Dr. Chattopadhyaya has done a wonderful job presenting the theory that in the earliest days proto-Tantra and proto-Lokayata were a single system, which either originated or was most prevalent in northeast India. His argument rests on a theory of magic as a necessary precondition of religion, similar to the theories of anthropologists such as Andrew Lang and J.G. Frazer:  
The theory goes as follows: magic precedes, and then accompanies religion in early human societies. Magic does not necessitate Gods; it is simply an attempt for early humans to manipulate the forces of nature without knowing the actual mechanisms of how nature works. For example, it makes sense for someone ignorant of physics, chemistry and the hydrologic cycle to attempt to generate rain by ritualistically pouring water on the ground and calling to the sky. It is worth experimenting with at the very least. In this sense, magic is a sort of proto-science.
The problem is that when these magic practices are empirically tested over generations, most of them will be found ineffective. Yet still perceiving order and regularity, the population generally will infer that other conscious agents called Gods are in control, and will adapt their magical beliefs and practices into a theistic system. By this process the magic becomes a religion, and “experimental” processes ossify into religious rituals.
The magic underlying Hindu religion is evident in all the early texts. The Rig Veda contain a huge number of passages asking the Gods for purely material things such as cattle, crops, prosperity, or protection from the elements.    These are likely magic rituals to induce crops, or protect cattle adapted into a theistic framework. Other passages frequently identify the Gods as a “powerful chief” “foremost amongst men,” “the bravest among all humans” (In the case of the Rbhus, this is made explicit: being mortals they earned immortality”, RV, I.110.4) which is perhaps a clue that these first Gods were in fact God-kings or the deified spirits of ancestors, who were perhaps thought to be able to control physical processes from the next world. The Arthava Veda consists almost entirely of magic techniques, mantras and rituals, not dissimilar from what we find in Tantra. Several early Upanishads also espouse a belief in the magic power of breath manipulation.   It seems likely that many of these writings exist because early experimenters with meditation found them to be materially effective in inducing samadhi states. In other words, early meditative practices can be thought of as successful early attempts at quasi-scientific experimentation with the human body and brain. Sinha argues that the materialist trend represented in the Vedas culminated in Lokayata in the 7th century BC. As evidence he cites many examples similar to those above, but also emphasizes the fact the in the Vedic canon, the progenitor of materialist philosophy is the deified guru named Brihaspati. Another word for Lokayata is Brihaspati Darsana.
Given its prevalence amongst the general population, it is likely that Lokayata had some roots in non-Aryan magic as well. Chattopadhyaya argues that the “chanting dogs” episode of the Chandogya Upanishad is actually an adaptation of a non-Aryan magic ceremony. He also exhaustively documents the fragments of magical beliefs we can ascribe to tribal societies and those otherwise not integrated into Vedic society. Theirs however were largely centered on agriculture, fertility, and totemism, and were eventually transformed, through synthesis with Vedic religion into the Ganapati and Devi worshipping sects, and into more modern Tantra.
These are all examples of magic, transitioning into religion. However, this makes no account for for magic, which resists the religious impulse. Chattopadhyaya asserts that this occurred in India, and explains the existence of early Tantra and Lokayata. These practitioners would have been members of the poorer sectors of society. They would have belonged to the mass of the population, which remained rural, and less influenced by increasingly urban, Brahminical philosophical trends. The continued magic experimentation, which these villagers practiced, constitutes proto-science and proto-materialist philosophy.
It might initially seem far-fetched to believe that the hard materialist Lokayatas, and the somewhat mystical Tantrikas have a common root, but upon inspection of their earliest beliefs the differences are not particularly great. At first, Lokayatas and Tantrikas both engaged in rituals and magic. Comments in the Lankavatara Sutra imply the use of magic and nature-lore by Lokayatas. They are described as “skilled in varieties of incantations and in the art of eloquence” and as ” knowing well even the minds of the animal world…” The Saddharma Pundarika also mentions “lokayata-mantra,” and the Divyavadana mentioned “lokayata-yajna” (sacrifice).  Kumarilla’s Slokavarttika and Parathasarathimisra’s Nyayaratnakara both identify the Mimamsas who rejected the universal efficacy of rites as almost Lokayatas. The Mimamsas were a sect concerned primarily with the accurate recitation of mantras and performances of rituals. Therefore, Kumarilla and Parathasarathimisra are implicitly stating that Lokayatas had some rituals and mantras, though they probably did not believe in their universal efficacy.
