Tocharian

Hello to everybody here!

As a linguist-hobbyist, I am very pleased to have been invited to this
group. For a start, some questions.

The very first of them is why Tocharian is thought "primitive".
Namely, the Anatolian group is thought to be the first to be separated
from the other IE lnaguages and OK about it. But why Tocharian is
thought to be the next branch to have been separated from all the
others?