Hi, Joshua. Am I as dismissive of science as you are of me? Hmmm.
I read your article yesterday, as I said several times. What I didn't have time to do was look at the original articles by Rohde, here in Nature and here in the unpublished working paper. In any case, comparing your paper with Rohde's originals raised some questions in my mind. I'll just throw them out there and you can address them (or not):
Swamidass: "Peer-reviewed estimates of these dates for all required descendants are not available in the scientific literature. Estimates are nevertheless possible. Currently, only one study models migration, geographic barriers, and population structure to estimate dates for all humans alive today (17). The same first author also released an unpublished and un-reviewed report with expanded results using a variety of parameters. These two studies represent the most realistic simulations of UGA (18). "
The footnotes 17&18 refer to the papers by Rohde linked above. You note that there are no peer-reviewed estimates that set the "required descendants" at A.D. 1. Rohde's model set the required descendants at present day and included many post-1500 changes.
Question 1: If your required descendants lived A.D. 1, is it valid to apply the estimate generated by Rohde? At the very least, this seems to add another layer of uncertainty until you are able to run the simulation afresh with new parameters, a new start date for the simulation, and a new target date for required descendants.
Joshua's article: "In the best simulations (17, 18), the MRUGA is estimated to arise 3,000 years earlier than the required descendants. The IAP is estimated to about 5,000 years earlier than the required descendants. The nearly IAP for Mesopotamia is likely closer to the MRUGA data than the IAP; a conservative number is 4,000 years. (Footnote 7: As the simulation author notes, 'the [nearly IAP] and the [IAP] are separated by perhaps 1000 years.')"
UGA – Universal genealogical ancestor (Common Ancestor, or CA in Rohde)
MRUGA – Most Recent universal genealogical ancestor (MRCA in Rohde)
IAP – Identical Ancestor Point (everyone farther back is a UGA)
To be specific, the dates generated by Rohde were 1415 B.C. for the MRCA, and a mean IAP of 5353 B.C., from which Swamidass deducted 1,000 years to arrive at an estimate of the "nearly IAP" in Mesopotamia of 4353 B.C. (~4000 B.C.).
As Swamidass explains it, the "nearly IAP" is akin to the amount of time it would take an "Adam UGA" in Mesopotamia to become related to everyone. Thus, "it will take between 3,000 and 5,000 years for a specific ancestor to become a UGA (Figure 2). The quickest time, 3,000 years, corresponds the time to the MRUGA and applies to very few, lucky and ideally located individuals. The longest time, 5,000 years, corresponds to the time to the IAP and applies to very few, unlucky, and poorly located individuals, like those in the Americas or Australia. More likely, especially for those in central locations like the Middle East, the wait time is between 4,000 and 3,000 years (Figure 3). A cautious estimate, therefore, of the wait time for typical individuals is 4,000 years, even though a more accurate estimate might be 3,500 years."
Here is what Rohde had to say about his estimate: "The point beyond which everyone alive today shares the same set of ancestors is somewhat harder to predict, but it most likely falls between 5,000 and 15,000 years ago, with a significantly more recent date for the point at which we share nearly the same set." (P. 27 of working paper.)
Deducting the same 1,000 years as Swamidass, Rohde's estimate would be between 2000-12000 B.C. for today's population. However, we are after the "nearly IAP" for A.D. 1, not A.D. 2000, so the dates provided by Rohde range between 4000-14000 B.C.
Question 2: Don't you think it would be more accurate to give a range, as Rohde has done, rather than fixating on the earlier date?
So, I was reading the unpublished paper where Rohde talked about tracking some of the "sims" in his computer simulation.
"This particular MRCA was born in Taiwan in 1536 BC. She had a remarkable advantage
in that one of her great grandchildren migrated up the coast to Chukotka. ... Her lineage first reached Indonesia in 1206 BC, North America in 1091 BC, Africa in 838 BC, Australia in 652 BC, South America in 95 BC, and Greenland in 381 AD.
I concede the sea voyage to S. America was not the best choice, but I'm claiming at least a TIE on the back of this poor sim's family. haha