Metal originated even earlier than Hardcore. The founding fathers would be Led Zeppelin and Black Sabbath. Black Sabbath had an especially strong influence on the genre since they distanced themselves even more from Blues and Jazz influences (still common in the rock genre) in their guitar-work. Their guitarist Tony Iommi is to this day considered as a huge idol for many guitarists despite two of his fingers missing their tips.
Said guitar-work also influenced the term “Heavy Metal” due to the low tune of their music as well as setting the foundation for the subgenre Doom Metal. Today, Heavy Metal is considered its own subgenre. Led Zeppelin on the other hand is to be less understood as a founder, but more as an inspiration, due to their virtuous guitar-play and distinctive drums.
It was 10 years after Black Sabbaths first album before bands like Iron Maiden and Saxon came along to start the further development of Metal and push Punk away in its popularity. Said bands achieved that by combining the heavy guitar-riffs of Black Sabbath with an increased dynamic and speed in their music.
After those long 10 years, the following development was much more rapid. Venom started to take the occult lyrical content of Black Sabbath to a next level and included satanic topics while adapting their music to those lyrics with an increased level of aggression. Furthermore they started three important trends with their first two albums. The search for even harder music; naming their second album Black Metal and thus inspiring the start of new subgenre of the same name which to this day involves extreme lyrical contant and they the founding of Thrash Metal.
This is when youth protection started taking a stance against Metal and labeled it as dangerous to young people as well as claiming it to be unnecessary noise. Due to the lyrical content of the first Metal bands they probably had a point.
Shortly after Venom, bands like Slayer, Megadeth, Metallica and Anthrax came along which to this day shaped and characterised Thrash and Speed Metal. Metallica's Kill 'em All is considered the first pure Thrash Metal album, which was followed 3 years later by Slayers Reign of Blood.
Meanwhile in Germany, independent of the previous stated bands, developed the internationally important Thrash Metal bands Kreator and Sodom.
Lyrically all those bands vary greatly and are independent of each other by distancing themselves from the occult and satanistic influence of earlier bands. Only Slayer decided to use rather violent lyrics which they later admitted to do in order to get and create a certain image since their first two albums were closer to what was later considered Power Metal, by Bands like Blind Guardian and Helloween.
During the mid 80s Glam Metal came along which was often laughed at by most Metal listeners. Bands like Bon Jovi, Alice Cooper and Guns'n'Roses had more to do with Rock than what Metal was and is. Their influences derived more from Led Zeppelin than Black Sabbath. The guitar-playing of Slash and Van Halen is paramount in showing that.
Ironically shortly after the rather soft-sounding Glam Metal appeard, Death Metal also developed. The term was inspired by the band Death. The band not only influenced the naming of a new subgenre, but also was one of the first which used screaming, hence becoming the foundation for the now typical Black and Death Metal screaming and guttural vocals as well as helping the development of growling within the Metal and Hardcore scenes. That being said, Kreator already used a very Death Metal like screaming a year prior.
In the subsequent development of Black and Death Metal it is important to note that these two subgenres mostly flourished in the east of Europe and thus were strongly influenced by ideologies during the time of the iron curtain. On the other hand, Thrash Metal established and developed itself in the rest of the world more freely and independently.
Transitioning to the 90s, Grunge started to fend off Glam Metal. Due to the start of the Grunge-Wave, Metal went back down into the underground. Pantera was one of the only original Glam Metal bands that managed to transcend into Thrash Metal and increase their popularity. They also were one of the few to have some Hardcore elements to their music in terms of lyrics, vocals and their drum-playing, while going by the motto of Venom to try and become harder and faster in their music.
At the same time Pagan and Viking Metal developed underground, inspired by but distancing itself from Black and Death Metal as well as Sweden giving birth to Melodic Death Metal (Göteborg School) which later became important for Metal and Hardcore overlapping with each other.
