How was DNS originally implemented?

In which I capture a bit of DNS history to remind myself

Back in the day, there was just one file: HOSTS.TXT.

It contained a name-to-address mapping for every entity within ARPANET.

/etc/hosts used to be compiled from HOSTS.TXT

It didn’t scale for a number of reasons:

As soon as administrators pulled the latest version of HOSTS.TXT, it would already be out of date
There was no way to enforce constraints eg; no duplicates on hostnames
It took a lot of resources to serve it up to every administrator