« In Somalis, the
Time to Most Recent Common Ancestor (TMRCA) was estimated to be 4000–5000 years (2,500
BCE) for the haplogroup
E-M78 cluster γ and 2100–2200 years (150 BCE) for Somali
T-M184 bearers. »
So far the data that these studies published, their results and what we can witness on YFull are not conflicting. In fact they seem to complete one another.
We know for sure that the TMRCA won’t exceed a lot more than 2k. This lineage came without a doubt directly from Arabia.
The latest information we have from yfull is the tmrca for dir/isaaq T is 1550ybp. But even if we extend it back to 100 b.c, we still don't know if it came from arabia. This time frame would have coincided with increasing commercial ties between the somali coast and the hellenistic world. Why couldn't this lineage be brought by mercantile groups living in north-east africa? We know kulubnarti nubians for instance had t-l208. And some specific T lineages they had like T-Y31477 is dominated by saudis, but these nubians having arab origins doesnt make any sense.
There are a couple things that would convince me if it actually was an arabian lineage.
1. Diversity of this lineage represented by many arab tribes and countries. We know arabs are over-represented on yfull so this shouldnt be an issue, yet we don't see this.
2. TMRCA being quite old, talking about 4k ybp atleast. Old lineages make it clear that originated from that area. the arab samples are not this old so far with the oldest one being 2900ybp, similar to the nubian T-Y31477 which is 2800ybp.
3. Historic interaction with people. Yes we interacted with himyarites and sabaeans and have evidence of that.
4. Distribution of the lineage within the horn. So far the dir/isaaq branch of T is pretty exclusive to them, making it likely that these were lone individuals/related family members who settled in SL. arab branches would probably have greater distribution among other ethnic groups. There is a specific rahanwayne T lineage that is related to eritrean T, which is also related to various arab T lineages as well. That is more clearly an arab lineage than the dir/isaaq T one.