The oldest surviving evidence of Egyptian writing survives because it was carved into stone. The oldest surviving evidence of Sumerian writing survives because it was cut into clay and dried. The earliest evidence of Chinese writing writing survives because...wait, tortoise shells? Seriously?

3,500 years ago (for the sake of argument), the inhabitants of what is now China were scratching things onto tortoise shells or the shoulder blades of oxen. But, why is it just bones and shells?

Certainly an aspect of it is that those are the things which survived. Unlike other and more ephemeral ways of writing things down, the bones and shells have lasted for thousands of years. That's not anything people at the time were thinking of, though. What they were thinking about was...magic.

When you take a fresh shell or scapula (with carved information) and heat it up in a certain way, it will crack. Pits and depressions would be carved in a pattern, and then heat would be applied to those bits. Where and how it cracked would be used for the purposes of divination. Divination by fire being pyromancy, which I mention not because I think it's an important word to understand these things, but because it's an awesome word and lets me get around to pyro-scapulimancy, which is the term for when a shoulder blade(scapula) was used. I've yet to find someone who wasn't a crackpot claim to know exactly how the cracking of the bone was read.

In a move helpful to scholars, the shells tended to be carved with a preface, stating the date and the diviner involved. Even more helpful to scholars, oracle bones tended to be buried into pits with hundreds or thousands of other samples from around the same time.

The script used on the oracle bones is a precursor to Old Chinese, and seems to represent an intermediate step between a fully pictographic script and Old Chinese.

Now seems to be as good as any to transition to what initially got me interested in oracle bones. When I was studying Mandarin, my group won a talent competition. We performed a version of Little Red Riding Hood that was deemed sufficiently hilarious, and were awarded departmental t-shirts. These t-shirts had 4 characters on them, none of which we could read, in a style none of us recognized. We button-holed our professor for an explanation.

The characters turned out to translate to "middle language big learn" or "Chinese University." After we were told, two of the four characters were pretty obvious, and the other two inscrutable. They explained that the characters were Old Chinese, but that the characters that we could read were basically identical to what was used 3,500 years ago on oracle bones.

That led me to a small pile of reading that shifted the way I thought about history.

The first time I visited Beijing, I had a a couple of hours to burn and visited the National Museum of China. I was with a few people who were copasetic with visiting the Museum, until I found the exhibit with the oracle bones. My insistence on lingering and gawking started to annoy, until I explained to them exactly what we were looking at. It didn't hurt that ones on display were quite likely the most intact and pleasing ones around.