In short, it was the Sumerian civilisation, in what is now the Iraqi region. This topic has been explored in detail within Atheism Delusion and other Ansari programmes and literature.

If you research further into the topic, you will find that Atheism Delusion describes Adam (as) and his community as being the first that migrated out of Africa and across the Southern cost of the Arabian Peninsula and into modern day Iraq.

An excerpt is given from Atheism Delusion below…

“We can regard the dwellers of this valley (present day Gulf) the people of the first human civilization on this earth, and they are the ones that began forming Sumerian or Akkadian villages in South Iraq which preceded the Sumerian Akkadian civilisation that has reached us, which is why we can say about them that they are the first Sumerians, or they are the grandparents about whose books we read in the clay tablets, and they are proud of these and their civilisation and their higher moral values.”

(Atheism Delusion – p 117)

يمكننا اعتبار سكان هذا الوادي (الخليج الحالي) أصحاب أول حضارة إنسانية على وجه الأرض، وهم من بدأوا بتكوين القرى السومرية أو الاكادية في جنوب العراق التي سبقت الحضارة السومرية الاكادية التي وصلتنا، ولهذا يمكن أن نقول عنهم: إنهم السومريون الأوائل، أو هم أولئك الأجداد الذين نقرأ للسومريين كتاباتهم في الرقم الطينية وهم يفتخرون بهم وبحضارتهم وقيمهم الأخلاقية العليا.

(كتاب وهم الإلحاد ـ ص 117)

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Atheism Delusion (2014 edition)