"But the American X lineage turned out to be pre-Columbian and its owners would have arrived in America either 15,000 or 30,000 years ago, depending on certain genetic assumptions.
"The European X lineage seems to have originated in Western Asia around 40,000 years ago. Dr. Wallace suggests a part of this group may have made their way to America via Siberia, even though no traces of the X-lineage have yet turned up in eastern Asia. A trans-Atlantic route is a possible alternative.
"When modern humans first started to leave Africa, about 50,000 years ago by present reckoning, they probably consisted of small groups of hunter-gatherers a few hundred strong."
What is truly amazing about the above is that it presupposes African origin and dispersal. This assumes similar climatology over this span of time, when we know there was an intermedial ice age from ~10k BCE to 4KBCE. Some assume an earlier appearance ~18k+ BCE.
What is omitted is Antartica from the hypothesis. Yet we know it was once free of Ice! And, given it's location, in a more favorable climate period, could easily be the homeland for human dispersion to ALL Continents!
While it is true that antarctica was indeed lush at one time, that was on a geological scale. You are talking about hundreds of millions of years ago as opposed to the hundreds of thousands of years of humans. Antarctica has been ice covered longer than any evolutionary evidence of even proto-humans nevermind homo sapiens sapiens.