It is an error, or issue caused by excessive server load. It stops players from playing online
Instead of chasing after their prey, homo sapiens developed spears out of flint and rock to kill their prey. The discovery of fire also helped these homo sapiens cook their food, which helped them to avoid sickness as a result of food poisoning.
Slate is much more commonly found in wet areas, especially around rivers in the United States, so it wouldn't be very available to Natives who were more inland.