A bit later during the mid 90s, the Finnish band Nightwish started to become more famous and helped to develop another subgenre of Symphonic Metal while the biggest European Metal Festival Wacken had it's first event ever. At this time the American scene stagnated which resulted in more genre crossovers like Nu-Metal, inspired by Faith No More, and it's huge hype during the end of the 90s which many should still remember. A genre that many Metal listeners are critical to, due to it's high Hip-Hop and Hardcore and low Metal influence.
Lyrically, there was still no emphasis within the huge number of subgenres except for Black and Death Metal. Only Thrash Metal stayed a bit more prone to violence while Power Metal had more fantasy and fable-oriented lyrics. Vocally only Thrash Metal involved the growling and screaming from Death Metal while most other subgenres go without these vocal techniques.
Important chronological song-recommendations for the key-developments of Metal:
Led Zeppelin - Whole Lotta Love - 1969
Pay attention to the song-structure and guitar-playing.
Black Sabbath - Paranoid - 1970
Listen to the grooving guitar as well as the deep-tuned instrumentalisation.
Iron Maiden - Running Free - 1980
Saxon - 757 - 1980
Venom - Black Metal - 1982
Pay attention to the dark, gloomy mood and the fast guitar-playing.
Metallica - Hit the Lights - 1983
Compare the development of Thrash Metal within the Iron Maiden and Saxon songs to this Metallica and the later linked Slayer songs.
Helloween - Walls of Jericho - 1985
German band and first known Power Metal band.
Slayer - Raining Blood - 1986
Taking things up a notch compared to Metallica - especially through the more intense drums.
Bon Jovi - Runaway - 1986
Personally I prefer Guns'n'Roses and Van Halen in terms of Glam Metal. Pay attention to the clear guitar-playing trhoughout.
Kreator - Ripping Corpse - 1986
German Band and first known with “grunting” vocals, which became more typical for Death rather than Kreators Thrash Metal.
Death - Infernal Death - 1987
Taking the grunting to a whole different level and with it being the founding father of Death Metal.
Pantera - Walk - 1992
Focus on the punkish build-up in the refrain as well as the drum-playing which is similar to some of the earlier mentioned Hardcore songs while the guitar-playing is pure Thrash Metal.
At the Gates - Blinded by Fear - 1995
Pay attention to the melodic guitar-playing which is the beginning of Melodic Death Metal that also brought up bands like In Flames, Soilwork and Dark Tranquility.
Faith No More - Ricochet - 1995
Crossover that is fairly similar to what later turned into Nu-Metal.
Korn - Good God - 1996
Nightwish - Sacrament of Wilderness - 1998
Towards the millenium things suddenly went up a notch and a lot of things happened almost simultaneously. Firstly the Metalcore-Hype started initiated by bands like Killswitch Engage and Chimaira which combined elements of the heavier Hardcore, Post-Hardcore, Melodic Death Metal and Thrash Metal. Already in the mid 90s, bands like Vision of Disorder, Hatebreed, Biohazard and later The Dillinger Escape Plan combined Metal and Hardcore, but simply didn't have as much success. Lyrically, this trend was a lot closer to its Hardcore roots where Killswitch Engage originally started with political and philosophical content while Chimaira initially was very close to Post-Hardcore dealing with personal topics.
As the Metalcore-Hype started turning up its gears towards Killswitch Engage's third Album, bands like Bullet for my Valentine, As I lay Dying, Underoath (later getting closer to Emocore), Shadows Fall and All that Remains came along.
Suddenly with all the success Metalcore bands had, it became more accepting for Metal and Hardcore to be combined. That created a whole new wave of bands eager to experiment and push previously existing boundaries. The experimenting led to an increased influence of progressve songwriting and subgenres as well as rediscovering Venom's motto of becoming faster and harder resulting in extreme subgenres like Deathcore which is a mix of Death Metal and Hardcore elements. The Metalcore-Hype is also directly responsible for the upcoming Emo- and Emocore-Hype a few years later.
