Inbreeding among Neanderthals-
https://www.sapiens.org/column/field...x-lives-bones/

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I
n a cave tucked into the limestone hills of the Asturias region of Spain, there lie the remains of a group of 13 Neanderthals that date to between 50,600 and 47,300 years ago. The site is infamous among anthropologists who study the Paleolithic period for the evidence of what appears to be the massacre and possible cannibalization of a family: Their bones seem to have been hacked at by stone tools and hammers, probably by another group of Neanderthals, to remove their flesh and marrow.
But more importantly, for this story, those bones also reveal something of the sex life of the cave’s inhabitants. Anomalies and deformations, along with the DNA buried within their bones, suggest that the members of this group (and their parents) were mating with their close kin."

"DNA tests from these bones indicate that inbreeding and a small population size were likely factors contributing to the physical peculiarities in this family. The 13 El Sidrón Neanderthals share much longer segments of their DNA than would be expected if they were the offspring of non-relatives.

Genetically, the three adult males in the group were closely related enough to be brothers, cousins, or uncles, while the four adult females in the group came from three distinct genetic lines. While all individuals were likely distantly related to one another (think third or fourth cousins), it is likely that the males exchanged females with another local, slightly less closely related group."

"Neanderthals were not the only ancient hominins to mate with their close relatives. Anatomically modern humans have also been found with skeletal evidence of inbreeding, such as abnormally bowed thigh bones, deformed arm bones, and even a case of a toddler with a swollen brain case consistent with hydrocephalus."