Re: human ancestors never passed through a knuckle-walking phase

I'd agree with Marc's statement that human ancestors did not pass
through a knuckle walking phase. Infant humans pass through a knee
crawling phase, which is significant.

Regarding stone erosion at coasts, low density stones (eg. pumice) are
more likely to erode into a sphere like pebble, (being closer in
density to water) before eroding into dust. More dense stones may wear
into a spherical form or a flat disk form, depending on circumstances,
either way they may erode to dust quickly.

.