Question about Denisovan DNA. Once introduced into a population, beginning many millenia ago, what keeps it from being in the DNA of everybody in the area? I exclude new arrivals, but what kept the Denisovan DNA from being spread to the homeland of the new arrivals what with the traveling salesmen, the refugees from tribal pushing and shoving, armies marching, cross marching and countermarching? It isn't as if Denisovan genes cause assortative mating by making the possessor either a hell of a catch or a last-man-on-earth scenario. Is it? Selective survival against diseases that come and go, while not so good in between, a la sickle cell? Is the blender model of human reproduction faulty somehow. As to potatoes, I'd heard that one advantage is that armies, used to pasturing their horses in the grain of the enemy's peasants' fields, had to move on more quickly when the supply officers gave up trying to get their foraging parties to dig potatoes. If, as Keegan hypothesizes, the ration was one pound of meat and two of bread (requiring two pounds of firewood) per man per day, an army of 30,000 ate out a location pretty quickly. If spuds were the local staple, they'd have to move. You just can't feed 30,000 guests who arrived unannounced by digging potatos. Not fast enough. Do horses like potatos? So, the army moves on--win--and the peasants get out the potato forks and do okay, more or less. Win.
Re: potatoes – I think you’ve pointed to an important factor – also, they can’t be burned when the army retreats. The sheer productivity of tubers really does outweigh the available grain crops in Northern Europe.
Re: Denisovan DNA – The genes should have diffused into other populations, all things being equal. That they did not do so is a pretty strong indication that SE Asia today shares little genetically with SE Asia 30,000 years ago. There must have been a massive influx of people who lacked Denisovan ancestry, well after the initial mixture with Denisovans happened and Denisovans themselves left the scene.