The first shapes

Circle, square, triangle, rectangle. Shapes are an extremely basic thing taught to toddlers, alongside letters and numbers and colours and animals.

But they're also rather an artificial human construct.

I mean, some shapes do exist in nature, but most of nature is irregular in shape. Prehistoric humans could have gone a lifetime without ever seeing a triangle, so they probably didn't feel the need to name it. And if they did name it, they wouldn't feel the need to teach it to children as a basic core concept.

Shapes would probably become more common as humans started making things, and standardizing the thing they're making. Then they would need to communicate what the things should look like, so they'd need names for shapes.

Would some time have elapsed between the naming of shapes and teaching them to children as a basic concept?  On one hand, if they were a fairly new human invention, people might not feel they're a basic concept that small children need to know. On the other hand, in the past children have been closer to their parents' livelihoods, so they would likely learn the vocabulary of their parents' trades and/or the tasks of everyday living early on simply as a matter of course.

It's also interesting to think about the time between when people started making things but hadn't yet named shapes.  If you don't have a word for a shape, you're less likely to be thinking in terms of shapes. Therefore, people probably didn't see it as necessary for things to be a particular shape in the way we do today.  For example, you probably aren't going to think that the rocks around the fire need to be in a circle if you don't have a word for a circle. You probably aren't going to think your room needs to be rectangular if you don't have a word for a rectangle. Why should flatbread be round? Why should a tent be triangular? Things might have been all kinds of funny shapes simply because people didn't have the vocabulary for standard shapes!

Also, what constitutes a "standard" shape may well have varied depending on what kinds of things a particular society made. If you live in a teepee, you might have a word for triangle or cone, but not a word for rectangle or cube. If you live in an igloo, you might have a word for circle or dome, but not triangle.  The internet tells me that igloos are made of blocks of ice, but perfectly cubic blocks won't make a perfectly hemispherical dome, so maybe your concept of what constitutes a "block" or a "dome" is affected by this fact.

And then, after millennia of no shapes, and millennia of shapes being the result of what people made, we somehow transitioned into theoretical shapes - perfect circles, squares, triangles and rectangles.  Which affected the shapes of things people made, and eventually became so commonplace that they were no longer just for mathematicians and architects and engineers, but instead taught to preschoolers.