“When life is yours, live joyously;
None can escape Death’s searching eye;
When once this frame of ours they burn,
How shall it e’er again return”
“There is no heaven, no final liberation, nor any soul in another world”
– Sarva Darsana Samgraha by Vidyaranya 
When I’m fortunate enough to get out of the city, I like to take time to stare at the stars. Thanks to our scientific apparatus and educational system that explains certain scientific findings, I know that our sun is a star, same as all the stars seen in the night sky. I’ve long considered the sheer scale of the universe to be a powerful argument against a Creator.  Why should there be so much matter in the vast emptiness of space if humanity is the all-important center of the everything?
If I were a Scythian nomad, or an aristocratic medieval prince, or a pre-Colombian Amazonian hunter-gatherer I have little doubt I would accept whatever wisdom and knowledge I received from the culture I was born into in regards to the universe and humanity’s place in it. I would gaze at the stars and likely never conceive that they were made of the same stuff as our sun if it weren’t conventionally known. I would fully believe in the deities of the culture and that there was some form of life after death.
It’s with that in mind that I enjoy reading about the metaphysical beliefs of pre-modern peoples, especially those that are iconoclastic with regards to their time period and lay adjacent to the current scientific conception of reality. Roughly contemporary with the Pre-Socratics, a sect of philosophers in Vedic India espoused a view that is recognizably atheist from our modern perspective.  I’m referring to the ancient Indian school of Charvaka. I did a search on FtB and it appears no one has written about it. What follows is a brief and very broad synopsis, though every subject briefly described is deserving of far more explication. I should point out that I have a layperson’s understanding and am certainly open to those with more knowledge of Indian philosophy pointing out errors and misconceptions.
Charvaka is seen as heterodox in terms of arising from the philosophical/theological framework of the Rg Veda, Upanishads, and Mahabharata but neglects to provide justifications for the teachings from those traditions. Arising during the Vedic and Epic periods in Indian history (roughly 1500-500 BCE), Charvaka is grouped spatiotemporally with Buddhism and Jainism as standing opposed to the six orthodox Hindu philosophies: Nyaya, Vaisheshika, Yoga, Samkhya, Mimamsa, and Vedanta. Unfortunately, the primary Charvaka document, the Brhaspati Sutra, dated to roughly 600 BCE, is lost. The primary evidence for its tenets come from rival sects and are preserved in writings dated a thousand years after its founding. The seventh century CE Tattvopaplavasimha by Jayarashi Bhatta is the earliest complete account, though there are arguments for and against its association with Charvaka. 
Samkhya and Mimamsa are both atheistic in terms of not positing a creator, but they adhere to the atman/prakrti (roughly equivalent to soul/matter) dualistic conception of the cosmos as the rest of the orthodox philosophies do, with the exception of the Advaita sub-discipline of Vedanta. Buddhism, while atheistic, has both dualistic and monist characteristics that vary by sect, but all reject the soul/atman. However, there is belief in supernatural elements like reincarnation, different dimensions inhabited by gods and demons, ghosts, etc. As far as I can tell, Charvaka is the only school of thought from that fertile philosophical time period to be both atheistic and nominally monist vis-à-vis the atman/prakrti dichotomy  while repudiating the fantastical elements contained in the other systems. There is no concern with breaking the karmic cycle of samsara that their contemporaries strive for, since death is final.
There is a sense of hedonic nihilism embedded within the doctrine:
“The enjoyment of heaven lies in eating delicious food, keeping company of young women, using fine clothes, perfumes, garlands, sandal paste, etc.” – Sarva Siddhanta Sangraha, by Shankara
Given the obsession of Buddhism and the orthodox Hindu traditions with suffering and the best way to cope with it, it wouldn’t be too surprising if the impoverished masses didn’t gravitate towards a hedonistic lifestyle they didn’t have access to. Moreover, the ruling classes probably weren’t likely to exploit a “religion” that didn’t advocate piety and obedience, with the promise of a better subsequent life to make up for one’s present shitty life. These could be two of the reasons why Charvaka didn’t last.
It’s wild (to me anyways) to think that during the life of Thales of Miletus in ancient Greece there were dissident proto-atheists half a continent away. I’m humbled by the the thought of early humans being able to cast aside what current atheists regard as illogical beliefs, something that’s fairly easy to do nowadays given widespread access to scientific information. I’m pretty sure if I existed in an earlier era I wouldn’t be able to do the same.
1. Most of the following information comes from A Sourcebook in Indian Philosophy, edited by Sarvepalli Radhakrishan and Charles Moore.
2. It turns out that our observation bubble is even larger than previously thought: http://www.space.com/34382-universe-has-10-times-more-galaxies-hubble-reveals.html
3. I’ll reductively classify atheism as the denial of a Creator coupled with a monist conception of reality (i.e. only physical reality is real).
4. The book referenced above definitively places it within the Charvaka paradigm, but the Wikipedia entry for Bhatta cites proponents of arguments against this.
5. I should note Charvaka describes the principle elements of matter as listed as air, fire, earth and water, so perhaps monist is not the best description. However, the existence of a soul/atman is explicitly denied. Consciousness is said to arise from a mixture of the aforementioned elements and ceases to exist upon the body’s dissolution. As yet another aside, Vaisheshika, while still dualistic in nature, has an atomist conception of matter/prakrti.