What is the origin of yagna that has been said in Gita and what does it give?

In the Gita before telling what is yagna lord krishna tells us where does yagna come from ...
        saha-yajnah prajah srstva, purovaca prajapatih
       anena prasavisyadhvam, esa vo ’stv ista-kama-dhuk
                                             -B.G.CHAP3 SLOKA 10
English translation- “At the beginning of kalp-the course of self-realization Prajapati Brahma shaped yagya along with mankind and enjoined on them to ascend by yagya which could give them what their hearts aspired to.’’


Prajapati Brahma, the god presiding over creation, made mankind along with yagya at the beginning and told men to progress through yagya. This yagya, wholly propitious, was prescribed or ordained as the action which would satisfy their hunger for realization of the eternal God.Who was the creator of mankind along with yagya? Was it Brahma and who is he?



The sage who has realized and become one with the 

Supreme Spirit, the fountainhead from which all mankind has 

arisen, is ‘‘prajapati.” Wisdom that results from knowledge of God 

is itself Brahma. At the moment of this realization the mind becomes 

a mere instrument. It is God himself who then speaks through the 

voice of the sage.

There is constant growth of wisdom after the commencementof spiritual adoration, or worship. Since at the beginning this wisdom is endowed with knowledge of God, it is called brahmvitt. Gradually,as evil impulses are subdued and the knowledge of God is enriched,this wisdom is said to be brahmvidwar. As it ascends yet higher and gets more refined, it comes to be known as brahmvidwariyan.At this stage, the sage who is blessed with knowledge of God also achieves the capacity to bring others on to the way of spiritual growth. The highest point of wisdom is brahmvidwarisht, that state of divine inundation in which the spirit of God flows through it like a crystal current. Men who have attained to this state enter into and dwell in the Supreme Spirit from whom all mankind is born. The minds of such sages are mere instruments and it is they who are called “prajapati.” By dissociating themselves from the contradictions of nature, they create the Self who is yet unaware of the process of meditation or God’s worship. Conferring perfection
which is in accordance with the spirit of yagya is the creation of mankind.



Sanskar has always been 

there: but before the sages conferred perfection on it, it was 

deformed and in a state of anarchy. To shape it in accordance with

the requirements of yagya is the act of refining and adorning.


Some such accomplished sage or sages created yagya besides 

The true cure is that which provides liberation from the general 

creating mankind at the beginning of kalp, the course of Selfrealization.(kalp- 

malady of the world. The beginning of worship or adoration is the 

commencement of this remedy. When meditation is complete, we 

are wholly cured.)

Thus sages with their beings in the Supreme Spirit gave a proper 

shape to spiritual excellence and yagya, and instructed men that 

they would prosper through the observance of yagya.

They rather wished men to know that yagya would fulfil their God-inclined aspirations. A logical question that confronts us here is whether yagya leads to immediate attainment of God or only by gradual steps. Brahma further said to mankind :


                 devan bhavayatanena, te deva bhavayantu vah
               parasparam bhavayantah, sreyah param avapsyatha
                                                     -B.G.CHAP-3 SLOKA 11
English Translation- “And may you cherish gods by yagya and may gods foster you, for this is the means by which you will finally achieve the ultimate state.”

Cherishing gods by yagya means fostering sacred impulses. 

And that is also how gods foster mankind. Thus, by mutual 

augmentation men will ultimately achieve that final bliss after which 

there is nothing more to achieve.

And what will yagna give? This is stated in the following sloka no. 13 of chapter 3 and chapter 4 sloka 31.

                  yajna-sistasinah santo, mucyante sarva-kilbisaih
           bhunjate te tv agham papa, ye pacanty atma-karanat
                                             -B.G. CHAP-3 SLOKA 13
English Translation- ‘‘The wise who partake of what is left over from yagya are rid of all evil, but the sinners who cook only for the sustenance of their bodies partake of nothing but sin.”
                 yajna-sistamrita-bhujo, yanti brahma sanatanam
             nayam loko ’sty ayajnasya, kuto ’nyah kuru-sattama
                                            -B.G. CHAP-4 SLOKA 31
English Translation-“O the best of Kuru, the yogi who have tasted the nectar flowing from yagya attain to the eternal supreme God, but how can the next life of men bereft of yagya be happy when even their life in this world is miserable?”


So, lord krishna clearly states that  "yajna-sistamrita-bhujo, yanti brahma sanatanam" What yagya generates-what results from it, is nectar the substance of immortality. A direct experience of this is wisdom. The one who feeds on it becomes one with the eternal God. So yagya is something which with its completion unites the worshipper with God.


So, the question that now remains is what is yagna?