I'm not obviously talking about small groups or individuals but about an actual civilization that never believed about the existence of a supreme essence. Is atheism just a modern concept?
Also, Jainism seems to be a kind of a "no "supreme essence" type of religion, but I'm not an expert.
A pretty good set of sources on the history of atheism and some analysis can be found here: Investigating Atheism (affiliated with University of Cambridge)
There are many, many different directions this could go. You want an example of an ancient society which didn't believe in a single over-arching "supreme essence"? The Greeks and then the Romans immediately come to mind. Sure, some gods were more powerful than others, and had bigger and better positions, but the Greeks did not believe that Zeus had the same level of omnipotence/omnipresence that the Judeo-Christian god does, and they would very often choose to worship one of the supposed "lesser gods" that dealt in arenas that mattered more to them instead.
Or, for that matter, the god of the Old Testament doesn't really seem to be a "supreme essence". The Bible seems to indicate that YHWH was better than the other gods that were out there but I don't get the sense from the OT that they ever considered him to be all that there was out there. The first commandment itself doesn't say "don't worship other gods because there aren't any other gods and that's just stupid", it says "don't worship other gods because I'm jealous and I don't like it".
If you meant to include polytheistic religions, that is not without its issues either. One thing about atheism, particularly skeptical, science-based atheism, is that it tends to answer "I don't know" to a lot of questions. That's something that causes enough discomfort when we ask questions like "what happens to the mind after we die?" or "how did life first occur on this planet?"; when your level of technological advancement is such that the non-theist has to answer questions like "where does the sun go at night?", there are so many uncomfortable questions that a lot of people simply aren't going to accept not knowing and will create deities and such to explain this.
On top of that, how do we get to the question of knowing and not knowing? We have the scientific method now as well as a lot of other heuristics to help lead us to knowledge, but we've had to build these up over thousands of years. What's to stop a perfectly reasonable person who doesn't have access to these heuristics from coming to an incorrect conclusion (from our perspective)? Heck, it wasn't that long ago that learned men really thought you could turn lead into gold, as silly as that sounds to the modern scholar.
Finally, humans are really, really good at recognizing correlations, and our creativity helps us to align those with causes. It's what we do. We've got millions of years of living in the bush and determining whether a rustling in a nearby patch of grass was a lion or if it was just harmless rustling to help shape our brains to perceive these. We can create small mountains in the sand, therefore it is perfectly reasonable to think that some bigger version of us created the mountains in the distance. Eating the meat of the pig can make you very sick in some parts of the world, therefore whatever god we have created to answer some of those other discomforting questions doesn't want us to eat pig meat. And so on and so forth.
Where I'm leading with this is that in a large sense asking if there are examples of ancient peoples who lived without "gods" is basically asking in one way or another if there are examples of ancient peoples who lived with the same sense of the universe that we have today. And the answer is "no, of course not". That's impossible. If our viewpoints today are more enlightened, it's because we stand on the shoulders of giants who themselves stand on the shoulders of titans.
Certainly there are major religions in the East in ancient times that never believed in supreme creator/being. Two of these major religions still exist today. They are Buddhism and Jainism. The startling thing is they grew along side Hinduism in India which is a religion full of hundreds of gods! This was mainly because of sensible patronization from kings at that time like Ashoka the great. This was about 1000 - 2000 years ago. But later on probably during the middle ages they lost in popularity thanks to resurgent Hinduism again thanks to kings who started patronizing Hinduism more this time. But still they exist in India as minorities and actually as you know Buddhism is a world religion which spread to various other Asian countries again thanks to kings at that time. But Jainism still survives as a vibrant minority in India.
Now coming to how exactly these religions are Atheistic compared to their rival Hinduism which is polytheistic. As I said both of these religions as taught in their ancient texts don't believe in supreme God. They are called 'Shramic' or 'Shramana' religions which literally means self-help suggesting your 'karma/deeds/conduct' is responsible for your well being, no supreme being can elevate you. And both incorporate non-violence as a major principle particularly Jainism. There is evidence to suggest that they prescribed democracy and republican principles back then in India in a country mainly ruled by kings. Influenced by them there were some ancient Indian democracies/republics just like in Greece.
Having said that, though Buddhism and Jainism both have strong atheistic principles at their root, these religions are not 100% scientific or Atheistic in modern sense which is based on reason and science. They believe that there is a purpose to life, they believe in Karma, soul and reincarnation. They suggest if you do good 'Karma', eventually after many rebirths your soul can be liberated from the eternal suffering in the cycle of births and rebirths... If you do bad 'karma' then you will forever be lost in the cycle of births and rebirths..
Meaning they actually see that the very fact of birth and rebirth and existing and living on earth is a suffering and the ultimate goal of any soul should be to be liberated from it which is what is called 'Nirvana' or 'Moksha' which is possible only through doing good deeds, being non-violent and being very strict and ascetic. This they believe can take hundreds and thousands of birth-rebirth cycles of good 'karmic ' life finally culminating in last few births where one turns to being very ascetic in lifestyle to finally attain liberation/salvation... So in that way these religions are a bit of renunciatory. But in practice only monks of these religions tend to be that extreme and ascetic, ordinary folk practice asceticism in a far more liberal way.
The liberated souls are regarded highly with special status.. in Jainism they are called Thirthankaras/Siddhas etc. But over time these have reduced into being worshiped as gods actually defeating the Atheistic principles.
Although much of what I said in last para may not appeal to modern Atheists, the fact that good conduct/morals and non-violence all of which make sense anyday are core of these religions.. than glorifying supreme deities.. is a good thing.
But much of what I said is the nature of these religions in their purest form as taught 2 thousand years ago. Throughout history since these religions have had to compete with Hinduism for survival, over time they have been influenced by it and have borrowed Godly things from it. Though they are atheistic religions in their purest form, over the course of time particularly in middle ages, they too incorporated many God ideas and today they can be seen as worshiping gods albeit in a incomplete way.
Atheism is definitely not a modern concept.
Wikipedia states that the word itself become popular in 18th century, but throughout the history for sure there were individuals without any belief system which included spiritual beings, or any supernatural phenomena.
Also, common sense tells us that without teism or some other more complex form of belief in supernatural, there is no real atheism in today's sense. In other words, we can conclude that modern atheism could not exist before organized religion emerged. Religion, as a concept emerged very early, probably in parallel with civilizations themselves serving multiple cohesive roles within group, tribe or clan helping early civilization to emerge in the first place. So, atheistic civilizations are highly unlikely to have ever existed.
In prehistoric times, before first civilizations emerged, the pseudo-religious practices included animal worship or totemism, ancestral worship, worship of natural phenomena etc. In that case in practice you could worship or not, but you couldn't not believe, because the objects of worship were real, not imaginary and as such did not require belief itself.
I'd say most primitive groups, practicing nature worship in its simplest forms, have no concept of "god". They revere nature itself, its processes and phenomena, but don't usually personify them. That comes later, when society grows.