Wrong, wrong, WRONG!
Firstly Benaadiris (reer xamar and Barawaanis) do not share common origins at all.
There are Arab, Indian, Persian, Hebrew and Greek (who are usually found in Xamar) Benaadiris. I only heard of one tribe that have Greek ancestry.
@HalyeeyQaran Persians can be found in Barawa too saaxib. Many Shanshiyos migrated to Baraawa centuries ago, I believe Shirazi Persians have settled among them too.
Most Benaadiri tribes can be found in Xamar AND Baraawa (for example the Shanshiyo), some tribes are more concentrated in Xamar whilst others are to be found mainly in Baraawa. The Omanis are mainly to be found in Baraawa, the Asharaaf are to be found in both Xamar and Baraawa. The Asharaaf that are descendants of Hasan ibn Ali (RA) the grandson of the prophet (SAW) are mainly found in Marka, Xamar, they are from the Al-Ahdali family. The Asharaaf that are descendants of Hussein (RA) younger brother of Hassan (RA) are to be found mainly in Shingaani, Xamar (where I'm from).
There are various of Arab and Persian tribes that have settled in Benaadir. Asharaaf are one of the Arab tribes, there is also the Hatimis (my tribe) who trace their lineage to Adi ibn Hatim (RA), there are also the Omanis, other Qahtaanis and another tribe from Quraysh in addition to the Asharaaf Hashimites called the Al-Zuhri who are from the Banu Zahrah branch of Quraysh. The only Hebrew tribe I'm aware of are the 'Amraani who descendants of Yemeni Jews but converted to Islam.
Reed Xamar Benaadiris have a dialect that can be understood by the local ethnic Somalis.
Barawaanis speak chimini (which I can understand) and it's not the same thing as Swahili.
Source of my information? I ask the elders of Benaadir a lot of questions regarding these issues (sometimes they get annoyed with me).
Over 90% of the Benaadiris left Xamar and around 3000 remain. The estimation was 2000 in 2004.
http://www.justice.gov/sites/default/files/eoir/legacy/2013/06/11/Mogadishu.pdf
The vast majority have migrated to the Middle East, Far East, Europe and neighbouring East African countries.