Actually, there were serial-to-ethernet adapters. I'm aware of two- the Farallon EtherWave Mac/PB adapter and a similar one from Dayna.
Basically, you plugged it into the serial port and loaded a driver. IIRC, they usually only worked with classic networking (not OpenTransport). The Farallon could also work in a "default" mode as a LocalTalk bridge.
I use to use one of the Farallon adapters with a Duo 230. It was okay, albeit a bit slow.
John