The earliest foundational beliefs of Tantra also line up remarkably well with those of Lokayata. For instance, the notion of deha-vada is as strongly associated with Tantra as it is with Lokayata. It was their body-centric approach which inspired Tantrikas to pursue alchemy and chemistry. The main Indian alchemical text, the Rasarnava is a Tantric text, which espouses:
Liberation is declared in the six systems to follow the death of the body.
Such liberation is not cognised in perception like an emblic myrobalan fruit in hand.
Therefore a man should preserve that body by means of mercury and of medicaments
Tantrism of this sort is purely in line with Lokayata views on the epistemological supremacy of perception, skepticism, especially skepticism of the orthodox schools, and the desirability of preserving and utilizing one’s body in the real world.
Deha-vada combined with the lower class standing of the Tantrikas made them more inclined to handle corpses despite the Brahminical injunctions against doing so. This allowed them to garner a greater knowledge of human anatomy than the members of the orthodoxy. For instance, the Tantrikas were the first to identify the cerebro-spinal system as the seat of consciousness, a discovery that is most obvious today in the form of kundalini meditation. 
Materialism in modern Tantra influenced systems:
Though Lokayata has died, the early substratum, which produced both Tantra and Lokayata, still survives albiet in permutated forms. The most prolific of these is modern, mainstram Vedanta-influenced Tantra, which we have already alluded to. However, the practices of poorer or socially excluded groups typically retain the materialist character more strongly. If we are to believe Kancha Ilaiah‘s account of his community, the materialist notion of deha-vada survives amongst Dalits. He says: ” For Dalitbahujans life now and life after death has a different meaning from that of the Hindus. For them, life is a one-time affair. The philosophy is expressed in the proverb, puttindokasaare sachindokasaare (‘we are born only once and die only once’).
Tantrikas obviously retain deha-vada to this day, but in their more heterodox forms they also exhibit other features of proto-Lokayata. Baul music for instance is rife with philosophical hedonism, and criticism of religious authorities. Chattopadhyaya quotes Dasgupta who is summarizing a Sahajia Tantrika named Saraha Pada in terms strikingly reminiscent of the Lokayata:
The formal rules and regulations of religion were also severely criticized by the Sahjias. The most penetrating and scathing criticism was made by Saraha Pada in his Dehakosa. His first revolt is against the orthodox system of four-fold division of colors (caturvarna) placing the Brahmins at the top. Saraha says that the Brahmins as a caste cannot be reasonably recogised to be the highest of men – for the saying that they dropped from the mouth of Brahma is a myth invented by a section of clever and cunning people… The Brahmins take earth, water, kusa grass and recite mantras and perform fire-sacrifices in their houses– in vain do they offer ghee to the fire, for thereby their eyes will only be affected with intense smoke. They become holders of single-fold or of three-fold sacred threads,– but this is of no avail unless truth is realised. Deceived is the whole world by false illusion– none does know the all-excelling truth were both religion and non-religion become one. The devotees of the Lord (Isvara), again, anoint the whole body with ashes, wear matted hair on their head, sit within the house and light lamps and ring bells seated in a corner: they take a yogic posture (asana) with their eyes fixed; they whisper religious doctrines into the ears (of the credulous people) and deceive them thereby. The widows, the mundis (women taking the vow of fasting for the whole month) and others taking different vows, get themselves initiated by these devotees who do it only in greed of money (daksina).
Though it would be improper to call these systems of belief modern manifestations of Lokayata, they are the existing cousins systems, which retain the most salient features of original proto-Lokayata or proto-Tantra.
More to come
 Chattopadhyaya, Lokayata: A Study in Ancient Indian Materialism, ch. 5.
