This might also help those who want certainty and more info:
http://storage.microsemi.com/en-us/_whitepapers/tech/sata/sas_sata_unprlcompat.htm (from early 2000s)
[T]he SAS interface will also be compatible with lower cost-per-gigabyte SATA drives, giving system builders the flexibility to integrate either SAS or SATA devices and slash the costs associated with supporting two separate interfaces.
The SAS connector is [..] form-factor compatible with SATA, allowing SAS or SATA drives to plug directly into a SAS environment whether for mission critical applications with high availability and high performance requirements or lower cost-per-gigabyte applications such as near-box storage.
SATA connector signals are a subset of SAS signals, enabling the compatibility of SATA devices and SAS adapters. SAS drives will not operate on a SATA adapter and are keyed to prevent any chance of plugging them in incorrectly.
[T]he similar SAS and SATA physical interfaces enable a new universal SAS backplane that provides connectivity to both SAS drives and SATA drives [..]
SAS consists of three types of protocols, each used to transfer different types of data over the serial interface depending on which device is being accessed. Serial SCSI Protocol (SSP) transfers SCSI commands, SCSI Management Protocol (SMP) sends management information to expanders and SATA Tunneled Protocol (STP) creates a connection that allows transmission of the SATA commands. By including all three of these protocols, SAS provides seamless compatibility with [...] SATA devices.
Also these example product spec:
"ExpressSAS 6Gb/s SAS/SATA RAID Adapters provide high performance data protection to direct attached SAS and SATA JBOD storage [..]"
"Support for 6Gb/s and 3Gb/s SATA and SAS drives to balance cost and performance"