The shield wall was first attested in the Stele of the Vultures, as seen here, but it is likely to have existed far before even Sumer considering the simplicity of the tactic: http://sumerianshakespeare.com/media/eff4fb62c807457effff80c1ffffe417.jpg
The shieldwall is still used today by riot police, but military usage died out in the late middle ages with the advent of gunpowder and polearms, although it has been rendered obsolete and revived many times.