Perhaps if you see such a piece of rock outside a place where it is likely it is an artefact (eg. a camp or burial site among other belongings) you wouldn't even consider it as man made.

Maybe you have passed such things multiple times without ever recognizing their true origins!

Spheres are an unlikely shape in nature according to the article.

"Spheres"

Have look at the pic in the article, not exactly polished ball bearings.

Those would be easy to ignore and nature makes plenty of more spherical rocks. Most rivers have shores with better looking "spheres".

The article claims that the rocks while not as polished are much closer to perfect spheres than anything nature produces.

I don't have the expertise to comment on if that is correct or not. I'm just repeating what 'facts' that I trust the reporter verified, though i'll admit to seeing enough examples of reporters not verifying facts to have doubt.

A very interesting point

There's a huge difference in population density between the two groups; there was no place in the US that wasn't inhabited even before projectile points were fully diffused across the area.

Combine that with the fact that we're temporally very close to the Indigenous occupation. Whereas there's over a million+ years of geological processes to affect those artifacts original provenance.

Also, if we haven't been looking for crudely rounded stones we aren't going to have much of a sample size to say whether or not they are, to use archeologist's favorite word, ubiquitous.

I'd wager most people aren't aware that spheres aren't common in nature.

Interesting point - population density when such spheres were being manufactured was low. Which leads to the next idea: when population density increased, spheres were no longer manufactured. Why? Something replaced them. Arrowheads?

We can start to unpick the mystery with insights like that.

Kind of makes you wonder if the first arrow heads were formed when someone was trying to make a sphere and struck the rock with flint (or one of the other suitable stones) and accidentally cut themselves on the sharp resulting shard.