Radiocarbon and genomic evidence for the survival of <i>Equus Sussemionus</i> until the late Holocene.

Abstract

The exceptionally rich fossil record available for the equid family has provided textbook examples of macroevolutionary changes. Horses, asses, and zebras represent three extant subgenera of <i>Equus</i> lineage, while the <i>Sussemionus</i> subgenus is another remarkable <i>Equus</i> lineage ranging from North America to Ethiopia in the Pleistocene. We sequenced 26 archaeological specimens from Northern China in the Holocene that could be assigned morphologically and genetically to <i>Equus ovodovi</i>, a species representative of <i>Sussemionus</i>. We present the first high-quality complete genome of the <i>Sussemionus</i> lineage, which was sequenced to 13.4× depth of coverage. Radiocarbon dating demonstrates that this lineage survived until ~3500 years ago, despite continued demographic collapse during the Last Glacial Maximum and the great human expansion in East Asia. We also confirmed the <i>Equus</i> phylogenetic tree and found that <i>Sussemionus</i> diverged from the ancestor of non-caballine equids ~2.3-2.7 million years ago and possibly remained affected by secondary gene flow post-divergence. We found that the small genetic diversity, rather than enhanced inbreeding, limited the species' chances of survival. Our work adds to the growing literature illustrating how ancient DNA can inform on extinction dynamics and the long-term resilience of species surviving in cryptic population pockets.