Their children were viable.

Those children could reproduce, and the genetic contributions of ancient encounters persisted in the genomes of their descendants for hundreds of thousands of years.

But the interbreeding episodes that had been documented before 2018 were all inferred indirectly — from the presence of ancient DNA in modern human populations, traced backward through time to identify when and where the encounters that produced those genetic contributions must have taken place.

No one had found direct evidence of a first-generation hybrid.