I am pleased someone has raised this issue; I've been noting it for many years. Considering that genetic drift is a constant process, the transition between species must be a gradual shift. Many people, particularly opponents, seem to conceptualize these abrupt, dramatic transitions which simply aren't what evolution is all about. It is conceivable that a modern Briton and an ancient Roman would be genetically incompatible, and therefore of different species (or at least subspecies).
Granted, there's a compelling reason to deduce that Julius Caesar and Princess Diana were indeed of the same species. That is, groups of humans isolated from each other since long before Julius Caesar's time are nonetheless capable of reproducing with one another, and therefore are of the same species.* There is no plausible reason to think that genetic drift since the 1st century BC Italy to 20th century AD Britain was greater than that from 10,000 BC Siberia or Polynesia.
* please spare us the mules..
Guess what? We're all one species. Maybe a bunch of local variations (races), but--we're all one species!
If you look at the migration patterns, you have diversity of many tens of thousands of years. With this time span, Julius Caesar and Princess Diana are chickenfeed. One species, no question. Before we need to test that they were members of the same species of Homo sapiens, lets see any evidence they were not. Otherwise you are just wasting our time.
Hmm.. I should not have said "10,000 BC Siberia or Polynesia" because Polynesia wasn't settled until after 1600 BC (beginning with the Solomon Islands, and that is more strictly Melanesia). I should've said Oceania, and I was thinking New Guinea and Australia, to be exact.