Avestan anthropogonies

what's the etymology of Siyamak, Siyamek, ancestor of men in Avestan mythology?

< *syama "black" ? akin to Skt s^yama-?

Gayo: Mar&rtan (Keyumars, Kayo:mart, Gayomart) (<*gWoyo-morton)
|
Mas^ya and Mas^yana (<*Martiya)
|
Siya:mek
|
Haos^yaNha (Hushang, Hoshang) (<*Haus.yasa)
|
Tahmuras
|
Yima Xs^aeta (Jemshid) cf. Yama

1) Mas^ya and his sister/wife sp born from trees, like Ash and Embla

2) The she-demon of lusciouness Jahi...what's her etymology? Any Sanskrit cognate?

Joao SL