You cannot. You need at least 2 pairs to have ethernet work.
This is true if you are using twisted-pair based ethernet (10baseT, 100baseT, 1000baseTX), but remember that the original 10mb ethernet was based on a single coax cable bus running between stations, tapped at each location.
Coax is not used for new installations, but it is still out there. It's generally considered to be obsolete, but it still works if you don't expect high performance.
Coax based ethernet is 10mb/sec, shared media, half-duplex, unlike 10baseT which uses dedicated tx/rx paths, and can be full-duplex if connected to switches.
Coax based ethernet comes in two types. ThickNet (10base5) uses a thick 50ohm coax similar to RG8/RG213/RG214 type. Normally yellow or orange in color, tapped with vampire taps, or N connectors. Maximum segment length 500m before a repeater or bridge is required. Each end must be terminated with a 50ohm terminator.
ThinNet (10base2) uses RG58A/U type 50ohm coax, with a maximum segment length of 185m. BNC connectors and T's are used at each station. Each end must be terminated with a 50ohm terminator, which may be built-in to a hub.
Note that these are both 50ohm coax types, not the 75ohm coax used for tv/sat/video.
If you are using a 10mb hub with a BNC connector on it, you can use RG58A/U to extend the network to other hubs.