While there are still fans within the Metal and Hardcore camps that look rather critical towards this blending, it is widely accepted by now since it brings more variety to the scene as well as helping already established subgenres to thrive.
As a result, Metal has become one of the most diverse genres of our time where even the subgenres are musically so far apart that they could stand alone as their own genre. The Hardcore genre naturally benefitted from this development a lot as well by moving away from the musically limiting Punk foundation of the genre. The wide diversity of subgenres includes those with high aspiration and technical quality, but also some rather simple-minded ones.
Overall Metal developed into a playground without boundaries, where bands can experiment with new things and push past limits, whilst Hardcore benefitted from an increased musical diversity for expressing raw emotion.
Milestones of the Metalcore-Trend and the resulting increase of creative songwriting:
Disclaimer: I shall put a space before and after the dot of .com to avoid a video-overload.
The Dillinger Escape Plan - Sugar Coated Sour - 1999
Pay attention to the genre-mixes as well as to the purposefully scattered guitar-playing which inspired progressive bands later on as well as being the foundation for many technical subgenres such as Mathcore.
www.youtube . com/watch?v=DtFoUboFyPk
Chimaira - Let Go - 2001
The song is pretty close to being a blueprint of what Metalcore became.
www.youtube . com/watch?v=p7DIpMPHKoU
Killswitch Engage - My Last Serenade - 2002
Similar to the Chimaira song, but with more melodic elements to it.
www.youtube . com/watch?v=Cr1H2Hmc3zA
Trivium - Rain - 2004
A lot more of a Thrash Metal influence.
www.youtube . com/watch?v=g3F4LXrIkNk
Alexisonfire - Control - 2004
Developed out of Metalcore as well, but a lot closer to Post-Hardcore as well as to what became Emocore.
www.youtube . com/watch?v=o_yaIUp-5fs
Between the Buried and Me - Backwards Marathon - 2005
Progressive mix that is closer to Dillinger Escape Plan than Chimaira and Killswitch Engage. Additionally implements quite a few Jazz elements.
www.youtube . com/watch?v=ggsC9bzZHJQ
Between the Buried and Me - Alaska Instrumental - 2005
Same band, different song. The instrumental gives a better feel about what's going on.
www.youtube . com/watch?v=MWHrQkpTy5I
Horse the Band - Black Hole - 2005
Horse the Band singel-handedly created a new subgenre and inspired a short-lasting trend that was called Nintendocore – a combination of Metalcore elements with trance-synthesizer sounds that reminds one of the old NES and SNES music. Enter Shikari would be the most popular band of that genre.
www.youtube . com/watch?v=4DsfkJGVdlM
The Devil Wears Prada - Gauntlet of Solitude - 2006
Metalcore that's based a lot more on Hardcore elements while taking on Melodic Death Metal typical keyboard elements. The heaviness is getting closer to what later became Deathcore.
www.youtube . com/watch?v=aCqZ02D_CdI
Carnifex - Slit Wrist Saviour - 2007
Becoming harder and heavier: leading to Deathcore.
www.youtube . com/watch?v=seEpR-S2NCk
Animals as Leaders - Tempting Time - 2009
Inspired by Meshuggah and with an increase of progressive influences, the subgenre Djent came to be.
www.youtube . com/watch?v=z5UBS-yrlfU
Veil of Maya - Punisher - 2012
A more progressive Deathcore approach.
www.youtube . com/watch?v=CrJOAZUCcqg
Naturally there are close to infinite more subgenres, genre-mixes and inbetweens while older genres still focus on what they have always been. The key is musical freedom and diversity combined with high technical skills. What many non-metal listeners forget is that in order to play louder and faster the musicians need an increased skill on their instruments in order to do so. Something that is unfortunately often missing in modern pop where it is all about marketing, money and being as popular as possible instead of focusing on music as an art. Real musical artists these days don't have much success in the mainstream and thus it is a pleasure to have exactly that artistic freedom in the wide scene that is Metal and Hardcore.