17th and 18th European war and war tactics interest me greatly. However, I'm not able to understand when the idea of line battles really began. Was the musket line the product of the musket itself, or a hold over from older pre-gunpowder tactics? Thanks!
While I'm no expert, I believe you are asking two questions at the same time, and I do not know if this is your intention. One question I got from your post was regarding volley fire. The second was on organizing musketeers on a line (linear formation).
To answer the first one, we need to look back to archery tactics. Volley fire has been used as far back as Agincourt (1415), and likely to have been used for much longer than that.
When firearms first came about, its early form was the Arquebus (literally "hook guns"). Given its cumbersome nature, reload times were long and can be variable. It was recognized early on that an arquebusier who is busy reloading his hand-cannon is vulnerable, so they were usually paired up so that one could have a loaded gun while the other was reloading. But they didn't yet fire by ranks, it was more a continuous fire to disrupt enemy formation which was dominated by pikes until the advent of the Thirty Years War.
During the Thirty Years' War (1618-1648) and the overlapping Eighty Years' War, Maurice of Nassau introduced significant reforms. He increased the ratio of firearms to pikes/halberds, eventually favoring the newer Muskets over the older Arquebus. He also drilled his musketeers and arquebusiers in reload and fire, well enough that they could now do so in sync.
Here's a pic of the famous "manual of arms" printed in that era.
So I would credit him and his era for "the first time bunch of musketeers lined up and fired at each other in volleys."
As for the second question, the thinning of the depth of shooters occurred over the same period and beyond. Gustavus Adolphus was often credited with thinning his formations to four or six ranks deep, drastically less than the prevailing practice of his time.
Anyway, the early modern era is really fascinating!
I'm surprised no one has said this, but you need to be looking at the Ottomans. I don't have all my books with me, but they were pretty much the first group to push for integrating gunpowder weapons into their military, notably the artillery. Janissaries were using matchlocks in the mid-1400s and Janissary formations consisted of bunching up and punching a hole in enemy ranks via counter-attacks, so it stands to reason that they did the same with the firearms.
Description of Janissaries battle formation:
Generally they positioned artillery at the center, and often a screen of Azabs covered their front and household cavalry positioned behind them or on the flanks. We also know that the Janissaries preferred several rows of deep formations and achieved a continuous barrage of fire by rotating rows forward. They were able to maintain this formation even against heavy enemy fire because of the ir discipline, courage, and training.
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Early, organized usage:
The Battle of Kosovo of 1448 was also a turning point for the Ottoman military. This was the first time that the Ottomans employed up-to-date battle tactics, such as war wagons and firearms, on a large scale [...]
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Some people think firearms were in use possibly late 1300s. Early 1400s is, apparently, when we first start seeing them getting used on larger scales with proper integration into armies, battle formations, logistics, etc. etc.
Also, as others have stated, volley fire predates firearms, and lined formations also predate firearms. It would make little sense for armies to suddenly abandon both at the advent of firearms.
The adoption of muskets into European warfare was more of a gradual transition than a direct jump. Early gunpowder weapons were quite slow, cumbersome, and inaccurate- it's odd to think of something like the Brown Bess as being a technological marvel, but it was in fact the result of centuries of development and innovation.
The most notable transitional stage was the pike and shot formation, best exemplified by the Spanish tercio. In its most prominent form, this consisted of a pike square with smaller squares of arquebusiers at its corners. The matchlocks were the ranged arm of the formation, with the four corners able to support each other with fire and react to an attack from any direction, while the pikes could protect the arquebusiers from cavalry and engage enemy formations in melee combat.
The bayonet was the real game-changer in terms of tactics, coming into its own in the late seventeenth century. A musket with a bayonet is not all that dissimilar from a pike, and this allowed musketeers to defend themselves against cavalry and engage other infantry at close range without need for supporting units.
As well, the development of the flintlock firing mechanism and other improvements in gun technology made the earlier square formations less necessary, as firearms were effective enough to not require the former arrangements in depth. From there, line deployments were a natural evolution, as they allowed more guns to be brought to bear with less exposure for each soldier, multiplying tactical effectiveness.
I'm aware that this doesn't answer your initial question of what the first instance of combat in line was; hopefully someones else will be able to speak to that.
