Śrīla Narottama dāsa Ṭhākura has sung,

karma-kāṇḍa, jñāna-kāṇḍa, kevala viṣera bāṇḍa, amṛta baliyā yebā khāya nānā yoni sadā phire, kadarya bhakṣaṇa kare, tāra janma adaḥ-pate yāya

He says that persons following the paths of karma-kāṇḍa and jñāna-kāṇḍa (fruitive activities and speculative thinking) are missing the opportunities for human birth and gliding down into the cycle of birth and death. Thus there is always the chance that he may be put into the Pūyoda Naraka, the hell named Pūyoda, where one is forced to eat stool, urine, pus, mucus, saliva and other abominable things. It is significant that this verse is spoken especially about śūdras. If one is born a śūdra, he must continually return to the ocean of Pūyoda to eat horrible things. Thus even a born śūdra is expected to become a brāhmaṇa; that is the meaning of human life. Everyone should improve himself. Kṛṣṇa says in Bhagavad-gītā (4.13) , cātur-varṇyaṁ mayā sṛṣṭaṁ guṇa-karma-vibhāgaśaḥ: “According to the three modes of material nature and the work ascribed to them, four divisions of human society were created by Me.” Even if one is by qualification a śūdra, he must try to improve his position and become a brāhmaṇa. No one should try to check a person, no matter what his present position is, from coming to the platform of a brāhmaṇa or a Vaiṣṇava. Actually, one must come to the platform of a Vaiṣṇava. Then he automatically becomes a brāhmaṇa. This can be done only if the Kṛṣṇa consciousness movement is spread, for we are trying to elevate everyone to the platform of Vaiṣṇava. As Kṛṣṇa says in Bhagavad-gītā (18.66) , sarva-dharmān parityajya mām ekaṁ śaraṇaṁ vraja: “Abandon all other duties and simply surrender unto Me.” One must give up the occupational duties of a śūdra, kṣatriya or vaiśya and adopt the occupational duties of a Vaiṣṇava, which include the activities of a brāhmaṇa. Kṛṣṇa explains this in Bhagavad-gītā (9.32) :

māṁ hi pārtha vyapāśritya ye ’pi syuḥ pāpa-yonayaḥ striyo vaiśyās tathā śūdrās te ’pi yānti parāṁ gatim

“O son of Pṛthā, those who take shelter in Me, though they be of lower birth — women, vaiśyas [merchants], as well as śūdras [workers] — can approach the supreme destination.” Human life is specifically meant for going back home, back to Godhead. That facility should be given to everyone, whether one be a śūdra, a vaiśya, a woman or a kṣatriya. This is the purpose of the Kṛṣṇa consciousness movement. However, if one is satisfied to remain a śūdra, he must suffer as described in this verse: tad evātibībhatsitam aśnanti.