Dtjytyk

Thieves World (ISFDb) by Robert Asprin is the first formal shared-world project I can remember; its success seemed to create a bunch of similar projects through the 1980s. I'm thinking of Heroes in Hell (ISFDb) by Janet Morris, The Man-Kzin Wars, Bolos, Merovingen Nights and probably others I'm just forgetting.

But was there an earlier instance of a shared-world where more than 2 authors wrote independent stories, with their own viewpoint characters, that shared a common universe and acknowledged the characters and events written by other authors?

I'm not considering series like Doctor Who where multiple authors serially wrote stories about the same set of characters, or comic books. (Specifically, I'm interested in "open universe" shared worlds as defined by the Encyclopedia of Science Fiction article.)

It will all depend on exactly how you wish to define the concept, but in short there were many before 1979.

In 1866, the magazine All the Year Round published a set of short stories at Christmas titled Mugby Junction. This contained related stories, including Charles Dickens' ghost story The Signal-man.

The Encyclopedia of Science Fiction article on Shared Worlds has many others with varying amounts of SFF content until 1952's The Petrified Planet anthology which is completely Science Fiction.

Arguably, the Cthulhu Mythos fits the mold. While H.P. Lovecraft was the primary author, he also encouraged and endorsed the work of other authors, such as Robert E. Howard and Fritz Leiber, in the same universe.

For example, Robert E. Howard's character Friedrich Von Junzt reads Lovecraft's Necronomicon in the short story "The Children of the Night" (1931), and in turn Lovecraft mentions Howard's Unaussprechlichen Kulten in the stories "Out of the Aeons" (1935) and "The Shadow Out of Time" (1936). Many of Howard's original unedited Conan stories also involve parts of the Cthulhu Mythos.

Argh! Fuzzyboots mentioned the Cthulhu Mythos before me.

I remember a shared world project that might possibly have a similar date as Thieves World. I remember reading a story in a magazine set on the same planet as a story by another author and wondering what was up. It turned out that it was a shared universe. I later read a collection of stories set on that world and noted that the writers did not agree on how many limbs the "fuxes", the people of that world, had.

I now believe the anthology was Medea: Harlan's World (1985), edited by Harlan Ellison. Several of the stories were published as early as 1978. And articles describing Medea were published as early as 1975.

In Tarzan Alive (1972) Philip Jose Farmer (1918-2009) created the "Wold Newton Family" by claiming that many fictional characters created by other writers had mutant powers because of being descended from a group of related and intermarried travelers who were exposed to radiation from the famous Wold Cottage Meteorite on December 13, 1795. The "Wold Newton Family" has since been expanded into the "Wold Newton Universe" by Farmer, Win Scott Eckert, and others.

Eckert also created a Crossover Universe in Crossovers: A Secret chronology of the World (2010).

https://en.wikipedia.org/wiki/Wold_Newton_family#The_Wold_Newton_Universe1

Fletcher Pratt (1897-1956) made his novel The Well of the Unicorn (1948) a sequel several generations later to Lord Dunsany's play "King Argimenes and the Unknown warrior" (1914).

Ambrose Bierce (1842-1914?) wrote "An Inhabitant of Carcosa" in 1886. Robert W. Chambers (1865-1933) mentioned Carcosa in several of the short stories collected The King in Yellow (1895). The King in Yellow was a character in a fictional play called The King in Yellow set in Carcosa, which may be on another planet. Carcosa became part of the Cthulhu Mythos.

And no doubt there are older examples.

Arrgh! Again! eshier beat me to mention the shared world anthology The Petrified Planet (1952) edited by Fletcher Pratt (see above).

And also see my answer and other answers to this question: Who was the first author to rework another scifi/fantasy author's character? 2

It is possible that the sequels to Amadis de Gaula, medieval Arthurian romances, medieval saint's lives, or ancient Greek epic poems could be considered the first shared fictional universes.

It's probably as old as civilization. Mythologies, such as Greek mythology, were the result of probably hundreds if not thousands of authors in a shared universe, and there are works of literature such as the Iliad that make reference to characters in those mythologies. The general consensus is that the Synoptic Gospels and the Gospel of John come from separate sources.