IBD Sharing between Africans, Neandertals, and Denisovans

Area of Science:

• Paleogenetics
• Human Evolution
• Population Genetics

Background:

• Interbreeding between human ancestors and archaic hominins (Neandertals, Denisovans) outside Africa is well-documented.
• The extent and timing of hominin interactions within Africa remain largely unexplored.
• Understanding early African hominin history is crucial for a complete picture of human evolution.

Purpose of the Study:

• To investigate the genetic history of humans, Neandertals, and Denisovans within Africa.
• To identify ancient interbreeding events using shared identical by descent (IBD) DNA segments.
• To date these events by analyzing the length of IBD segments.

Main Methods:

• Analysis of very short, identical by descent (IBD) DNA segments in the 1000 Genomes Phase 3 dataset.
• Focus on low-frequency and rare genetic variants to confidently identify ancient segments.
• Comparative analysis of autosomal and X chromosome DNA segments shared among human populations and archaic hominins.

Main Results:

• Two types of ancient IBD segments were identified: longer segments (primarily in Asians/Europeans) suggesting admixture outside Africa, and shorter segments (predominantly in Africans) indicating archaic admixture within Africa.
• Analysis of the X chromosome revealed a higher prevalence of Neandertal/Denisovan shared segments in African populations.
• Shorter IBD segments, indicative of older events, were more prominent in African populations.

Conclusions:

• Archaic hominin admixture was a common feature of human evolution occurring within Africa, predating the migration out of Africa.
• These findings necessitate a revision of the timeline and geography of hominin interbreeding.
• The study highlights the importance of investigating African genetic history for understanding early human origins.