Species evolution is often depicted as linear, neatly separated branches of a tree, an oversimplification that fails to capture the true complexity of evolutionary history. This is especially true for human evolution, where the prevailing belief has been that the modern human population that emerged from Africa was a distinct, isolated group. However, a recent study published in Nature by Ragsdale et al. 2023 challenges this view. https://doi.org/10.1038/s41586-023-06055-y

Previously, unexplained genetic diversity was attributed to interbreeding with “ghost archaic populations“. This study proposes a “weakly structured stem” model, where early Homo sapiens populations were not isolated but were semi-connected networks exchanging genes over hundreds of thousands of years. This interconnected framework challenges the idea of a single origin or reliance on ghost populations, theoretically isolated groups without fossil evidence, to explain genetic diversity. Instead, it presents a complex scenario where populations, including Homo heidelbergensis and archaic Homo sapiens, continuously interacted, shaping a shared genetic pool across different Homo members in Africa. These exchanges contributed to the vast genetic diversity observed today without needing contributions from unknown, isolated groups.

For this study, Ragsdale’s team have analyzed the genomic data from contemporary populations from eastern, western, and south Africa (including the Nama people), as well as Eurasia. The fossil record also supports their model. Fossils from sites like Jebel Irhoud (~315,000 years ago) and Herto (~160,000 years ago), which show both archaic and modern traits, fit into this interconnected network, highlighting the mosaic nature of human evolution.

Figure 3 in the study explains two models: Fig. 3a_a multiple-merger model with complex population loops, and Fig. 3b_a continuous migration model showing steady gene flow among ancestral groups for over a million years. The continuous migration model suggests that modern human population structures began forming around 120,000 to 135,000 years ago.

Conclusion

Our origins story is far from over. With each discovery, we inch closer to understanding the mosaic of human history, one that is as intricate and interwoven as the continent from which we all emerged. This dynamic perspective of human evolution emphasizes that our past is a complex tapestry of interconnected populations, reflecting the rich genetic diversity that defines us today.