Arguably the first Microsoft version of SQL Server was SQL Server 2005. Prior to this, in SQL Server 7.0 and SQL Server 200 there was quite a bit of legacy Sybase code. It was actually prior to version 7 that Sybase sold the code to Microsoft. Anyway more detail on this is availble on Wikipedia in their article Microsoft SQL Server which covers this genesis.
The best place I have found for establishing which version of SQL Server you actually have is or rather mapping the bulid number to a version, service pack and cumulative update package is SQL Server Builds. I usually get this version number from SQL Server Management Studio but you can also get it by running the following query:
SELECT @@VERSIONAlternatively you can use the query in Database Information.
As of 2016 Microsoft are working on SQL Server for Linux. This is not a port but a different approach, so read How an old Drawbridge helped Microsoft bring SQL Server to Linux | Ars Technica for details. It is not a short article but a good one and it covers SQL Server internals, so worth reading even if you are not interested in the Linux angle.