Tibetan verbs have at most two prefixes depending on the language). A past tense marker /n/ is found in Basque, Caucasian, Burushaski, Yeniseian, and Na-Dené (Haida, Tlingit and Athabaskan); in all of these except Yeniseian, it is a suffix or circumfix, which is noteworthy in these (with the exception of East Caucasian and Haida) suffix-poor language families. Another prefix /b/ is found in some Sino-Tibetan languages; in Classical Tibetan it marks the past tense and precedes other prefixes (if any). It may correspond to the Tlingit perfect prefix wu-/woo- /wʊ, wu/, which occurs in position −2, and the fossilized Haida wu-/w- /wu, w/ which occurs in verbs with "resultative/perfect" meanings. "There are also some commonalities in the sequential ordering of verbal affixes: typically the transitive/causative *s- is directly before the verb stem (−1), a pronominal agent or patient in the next position (−2). If both subject/agent and object/patient are referenced in the same verbal chain, the object typically precedes the subject (OSV or OVS, where V is the verb stem): cf. Basque, West Caucasian [see table above], Burushaski, Yeniseian, Na-Dené, Sumerian templates […]. [Footnote: "Alone in N[a]-D[ene] Eyak allows for subjects and objects in a suffix position."] In Yeniseian (position −5) [...] and Na-Dene (position −5) [...] noun stems or (secondary) verb stems can be incorporated into the verbal chain." (Bengtson 2008:108) The mentioned "transitive/causative" */s/- is found in Haida, Tlingit, Sino-Tibetan, Burushaski, possibly Yeniseian ("an 'empty' morpheme occupying the position of object in intransitive verbs with an animate subject"; Bengtson 2008:107) and maybe in Basque. A causative suffix *-/s/ is found in many Nostratic languages, too, but its occurrence as a prefix and its position in the prefix chain may nevertheless be innovations of Dené–Caucasian. Family tree proposals Starostin's theory The Dené–Caucasian family tree and approximate divergence dates (estimated by modified glottochronology) proposed by S. A. Starostin and his colleagues from the Tower of Babel project: 1. Dené–Caucasian languages [8,700 BCE] 1.1. Na-Dené languages (Athabascan–Eyak–Tlingit) 1.2. Sino-Vasconic languages [7,900 BCE] 1.2.1. Vasconic (see below) 1.2.2. Sino-Caucasian languages [6,200 BCE] 1.2.2.1. Burushaski 1.2.2.2. Caucaso-Sino-Yeniseian [5,900 BCE] 1.2.2.2.1. North Caucasian languages 1.2.2.2.2. Sino-Yeniseian [5,100 BCE] 1.2.2.2.2.1. Yeniseian languages 1.2.2.2.2.2. Sino-Tibetan languages Bengtson's theory John D. Bengtson groups Basque, Caucasian and Burushaski together