Dating - If we take R1a-Z93 as a proxy for steppic DNA South of the Caucasus, and take the expansion in the number of its extant lineages as a proxy for the expansion and dispersal of its populations, we see the following pattern. (I am using yfull's estimates, based on SNP variance. My own estimates based on STR variance show similar results.)
Expansion per 100 years -
3,000-2,850 BC - 0%
2,850-2,700 BC - over 1,000 %
2,700-2,250 BC - 4%
2,250-1,800 BC - 89%
1,800-1,500 BC - 8%
1,500-900 BC - 7%
900 BC-0 AD - 3%
It is clear that Z93 was a regular lineage for most of its history, apart from during two huge bursts of expansion/development - around 2,850-2,700 BC (consistent with CWC's initial expansion) and 2,250-1,800 BC. The latter of these expansions is presumably reflective of the expansion and spread of Z93 populations East and South of the Caucasus, and its greatest period of expansion is during the early part (2,250-2,200 BC).
This indicates that the expansion of the Mitanni and Hittites (dated to around 1,500 BC) were not proxies for the expansion of Z93 populations, which had mainly occurred probably 700-800 years earlier. Accordingly, I would propose that significant steppic DNA moved East and South of the Caucasus before 2,000 BC (as autosomal DNA suggests), and that subsequent IE/Aryan expansions during the middle 2nd millennium BC were of admixed populations (in which steppic DNA was already only a minor component, say up to 20%).
Expansion per 100 years -
3,000-2,850 BC - 0%
2,850-2,700 BC - over 1,000 %
2,700-2,250 BC - 4%
2,250-1,800 BC - 89%
1,800-1,500 BC - 8%
1,500-900 BC - 7%
900 BC-0 AD - 3%
It is clear that Z93 was a regular lineage for most of its history, apart from during two huge bursts of expansion/development - around 2,850-2,700 BC (consistent with CWC's initial expansion) and 2,250-1,800 BC. The latter of these expansions is presumably reflective of the expansion and spread of Z93 populations East and South of the Caucasus, and its greatest period of expansion is during the early part (2,250-2,200 BC).
This indicates that the expansion of the Mitanni and Hittites (dated to around 1,500 BC) were not proxies for the expansion of Z93 populations, which had mainly occurred probably 700-800 years earlier. Accordingly, I would propose that significant steppic DNA moved East and South of the Caucasus before 2,000 BC (as autosomal DNA suggests), and that subsequent IE/Aryan expansions during the middle 2nd millennium BC were of admixed populations (in which steppic DNA was already only a minor component, say up to 20%).