What were the 21 potential landing sites for the Mars 2020 mission?

Question

In August 2015 more than 150 scientists attended a conference to evaluate 21 potential landing sites for NASA's next rover, a mission called Mars 2020.

Out of those proposed landing sites the Mars 2020 Landing Site Steering Committee, appointed by NASA, came up with a list of 8 selected sites.
A workshop held in February 2017 discussed these sites and narrowed the list down to 3 sites for further consideration.

Because i'm interested in where on Mars past life could have existed, what were those initially proposed 21 landing sites in 2015 ?

Answer 1

Here they are, from the letter summarizing the results of the 2015 workshop:

The ancient habitable environment column sounds like what you're looking for.