It is believed that the gods themselves made the Brahmin seers of yore privy to the Vedas, the primordial rhythms of creation, and as the communion took place in Sanskrit, it is called devabhasha, the language of the gods.
It is another matter though that in the latter-day Brihadaranyaka Upanishad, the Brahmins themselves postulated that ".. since he (man) created gods who are better than he: and also because, being mortal, he created immortals, it is his higher creation. Whoever knows this, comes to be in this, his higher creation."
Be that as it may, if one were to read the Purusha Sukta (10.7.90.1-16) of the Rig Veda, the foremost of the four Vedas, it would be apparent that v11- v13 are clever Brahmanical interpolations though a clear give away. Given v13's alleged creation of the Brahmins from the creator's face, it can be inferred that this sloka, and its two facilitators, were inserted into the said sukta by them, the self-proclaimed guardians of the divine revelations. So as to grasp this Brahmin mischief, the relevant original hymns would come in handy.
V10
tasmad yajnat sarvahutaha
richassamani jijignire
chandhagamsi jijignire tasmat
yajus tasmad ajayata
From that yajna (or sacrifice) wherein the Cosmic Being was Himself the oblation, were born the riks (the mantras of the Rig-veda) and the samans (the mantras of the Sama-veda). From that (yajna) the metres (like Gayatri) were born. From that (yajna again) the yujas (the Yajur-veda) was born.
V14
chandrama manaso jataha
chakshoh suryo ajayata
mukhad indrash chagnishcha
pranadvayur ajayata
From His mind was born the moon. From His two eyes was born the sun. From His mouth were born Indra and Agni. From His breath was born the air.
V15
nabhya asidanta riksham
shirshno dyauh samavartata
padhyam bhumirdishash shrotrat
tada lokagamm akalpayan
From (His) navel was produced the antariksha (the space between the earth and the heavens). Dyuloka (or heaven) came into existence from His head. The bhumi (th earth) evolved out of His feet, and deek (or spacial directions) from His ears. Similarly (the demigods) produced the worlds (too).
V16
vedahametam purusham mahantam
adityavarnam tamasastu pare
sarvani rupani vichitya dhiraha
namani kritva abhivadan yadaste
"I know (through intuitive experience) this great Purusha (the Supreme Being), the wise one, who, having created the various forms and the nomenclatures (for those forms), deals with them by those names, and who is beyond darkness and is brilliant like the sun."
Thus, in this creative process, all are seen as arising out of the same original reality, the Purusha, which suggest human oneness, and that wouldn't have gone down well with the Brahmins, the self-proclaimed god's own angels. So, they set out to rectify the 'wrong' through the three following interpolations thus:
V11
tasmadashva ajayata
ye ke cobhaya dataha
gavo ha jijignire tasmat
tasmad jnata ajavayaha
From that were born the horses, as also animals (like donkeys and mules) which have two rows of teeth. From that were born the cattle. From that (again) were born goats and sheep.
V12
yatpurusham vyadadhuhu
kadhita vyakalpayan
mukham kimasya kau bahu
kavuru padavuchayate
(Now some questions are raised by the sages:) When the gods decided to (mentally) sacrifice the Viratpurusha (and produce further creation), in how many ways did they do it? What became of his face or mouth? What became of his two arms? What became of His two thighs? What were (the products of) the two feet called?
V13
brahmanosya mukhamasit
bahu rajanyah kritaha
uru tadasya yadvaishyaha
padhyagam shudro ajayata
From His face (or the mouth) came the brahmanas. From His two arms came the rajanya (the kshatriyas). From His two thighs came the vaishyas. From His two feet came the shudras.
So, His face (head) produced what– Heaven or Brahmins?
Who were born out of His belly (navel)? – Antariksha or Vaisyas?
What evolved from his feet – Earth or Shudras?
Often the Purusha Sukta with these contradictions gets chanted (and heard) without anyone raising an eyebrow for none knows Sanskrit and that's about the Hindu spiritual tragedy. Hence, it is obvious that V11 gave a mundane twist to the divine creation to facilitate the motivated question in V12 for the facilitation of the self-aggrandizing answer in V13.
So, one can take his pick and move on as the Kshatriyas keep the Creator's hands all for themselves! But the Brahmins couldn't have left it at that as there was also the Bhagvad-Gita to contend with; so, they applied their interpolative hands to handle it. As would be apparent from the following dissection of the Gita 'as it is', similar sukta tactics were adopted to make it call their mundane bidding. But then, what was the provocation for the Brahmins to dabble with this philosophical discourse as well with their interpolative verses?