 Andrew Lang, Magic and Religion, (London: Longman’s Green & Co., 1901) https://ia600400.us.archive.org/2/items/cu31924029075451/cu31924029075451.pdf (accessed October 25, 2013).
 J.G. Frazer, The Golden Bough: A Study in Comparative Religion , (New York: MacMillan and Co., 1894) https://ia600303.us.archive.org/20/items/goldenboughstudy01fraz/goldenboughstudy01fraz.pdf (accessed October 25, 2013).
 Internet Sacred Text Archive, trans. Ralph T.H. Griffith, “Rig Veda Hymn 1. Agni.” 1896. Accessed October 25, 2013. http://www.sacred-texts.com/hin/rigveda/rv01001.htm
 Internet Sacred Text Archive, trans. Ralph T.H. Griffith, “Rig Veda Hymn XXIX. Indra.”1896. Accessed October 25, 2013. http://www.sacred-texts.com/hin/rigveda/rv01029.htm.
 Internet Sacred Text Archive, trans. Ralph T.H. Griffith, “Rig Veda Hymn CXIV. Rudra.” 1896. Accessed October 25, 2013. http://www.sacred-texts.com/hin/rigveda/rv01114.htm.
 S. Radhakrishnan, ed. The Principal Upanisads, (New Delhi: Harper Collins, 2006), 341.
 Radhakrishnan, The Principal Upanisads, 341.
 A.K. Sinha, “Traces of Materialism in Early Vedic Thought: A Study,” Annals of the Bhandarkar Oriental Research Institute, 75, no. 1/4 (235-241): 675-682, http://www.jstor.org/stable/41694419 (accessed October 28, 2013).
 Chattopadhyaya, Lokayata: A Study in Ancient Indian Materialism, 77-117. Note: He argues that ancient Indian texts often use animal names to refer to humans or human groups, and use the word “dog” to refer to lower class people specifically. In the ritual described, the dog-priests “sing” together, rather than chanting or praying. They also sing exclusively to obtain food from the Gods.
 Chattopadhyaya, Lokayata: A Study in Ancient Indian Materialism, chap. 3-4
 Bunyu Nanjo, The Lankavatara Sutra, trans. Daisetz Teitaro Suzuki, (London: 2005) http://lirs.ru/do/lanka_eng/lanka-nondiacritical.htm (accessed October 25, 2013), chap. 3. LXIII, 173.
 Bunyu Nanjo, The Lankavatara Sutra, chap. 3. LXIII, 174.
 Chattopadhyaya, Lokayata: A Study in Ancient Indian Materialism, 37-38.
 Hajime Nakamura, A History of Early Vedanta Philosophy, Part 2, (Delhi: 2004), Google Books edition, 170-172.
 Sonali Bhatt Marwaha, “Roots of Indian Materialism in Tantra and Pre-Classical Saṃkhya,” Asian Philosophy: An International Journal of the Philosophical Traditions of the East, 23, no. 2 (2013): 180-198, 10.1080/09552367.2013.777582 (accessed October 25, 2013).
 Praphulla Chandra Ray, A History of Hindu Chemistry From the Earliest Times to the Middle of the Sixteenth Century, (Calcutta: B.C. Sanyal, 1903)https://ia600304.us.archive.org/14/items/historyofhinduch01rayprich/historyofhinduch01rayprich.pdf (accessed October 25, 2013), lxxii.
 Chattopadhyaya, Lokayata: A Study in Ancient Indian Materialism, 335-336.
 Chattopadhyaya, Lokayata: A Study in Ancient Indian Materialism, 335.
 Kancha Ilaiah, Why I am Not a Hindu, Electronic edition, 51.
 Solomon, Carol. Dr. Farooq’s Study Resources Page, “Baul Songs.” ed. David S. Lopez, Accessed October 25, 2013. http://www.globalwebpost.com/farooqm/study_res/bangladesh/carol_baul.html.
 Chattopadhyaya, Lokayata: A Study in Ancient Indian Materialism, 332.