Recent archaeological discoveries make Sumerian origins a little clearer. Explorations in Central Asia (as mentioned above p. 257) show that districts once well watered, and capable of supporting a large population, have been subject to periods of excessive drought, and this no doubt is the prime cause of the racial unrest which has ever been characteristic of the dwellers in these regions. A cycle of drought may well have prompted the Sumerian migration of the fourth millennium B.C., as it is shown to have prompted the later invasions of the last two thousand years[592]. Although there is no evidence to connect the original home of the Sumerians with any of the oases yet excavated in Central Asia, yet signs of cultural contact are not wanting, and it may safely be inferred that their civilisation was evolved in some region to the east of the Euphrates valley before their entrance into Babylonia[593]. Since Semitic influence was first felt in the north of Babylonia, at Akkad, it is assumed that the immigration was from the north-west from Arabia by way of the Syrian coastlands, and in this case also the impulse may have been the occurrence of an arid period in the centre of the Arabian continent. The Semites are found not as barbarian invaders, but as a highly cultivated people. They absorbed several cultural elements of the Sumerians, notably their script, and were profoundly influenced by Sumerian religion. The Akkadians are represented with elaborately curled hair and beard, and hence, in contradistinction to the shaven Sumerians, are referred to as "the black-headed ones." Their chief weapon was the bow, but they had also lances and battle axes. As among the Sumerians the sign of kingship was a boomerang-like sceptre[594]. Except for Babylon and Sippar, which throw little light on the early periods, no systematic excavation has been undertaken in northern Babylonia, and the site of Akkad is still unidentified. The chronology of this early age of Babylonia is much disputed. The very high dates of 5000 or 6000 B.C. formerly assigned by many writers to the earliest remains of the Sumerians and the Babylonian Semites, depended to a great extent on the statement of Nabonidus (556 B.C.) that 3200 years separated his own age from that of Naram-Sin, the son of Sargon of Agade; for to Sargon, on this statement alone, a date of 3800 has usually been assigned[595]. This date presents many difficulties, leaving many centuries unrepresented by any royal names or records. Even the suggested emendation of the text reducing the estimate by a thousand years is not generally acceptable. Most authorities hesitate to date any Babylonian records before 3000 B.C.[596] and agree that the time has not arrived for fixing any definite dates for the early period.