This is a slightly silly discussion because the 'civilization' the Silurian hypothesis is concerned with is something like human civilization as a whole.

Whether pastoralists are a 'civilization' (or even more fraught, 'civilized') seems like a completely arbitrary distinction to try to make, especially on the basis of something like 'cities'. The ability to organize, direct, coordinate, supply and project military power over great distances is a hallmark of plenty of things we think of as 'civilizations' and the Mongols were easily the world heavyweight champion of that, in their day. A great number of people found out the fact a stone wall is much harder to move than a yurt is not the tremendous civilizational advantage they thought it was.

> is a slightly silly discussion because the 'civilization' the Silurian hypothesis is concerned with is something like human civilization as a whole

Going back to the top comment: “Within the Silurian context, the urban distinction is almost demanding: if humans stopped at Neolithic pastoralism, there is a good chance all evidence of our tool use would have disappeared within a few millennia, let alone millions of years.”

I've done archaeological work in Mongolia. The oldest things I've found predate the last glacial maximum. Those rocks might not have survived a few million more years, but I can certainly think of lithics that would.