To start with, Krishna averred, as already noted,
Ch9, V6
Skies in rooted wind as spreads
Dwell in Me though disperse all.
yathākāśha-sthito nityaṁ vāyuḥ sarvatra-go mahān
tathā sarvāṇi bhūtāni mat-sthānītyupadhārayaand,
Ch6, V31
Me who sees in all beings
He's the one that dwells in Me.
sarva-bhūta-sthitaṁ yo māṁ bhajatyekatvam āsthitaḥ
sarvathā vartamāno 'pi sa yogī mayi vartate
and these are counter to the Brahmanical innovation in the Purusha Sukta that they were specially produced from the creator's face, which, if allowed to propagate, would undermine the false narrative of their preeminent birth.
Secondly, it was Krishna's stance that,
Ch 2, V42
Unwise use all enticing
Flowery language to further
Rituals Vedic in their scores
Not the knowledge of Vedas.
yāmimāṁ puṣhpitāṁ vāchaṁ pravadanty-avipaśhchitaḥ
veda-vāda-ratāḥ pārtha nānyad astīti vādinaḥ,
This is but an unambiguous deprecation of the Vedic rituals that accord the Brahmins their temporal power in the religious place that afforded them an undisputed social preeminence, which if gained ground could have hurt them where it hurts the most.
Hence, at some stage, they fiddled with the Gita the way they did with the Purusha Sukta, so to say, as shabbily at that, but surprisingly managed to get away with it for all time to come, so it seems, of course, aided in no small measure by the raise in the scriptural belief and the fall of the Sanskrit usage. But the hard rub, as is already seen, was the attribution of the false caste narrative to Krishna with its debilitating lower caste duties.
Ch4, V13
chātur-varṇyaṁ mayā sṛiṣhṭaṁ guṇa-karma-vibhāgaśhaḥ
tasya kartāram api māṁ viddhyakartāram avyayam
It is I who engineered the division of men into four varna (castes) based on their guna (innate nature) and karma (earthly duties) but yet although I am the creator of this system, know me to be the non-doer and eternal,
So, this, as noted before, is akin to that advanced by the Brahmins in the Purusha Sukata:
V13
brahmanosya mukhamasit
bahu rajanyah kritaha
uru tadasya yadvaishyaha
padhyagam shudro ajayata
From His face (or the mouth) came the brahmanas. From His two arms came the rajanya (the kshatriyas). From His two thighs came the vaishyas. From His two feet came the shudras.
Not only that, the Brahmins, through their interpolations in the Gita, sought to cement the caste walls by detailing the caste duties as well, cynically at that with -
Ch 3, V35
śhreyān swa-dharmo viguṇaḥ para-dharmāt sv-anuṣhṭhitāt
swa-dharme nidhanaṁ śhreyaḥ para-dharmo bhayāvahaḥ
It is far better to perform one's natural prescribed duty, though tinged with faults, than to perform another's prescribed duty, though perfectly. In fact, it is preferable to die in the discharge of one's duty, than to follow the path of another, which is fraught with danger.
Ch18, V45
śhreyān swa-dharmo viguṇaḥ para-dharmāt sv-anuṣhṭhitāt
svabhāva-niyataṁ karma kurvan nāpnoti kilbiṣham
It is better to do one's own dharma, even though imperfectly, than to do another's dharma, even though perfectly. By doing one's innate duties, a person does not incur sin.
Also should be seen in this interpolative course are the yoga classes, superstitious suppositions, tasteless assertions, and such that abound in the Gita 'as it is', absurdities all, seen in the context of it having been conceived to dispel Arjuna's reservations in joining the battle of Kurukshetra.
Next is the aspect of structural economy and one finds the similitude though of the benign content in many a sloka in the same or in a different context throughout the text. Obviously, some of them are interpolations but which were the originals and which are the imitations could be hard to find out for they smugly fit into the overall structure. Whatever, save lengthening the discourse, these do not belittle the same and fortunately, not even tire the reader / listener, thanks to the exemplary charm of Sanskrit, which, for the British philologist, Sir William Jones, 'is of wonderful structure, more perfect than Greek, more copious than Latin and more exquisitely refined than either.'
Boxed here in the 'in vogue' Gita's thirteen chapters are 110 verses of deviant character or digressive nature that can be taken as interpolations with reasonable certainty and so one may read the epic afresh by passing over them for a refreshing experience.