No, it is not an interpolation. The Taittiriya Samhita of the Krishna Yajur Veda says the same exact thing as the Rig Veda Purusha Sukta hymn about the origin of the castes and the Devas from the body of the Purusha, or in the Yajur Veda's case, Prajapati.
Here is what it says in 7.1.1:
Prajapati [or Purusha] desired, 'May I have offspring.' He meted out the Trivrt
from his mouth. After it the god Agni was created, the Gayatri metre,
the Rathantara Saman, of men the Brahman, of cattle the goat;
therefore are they the chief, for they were produced from the mouth.
From the breast and arms he meted out the Pañcadaça Stoma. After it
the god Indra was created, the Tristubh metre, the Brhat Saman, of
men the Rajanya, of cattle the sheep. Therefore they are strong, for
they were created from strength. From the middle he meted out the
Saptadaça Stoma. After it the All-gods as deities were created, the
Jagati metre, the Vairupa Saman, of men the Vaiçya, of cattle cows.
Therefore are they to be eaten, for they were created from the
receptacle of food. Therefore are they more numerous than others, for
they were created after the most numerous of the gods. From his feet
he meted out the Ekavinça Stoma. After it the Anustubh metre was
created, the Vairaja Saman, of men the Çudra, of cattle the horse.
Therefore the two, the horse and the Çudra, are dependent on others.
Therefore the Çudra is not fit for the sacrifice, for he was not
created after any gods. Therefore they depend on their feet, for they
were created from the feet.
When you rephrase the paragraph by keeping only the sentences related to caste, you end up with this:
Prajapati desired, 'May I have offspring.'
He meted out Brahmanas from
his mouth. Therefore, they are the chief, for they were produced from
From the breast and arms he meted out the Rajanya [or Kshatriyas].
Therefore, they are strong, for they were created from strength.
From the middle [waist or thighs] he meted out the Vaishyas.
Therefore, they are more numerous than others, for they were created
after the most numerous of the gods.
From his feet he meted out the Shudras. Therefore, Shudras are
dependent on others. Therefore the Shudra is not fit for the sacrifice
[Yajna], for he was not created after any gods. Therefore they depend
on their feet, for they were created from the feet.
First, note how this is the same concept found in the Purusha Suktam hymn of the Rig Veda Samhita.
When they divided Puruṣa how many portions did they make? What do they
call his mouth, his arms? What do they call his thighs and feet?
The Brahman was his mouth, of both his arms was the Rājanya made. His
thighs became the Vaiśya, from his feet the Śūdra was produced.
The Moon was gendered from his mind, and from his eye the Sun had
birth; Indra and Agni from his mouth were born, and Vāyu from his
breath. Forth from his navel came mid-air the sky was fashioned
from his head Earth from his feet, and from his car the regions. Thus
they formed the worlds.
Note that this same exposition of the caste system is found in the Yajur Veda Samhita as well as the Rig Veda Samhita. So, how can anyone call it an interpolation when it's found in another Vedic Samhita that's almost as ancient (according to Indologists).
But Indologists also say that the ancient Vedic society during Rig Vedic times did not support an elaborate caste system, and that the Purusha Suktam hymn that talks about caste, is a later interpolation because the language is closer to classical (Panini's) Sanskrit instead of Vedic Sanskrit. But even this is incorrect because the Taittiriya Samhita paragraph quoted above mentions the same caste theory and body-origination theory and it uses Vedic sanskrit, not classical Sanskrit! Read it for yourself.
Here is what Michael Witzel, who is the current leading scholar in the Aryan Migration theory, says:
The Yajur Veda Samhita prose texts ... may therefore be dated after c.
1200 BCE (until c. 800 BCE).
This time frame falls within the "Vedic period", before the modern/classical/post-Vedic period, and before Panini, who modernized/standardized Vedic Sanskrit and turned it into Classical Sanskrit.
This also refutes Susan Bayly's claim that,
the ancient texts did not in some way "create the phenomenon of caste"
So, the caste system is not a post-Vedic, classical era, Puranic/Smriti era creation.
This small paragraph from the Yajur Veda lays down the foundation of the birth based caste system that is so elaborately described in the Smritis, Itihasas, Puranas, and Dharma Shastras that so many people claim to be interpolated or fabricated with the goal of oppressing non-Aryas.
Let's first go through this paragraph and talk about how it supports the caste system.
The castes were created or produced from the body of Prajapati, just like how the animals mentioned in the same paragraph were created (such as goats, sheep, horses, and cows). Creation and production is a one-time event that is fixed. So this means that the sons and daughters of Brahmin parents are also Brahmins, and so on. This implies that caste cannot change through ordinary means in life.
The paragraph also describes the caste hierarchy starting with Brahmanas or priests first. It says: "therefore are they the chief, for they were produced from the mouth." And then the paragraph goes on to list the other castes produced from the other parts of Prajapati's body in order from top to bottom: kshatriya, vaishya, then shudra, which represents the hierarchy: castes that come from higher in the body have more authority than castes that come from lower in the body.
It ascribes qualities to castes, such as Kshatriyas: "Therefore they are strong, for they were created from strength." Vaishyas: "Therefore, they are more numerous than others" (since societies have lots of landlords and businessmen).
Shudras must serve Dvijas
It also says that the Shudras are dependent on others (Brahmins, Kshatriyas, and Vaishyas): "Therefore the two, the horse and the Çudra, are dependent on others." So this is why Smritis say that the duty of the Shudras is to serve the Dvijas.
Shudras cannot perform Vedic rituals
And finally it puts restrictions on Shudras performing Vedic rituals or yajnas: "Therefore the Çudra is not fit for the sacrifice, for he was not created after any gods." This is why Smritis say Shudras cannot undergo Upanayanam, etc.