Exodus and Metallica
The earliest documented roots of the Bay Area thrash scene date back to the formation of Exodus in 1979. By the time the group recorded their full-length album four years later, five different guitarists or bassists had already passed through the line-up, with some going on to join or form bands that were equally relevant to the area's burgeoning metal scene. In November 1982, Exodus opened a show at San Francisco's Old Waldorf venue for Metallica, a then-relatively unknown (and unsigned) band from Southern California who were recently discovered by Brian Slagel and had appeared on the first volume of his Metal Massacre compilation. Exodus (who were also unsigned at this time) had distributed an untitled demo the same year with a line-up that included guitarist Kirk Hammett.
Although Metallica had initially formed in Los Angeles, it wasn't until their February 1983 relocation to the East Bay area that Cliff Burton and Kirk Hammett joined as bassist and lead guitarist, sealing the band's first, formative line-up. The group (which then consisted of Burton, James Hetfield, Lars Ulrich and Dave Mustaine) had moved into a Carlson Blvd. house in El Cerrito, near Albany Hill, which had been rented by Mark Whitaker, then-manager of Exodus. After Mustaine was removed from the line-up that same year, then-Exodus guitarist Hammett would replace him following Whitaker's recommendation; in turn, Mustaine would move back to Los Angeles to form Megadeth.
Metallica's signing to Megaforce Records, the release of their first album and the songwriting process and rehearsals for Ride the Lightning and Master of Puppets had all taken place while the group resided at Whitaker's El Cerrito home.
Networking between local bands
Jeff Becerra of Possessed
Burton and Hammett's friendship with other local acts, notably Oakland's Exodus and Testament, and later, San Francisco's Death Angel—among others—strongly vitalized the scene, leading to intensive touring and tape trading that would cross borders and seas, and eventually graduate to record signings.
El Sobrante's Possessed would bring a turning point to the genre with 1985's Seven Churches, regarded as the first album to cross over from thrash metal to death metal for the largely "growling vocals" and subject matter dealing with horror and the occult. In addition to the inspiration of black metal, it would predate other albums, such as Los Angeles-based Slayer's Reign in Blood and Death's San Francisco Bay Area-written and rehearsed Scream Bloody Gore, which had also been regarded as influential to thrash and death metal.
Members from Possessed had also maintained a strong network with other integral musicians of the Bay Area thrash scene: vocalist Jeff Becerra and then-guitarist Larry LaLonde had played in a short-lived speed metal band called Blizzard, which featured former Exodus bassist Carlton Melson and Desecration guitarist Danny Boland (then-Possessed drummer Mike Sus and former Possessed bassist Bob Yost had also played in Desecration, a death/thrash band active between 1985 and 1989). Former Exodus bassist Geoff Andrews co-founded Possessed before Becerra replaced him in 1983, taking over both vocal and bass duties. Andrews went on to form the thrash band Sabertooth with Johnee McKelvey. 
After Possessed's first disbanding, LaLonde went on to join Blind Illusion, which featured former Exodus members Evan McCaskey and Tim Agnello, then-Heathen members Marc Biedermann and Dave Godfrey, and Blizzard drummer Mike Miner. Blind Illusion co-founder Les Claypool (who was a high school friend of Kirk Hammett at Richmond's DeAnza High School) would unsuccessfully audition as Metallica's bassist in 1986 after Burton's bus accident death in Sweden.
Networking between bands in other scenes
Heavy metal in Southern California had spanned back to the 1970s, and in the 1980s, was the home base of Slayer and Megadeth, two of thrash's "big four". However, from the mid-1980s until the early 1990s, glam metal had been the pervading and popular subgenre within the boundaries of the Los Angeles scene.
Slayer's Jeff Hanneman, Tom Araya
and Kerry King at the 2007 Hellfest. Although Slayer were not formed in the Bay Area, the band had gotten an early headstart in the Bay Area thrash metal scene, and are often associated (along with Megadeth
) as being a part of it.
Notwithstanding, both Slayer and Megadeth would receive recognition early in their duration by playing at Bay Area venues, including Berkeley's Ruthie's Inn with Exodus between 1984–85, a time in which current Slayer guitarist Kerry King had played alongside Mustaine in Megadeth. Oakland native Jeff Hanneman was a founding Slayer member; Paul Bostaph (formerly of Forbidden, and later Exodus and Testament) had been Slayer's full-time drummer for the balance of the 1990s. Similarly, drummer Dave Lombardo played on Testament's The Gathering album before returning to Slayer in 2004.
Before the 1989 dissolution of Desecration (which featured former members of Possessed), Concord drummer Chris Reifert had been a live guest musician at one of the band's shows. However, preceding this, Reifert had also been an acquaintance of Chuck Schuldiner, who relocated his band Death from Florida to Reifert's hometown in the Bay Area while building a line-up. Noticing the growth of the Northern California thrash scene, Schuldiner had earlier moved Death to the Bay Area in 1985 and briefly collaborated with drummer Eric Brecht, who recently exited Texas crossover thrash band D.R.I. and would eventually join Attitude Adjustment. However, it was the Schuldiner-Reifert pairing that would lead to Death's signing to Combat Records and release of the album Scream Bloody Gore, a debut regarded (alongside Possessed's Seven Churches) as one of the first releases to bridge the gap between thrash and death metal, as noted earlier. Both records had been produced by Randy Burns.
In 1987, shortly after the release of Scream Bloody Gore and Schuldiner's move back to Florida, Reifert would form his own endeavor, Autopsy, which has also been regarded as an early inspiration of the death metal genre. Autopsy's 1989 debut, Severed Survival, would feature bassist Steve DiGiorgio, founder of Antioch, California thrash metal band Sadus; Schuldiner had also met DiGiorgio while living in the Bay Area, who would play bass for Death on the albums Human and Individual Thought Patterns.
Reifert's legacy within the death metal genre would be further explored on the 1992 release of Acts of the Unspeakable, which featured bassist Josh Barohn, who also played bass on Effigy of the Forgotten, the 1991 debut album of pioneering New York death metal band Suffocation.