The Oxford History of Western Music, Pt. 11

The first appearance of pieces exhibiting the title "concerti" are by Andrea Gabrielli from 1585, and might be described as Masses for voices that are organised into antiphonally organised groups such that their contrasting timbres and musical characters form the main thrust of the work.
That only some of the voices are marked "a cappella" suggests that the other "choirs" are actually played by instrumental ensembles. Thus the earliest use of "concerto" suggests, simply, voices and instruments.
Ten years later the Bolognese organist Adriano Banchieri's piece Concerti ecclesiastici a otto voci was published including a seperate part for the organist that contained a harmonic "reduction" of the many vocal and instrumental parts. Accompanied songs soon after were published with an even more streamlined organ part, called the basso continuo.
The basso continuo was not "invented" at this time. Simply, publishers were now documented a practice that organised had long used in their "oral" tradition.
Interestingly, this new development gave rise to backlash among some organists who saw the published, "fully-realised" basso continuo part as an easy way for the lazy to play something that previously only the studied could.

The Oxford Histroy of Western Music, Pt. 10

The Madrigal (music set to secular poetry) became the hotbed for music experimentation in the sixteenth century. With Ars Perfecta composers such as Josquin streamlining composition to as perfectly express holy texts as clearly as possible (although not without incredible craftsmanship and feats of "composition"), composers used secular texts to create links between the expression in text and music.

Hence the representation of text became the challenge for many composers, and resulted in many different forms of radical experimentation in terms of texture, melody and harmony. Luca Marenzio's Solo e penso. Is the first example of the entire chromatic scale in a piece of music; it begins on F-above-middle-C and proceeds, by semitone, to cover the range of a major 9th, before descending again. Some semitone steps were diatonic, that is, between the 3rd and 4th or 7th and 8th degree of an ionian scale, or were chromatic, that, spelt using a different accidental on the same letter name. The text (Alone and thoughtful I pace the most deserted fields/with slow and heavy steps) is expressed through this mono-rhythmic and mono-intervallic soprano melody. The resulting harmony, if we were to use a system of harmonic analysis based on the 18th and 19th century, is unconventional. The fact of the matter is, however, that this harmony arises out of Marenzio's want to give life to text, rather than experiment with harmony for its own sake.

The use of the complete chromatic scale also suggested a refinement of the tuning system to equally space the 12 semitones through the octave while maintaining the 2:1 ratio of frequencies between octaves.