Somewhere around 9000 BC (or BCE if you prefer), the people of Mureybet, in what's now Syria, began making rectangular buildings. That's according to archaeologist Jacques Cauvin, as cited by Peter Watson in Ideas: A History of Thought and Invention, From Fire to Freud. Mureybet is also one of the oldest villages to be based around agriculture, and they may have been among the first to use tokens as symbols for communicating, but we'll get to all that later.
I don't know if the Mureybetians were the very first to make rectangular buildings, but they're on the list. I guess what seems like a breakthrough here is that they didn't simply build shelters and storage sheds in the easiest way they could think of ... they had a vision of what a building could be, and they followed it. It was designed!
There's a lot of speculation about the reason for making square or rectangular buildings. Once you have farms, you have food storage, and that's something of interest to invaders. Rectangular buildings could be built closer together, and would thus be easier to defend. It's also possible that moving goods around suggested streets, and that suggested rectangular blocks, etc. Who knows?
We have no way of knowing, of course, what were the very first human ideas. We don't even have a clear definition of idea that distinguishes it from reflex, recognition, and other mental activities. But it seems reasonable to consider that the first idea was consciousness itself ... the idea that there's a me that is somehow separate from everything that is not me. If I get hit with a rock, it hurts. If you get hit with a rock, it's funny.
Another candidate for really early ideas is change ... the observation that things don't stay the same. And close on the heels of change is the observation that very many things change in cycles. They repeat over hours, days, months, years, etc.
It's not surprising that human-like traits were associated with these changing things ... spirits, anima, gods ... all were credited with responsibility for the behavior of things in the natural world. What is ironic, though, is that some of the most regular cyclic events were ascribed to some of the most powerful gods. The cyclic nature of things would seem to suggest a lack of free will, a kind of submission to inevitability. Yet many of the most rigidly repeating phenomena (the sun's daily traversal of the sky, the moon's phases, the rise and fall of tides, etc.) were seen as gestures of the most powerful gods, who could have exercised free will even beyond that of humans. Maybe repetition and ritual were themselves seen as godlike qualities, to which humans could only aspire, in the same way we might admire some machinery or architecture as being too perfect and inorganic to be the product of human endeavor.
In any case, there must have been something reassuring about these cyclic events ... the knowledge that things would return to their previous conditions again. As Scarlett O'Hara puts it in Gone With the Wind, "tomorrow is another day."
It's hard to know if hierarchy is an idea or just a side effect of the way our brains organize information. It may have started as a side effect ... we tend to work from the particular to the general, which is how we recognize cartoon versions of everyday objects, etc. Children learn this very early.
But at some point, we clearly recognized that hierarchies are a useful way of organizing information, people, object-oriented software, etc. We create hierarchical organizations. Even primitive societies had leaders, priests, etc., and a chain of command.
So we use hierarchies to arrange our observations, such as the taxonomies used by Carl Linnaeus to classify plant and animal species. This is descriptive.
But we also explicitly create hierarchies. Businesses and the military both use hierarchical organizations to establish rank, precedence, etc. These structures are well defined and rigidly adhered to. So this would be considered a prescriptive hierarchy.
In object-oriented programming, you can have multiple hierarchies in place at once. There's an inheritance hierarchy that determines which classes are subtypes of which other classes, and what properties they inherit. There are also containment hierarchies ... which object is contained by which other object, etc. In XML terms, this is what determines an xpath.
Don't worry if that last paragraph didn't make any sense. It's all geek speak. The point is that hierarchies come into play in a very large portion of human thought. In fact, they could be considered what Charles Murray calls a meta-invention (Human Accomplishment: The Pursuit of Excellence in the Arts and Sciences, 800 B.C. to 1950), a cognitive innovation that enables other innovations or inventions to happen. Murry doesn't list hierarchies in his roster of meta-inventions, but it seems to fit his criteria.
Of course, if you ever had to do outlines in school, you know about hierarchies.
Depending on your point of view, sexual reproduction was either part of the plan by an all knowing power, or an adaptation that arose in the course of evolution. So it's either an idea or it's not. Clearly, though, sex is an important idea for many people, whether for reproduction or recreation.
Either way, sexual reproduction had a profound impact on biology. Two separate organisms could produce a third, with genetic properties based on those of its forebears. This is what allowed evolution to really take off, since it provided a mechanism for change, mutation and adaptation. Before this, organisms reproduced asexually, basically by splitting in half. Each half had genetic properties identical to the original, so they never had to argue over what to watch.
There's another important consequence of this, however, one that may have had a profound impact on our way of thinking and the things we think about ... mortality. When cells simply split in half, the material lives on in both halves, which eventually split again and again. Effectively, they're immortal. In the case of sexual reproduction, though, entirely new organisms keep being made. Eventually, some have to die, or the population grows forever and consumes all possible space, food and other resources.
But the fact that we die eventually may be the most important influence on our art, literature, philosophy, ethics and society, and even our science. Would you want to start a book or movie, knowing that it was never going to end?
There's some debate on whether sex is an idea or not.
(It certainly is something that gets thought about a lot.)
For reproduction, countless species have sex at their leisure,
Though humans are the only ones who do it just for pleasure.
But earth is just a spherically-shaped piece of real estate
And we should know empirically its resources, though great,
Are finite, and eventually will not support us all.
And yet we keep consensually filling up this ball.
And as the population grows and spreads through different lands,
These locales become homes to many different tribes or bands.
Each member looks on his or her own group with puffed up pride.
Alas, the more we multiply, the more we must divide.