* Triangles are fundamental shapes: They exist in nature and have been observed by humans for millennia. Ancient civilizations like the Egyptians and Babylonians used triangles in their architecture and for surveying.
* The concept of geometry evolved gradually: It wasn't until the ancient Greeks, particularly with mathematicians like Euclid, that the concept of a triangle as a geometric object was formalized and its properties were studied systematically.
So, rather than a specific date, we can say that:
* Humans have understood the basic concept of a triangle for a very long time.
* The formal study and definition of triangles as mathematical objects developed in ancient Greece.
It's also important to remember that different cultures and civilizations may have discovered and used triangles independently.
