In the 7th paragraph from the bottom, the writer speaks about the Mohs Scale of Hardness. This link contains a chart of hardness number of various natural objects, and human artifacts that can be used to test the approximate age of mineral substances. The author speaks of how hard the rock was that these hominids were cutting into. Actual quartz crystals (around 7) and other common rocks are often much harder than the 4.5 rock that was dug into, so it would not have been too hard for these hominids to do that work.
One anomaly I have been interested in is recent discoveries in the Kow Marsh in southern Australia. If I remember correctly, these were found to be of an age around 10,000 old, but of a skeletal type more like an erectus form of Homo, rather than the more modern native Australians who appear to have arrived around 40 to 50 thousand years ago. So were these a remnant earlier species who were driven south over 30,000 years and finally killed or died out?
Probably just died out...................