Because at one time,they were bored

Because at one time,they were bored of not doing any thing,so they started to do something that's not as boring as doing nothing,so they started to draw on walls and later their descendants became famous artists such as the one who drew MonaLisa.

I think it's one way to express their self, and they want to be existed in their environment. Or Maybe it could be a tradition to record and draw on walls using signs and symbols.

Hasil (Bahasa Indonesia) 2:

Karena pada satu waktu, mereka bosan tidak melakukan hal apapun, sehingga mereka mulai melakukan sesuatu yang tidak membosankan seperti melakukan apa-apa, sehingga mereka mulai menggambar di dinding dan kemudian keturunan mereka menjadi artis terkenal seperti orang yang menarik MonaLisa. Saya pikir itu salah satu cara untuk mengekspresikan diri mereka, dan mereka ingin ada di lingkungan mereka. Atau Mungkin itu bisa menjadi tradisi untuk merekam dan menggambar di dinding menggunakan tanda-tanda dan simbol-simbol.