For Denisova 11, the measured heterozygosity was comparable to present-day Africans—higher than would be expected for a typical Neanderthal or Denisovan individual, who came from populations with more limited diversity, and higher than would be expected for a later-generation admixed individual, in whom the initially elevated heterozygosity of a first-generation hybrid would have been diluted over subsequent generations.
This comparison is one of the key pieces of evidence for the first-generation interpretation. A second-generation individual—the child of a first-generation hybrid and a pure-ancestry individual from either group—would have heterozygosity roughly halfway between the first-generation level and the baseline level of the pure-ancestry population. A more distant admixed individual would have lower heterozygosity still. The high heterozygosity observed in Denisova 11, combined with the approximately equal proportion of Neanderthal and Denisovan ancestry, makes the first-generation interpretation significantly more likely than the alternatives.
The researchers did not rely solely on heterozygosity; they also modeled the data under multiple ancestral scenarios and found that the first-generation hybrid model fit the observed genome composition better than models involving admixture in earlier generations. But the heterozygosity comparison provides an intuitive and direct line of evidence that is worth understanding.
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## Comparison With Modern Hybridization in Other Species
The pattern of occasional, limited interbreeding between closely related but genetically distinct species is well-documented in the modern animal world. Understanding Neanderthal-Denisovan interbreeding in this broader context helps place it in perspective.
Modern wolves and domestic dogs interbreed where their ranges overlap, producing fertile offspring. The genetic signature of this interbreeding is detectable in both wolf and dog populations in regions where they coexist. The two groups remain distinct—they have different behaviors, social structures, and ecological niches—but they are not reproductively isolated, and the resulting hybrids are sometimes fertile and sometimes integrate into one parent population or the other.