Native lead is not found in nature. All findings of pure lead thus indicate the process of melting. The findings of lead are thus almost three millennia earlier than that of copper. Ones confronted with the possibility of melted metals, the next steps are now at least intuitively better understood. The next step must have been silver.
Pure silver does occur in nature, but is rare.
Most silver is obtained as byproducts, in particularly from lead ore. The lead compound becomes solid. Silver doesn't oxidize and is fluid. The process can be recognized by the remaining percentage of lead in silver. Physical properties of metals change dramatically when mixed with even small amounts of other metals.