The article adds that the Indians already celebrated a kind of Thanksgiving long before the Pilgrims arrived, in addition to being a very organized and civilized people. These additional details show that the Indians already knew the concept of Thanksgiving and were not taught by the pilgrims.
We can arrive at this answer because:
The text shows how the natives formed groups very well organized in terms of their political and social structure.
They knew how to use natural resources and moved around several times a year to protect themselves from the seasons and use natural resources more appropriately.
The article shows that indigenous people celebrated a type of Thanksgiving six times during a year, to celebrate the activities they had done successfully, even before the arrival of the pilgrims.
This shows us that the concept of Thanksgiving was not introduced on the continent by the pilgrims, as the indigenous tribes already knew about this type of celebration and already celebrated it more often than the pilgrims.
