Geometric design on ancient mussel shell

SciAm reports:

Now comes news that an even older, more primitive human ancestor — Homo erectus from Asia — showed signs of symbolic thought, too. Researchers have discovered a shell engraved with a geometric pattern at a H. erectus site known as Trinil, on the Indonesian island of Java, that dates to between 540,000 and 430,000 years ago. The find is at least 300,000 years older than the oldest previously known engravings, which come from South Africa.

Analysis of the engraving, made on a freshwater mussel shell, suggests that its maker used a shark tooth or other hard, pointed object to create the zigzag design.

John Hawks has more details.

Does that look like symbolic thought to you? Or primitive art? About ten scratches on a sea shell?

Maybe some dumb hominid was just trying to break open the shell. Maybe he was just trying to break the point off a shark's tooth. Maybe the scratches were formed naturally as some rock on top of the shell shifted with waves.

If this is symbolic, what does it symbolize? Why didn't they find any more shells with art?

This is a field where researchers can just dig up old stuff and make up stories. It sounds very dubious to me.

Update: Others dispute whether the shell are so old, and Homo erectus had anything to do with them.