One genome from one person at one moment in time.

The sample of ancient individuals that have been genetically analyzed from the period of Neanderthal and Denisovan existence is tiny — perhaps a few dozen individuals with significant genomic data, scattered across a period of hundreds of thousands of years and a geographic range of millions of square kilometers.

From this tiny sample, we are drawing conclusions about the behavior and interactions of populations that numbered in the tens of thousands at any given time, across territories spanning multiple continents, over periods of time longer than the entire history of anatomically modern human civilization.

The statistical inference required to go from this evidence to the conclusions that researchers draw is substantial, and with any statistical inference comes uncertainty.