Srimad-Bhagavatam: Canto 7 - Chapter 1 - Verse 25
यदृच्छया लोकमिमं प्रापित: कर्मभिर्भ्रमन् ।स्वर्गापवर्गयोर्द्वारं तिरश्चां पुनरस्य च ॥ २५ ॥
yadṛcchayā lokam imaṁprāpitaḥ karmabhir bhramansvargāpavargayor dvāraṁtiraścāṁ punar asya ca
In the course of the evolutionary process, which is caused by fruitive activities due to undesirable material sense gratification, I have received this human form of life, which can lead to the heavenly planets, to liberation, to the lower species, or to rebirth among human beings.
All living entities within this material world are undergoing the cycle of birth and death according to the laws of nature. This struggle of birth and death in different species may be called the evolutionary process, but in the Western world it has been wrongly explained. Darwin’s theory of evolution from animal to man is incomplete because the theory does not present the reverse condition, namely evolution from man to animal. In this verse, however, evolution has been very well explained on the strength of Vedic authority. Human life, which is obtained in the course of the evolutionary process, is a chance for elevation (svargāpavarga) or for degradation (tiraścām punar asya ca). If one uses this human form of life properly, he can elevate himself to the higher planetary systems, where material happiness is many thousands of times better than on this planet, or one may cultivate knowledge by which to become free from the evolutionary process and be reinstated in one’s original spiritual life. This is called apavarga, or liberation. Material life is called pavarga because here we are subject to five different states of suffering, represented by the letters pa, pha, ba, bha and ma. Pa means pariśrama, very hard labor. Pha means phena, or foam from the mouth. For example, sometimes we see a horse foaming at the mouth with heavy labor. Ba means byarthatā, disappointment. In spite of so much hard labor, at the end we find disappointment. Bha means bhaya, or fear. In material life, one is always in the blazing fire of fear, since no one knows what will happen next. Finally, ma means mṛtyu, or death. When one attempts to nullify these five different statuses of life — pa, pha, ba, bha and ma — one achieves apavarga, or liberation from the punishment of material existence. The word tiraścām refers to degraded life. Human life, of course, provides an opportunity for the best living conditions. As Western people think, from the monkeys come the human beings, who are more comfortably situated. However, if one does not utilize his human life for svarga or apavarga, he falls again to the degraded life of animals like dogs and hogs. Therefore a sane human being must consider whether he will elevate himself to the higher planets, prepare to free himself from the evolutionary process, or travel again through the evolutionary process in higher and lower grades of life. If one works piously one may be elevated to the higher planetary systems or achieve liberation and return home, back to Godhead, but otherwise one may be degraded to a life as a dog, a hog and so on. As explained in Bhagavad-gītā (9.25), yānti deva-vratā devān. Those interested in being elevated to the higher planetary systems (Devaloka or Svargaloka) must prepare to do so. Similarly, if one wants liberation and wants to return home, back to Godhead, he should prepare himself for that purpose. Our Kṛṣṇa consciousness movement is therefore the highest movement for the benediction of human society because this movement is teaching people how to go back home, back to Godhead. In Bhagavad-gītā (13.22) it is clearly stated that different forms of life are obtained by association with the three modes of material nature (kāraṇaṁ guṇa-saṅgo ’sya sad-asad-yoni-janmasu). According to one’s association with the material qualities of goodness, passion and ignorance in this life, in one’s next life one receives an appropriate body. Modern civilization does not know that because of varied association in material nature, the living entity, although eternal, is placed in different diseased conditions known as the many species of life. Modern civilization is unaware of the laws of nature. “The bewildered spirit soul, under the influence of the three modes of material nature, thinks himself the performer of activities that are in actuality carried out by nature.” (Bg. 3.27) Every living entity is under the full control of the stringent laws of material nature, but rascals think themselves independent. Actually, however, they cannot be independent. This is foolishness. A foolish civilization is extremely risky, and therefore the Kṛṣṇa consciousness movement is trying to make people aware of their fully dependent condition under the stringent laws of nature and is trying to save them from being victimized by strong māyā, which is Kṛṣṇa’s external energy. Behind the material laws is the supreme controller, Kṛṣṇa (mayādhyakṣeṇa prakṛtiḥ sūyate sacarācaram). Therefore if one surrenders unto Kṛṣṇa (mām eva ye prapadyante māyām etāṁ taranti te), one may immediately be freed from the control of external nature (sa guṇān samatītyaitān brahma-bhūyāya kalpate). This should be the aim of life.