Regarding your questions:

"1) some tool making hominid species spread from Africa all across Asia to north China 1.7 million years ago but apparently never reached the Americas. Yet horses, camels, cheetahs, mammoths, etc. migrated from the Americas to Asia."

I could list a whole series of animals of the Americas that failed to cross the Bering strait land bridge such has Giant Sloths. Why did Hominids fail to cross probably because at 1.7 million years ago they didn't have fire and sewn clothing. Further Northern, Eastern Siberia at this time was cold bleak and quite uncongenial to any but small groups of Hominids even if they had, had fire and sewn clothing. Hominids were not terribly well adapted to extreme cold climates and apparently from what we can gather did not evolve the cold weather adaptions that other animals did. Instead people eventually adapted to extreme cold weather through cultural adaptions like fire and clothing. And if those cultural adaptions did not exist the areas were avoided.

"2) why are the oldest human remains from the Americas dolichocephalic but later remains are mesocephalic?"

The bottom line is that the type of Cranial analysis you are talking about is about is about 70 years old. Modern analysis of this stuff tends to be a good deal more complicated and nuanced. There is also the fact that the amount of remains of early man in the Americas is frankly very small and making sweeping conclusions on such small samples is a bit dubious. Especially using something as weak as the cephalic index. So I wouldn't say that it indicates two migrations of two different populations for example. And it is also known that environmental changes along with genetic drift may be responsible for the changes. (See First Peoples in a New World, David J. Meltzer, University of California Press, Berkeley, 2009, pp. 170-181.)

"3) Where did Clovis culture originate and how did it spread across North America in only a few hundred years. Why did it suddenly disappear to be replaced with a multitude of regional variants?"

If your referring to the Solutrean hypothesis. It is frankly dubious. (The c. 5,000 year gap between the Solutrean culture in Europe and Clovis in the Americas is a rather large problem among many.) As for origin Clovis likely originated in the Great plains region / Southwest. Another possibility is Eastern North America.

As for its spread? In that respect is mirrors the spread of many other cultural characteristics in other parts off the world. For example the so called beaker culture in Europe. It used to be thought it denoted a migration of population but that does not appear to be the case. And after a while the spreading style evolved into local variations just like Clovis. There is some suggestion that Clovis may have spread because of it's adaptability for mega fauna hunting and that Clovis changed radically when mega fauna populations had declined significantly. (See First Peoples in a New World, pp. 242-280.)

"4) Why is zea mays the only grain crop with no wild precursor?"

This goes back to the old idea that Maize originated from a now extinct wild ancestor which was crossed with the wild plant teosinte. The idea was that once maize was domesticated the pollen from domesticated corn destroyed wild corn. This is because maize husks tightly wrap their seeds and don't shed them. Without Man's help domesticated maize would quickly go extinct. Thus the idea was that wild maize was supposedly in effect biologically raped by domestic maize pollen and unable to shed it's seeds died out.

The above appears to be totally wrong it appears that maize is in effect a domesticated version of teosinte which still exists in the wild. the debate is over what particular subspecies of teosinte was domesticated. Also whether or not there was back breeding, other subspecies of teosinte involved etc. A rather good overview of this is the following [en.wikipedia.org].




Edited 2 time(s). Last edit at 01/13/2017 11:55AM by Pacal.