Free The Ascent of Man by Jacob Bronowski

Christ, the huntsmen were settling to agriculture in theCanyon de Chelly, and starting along the same steps in the ascent of man that had first been taken in the Fertile Crescent of the Middle East.
    Why did civilisation begin so much later in the New World than in the Old? Evidently because man was a latecomer to the New World. He came before boats were invented, which implies that he came dry-shod over the Bering Straits when they formed a broadland-bridge during the last Ice Age. The glaciological evidence points to two possible times when men might have wandered from the easternmost promontories of the Old World beyond Siberia to the rocky wastes of western Alaska in the New. One period was between 28,000 BC and 23,000 BC , and the other between 14,000 BC and 10,000 BC . After that the flood of melt-water at the end of the last Ice Ageraised the sea level again by several hundred feet and thereby turned the key on the inhabitants of the New World.
    That means that man came from Asia to America not later than ten thousand years ago, and not earlier than about thirty thousand years ago. And he did not necessarily come all at once. There is evidence inarchaeological finds (such as early sites and tools) that two separate streamsof culture came to America. And, most telling to me, there is subtle but persuasive biological evidence that I can only interpret to mean that he came in two small, successive migrations.
    The Indian tribes of North and South America do not contain all the blood groups that are found in populations elsewhere. A fascinating glimpse into their ancestry is opened by this unexpected biological quirk.For the blood groups are inherited in such a way that, over a whole population, they provide some genetic record of the past. The total absence of blood group A from a population implies, with virtual certainty, that there was no blood group A in its ancestry; and similarly with blood group B. And this is in fact the state of affairs in America. The tribes of Central and South America (in theAmazon, for example, in the Andes, and in Tierra del Fuego) belong entirely to blood group O; so do some North American tribes. Others (among them the Sioux, the Chippewa, and the Pueblo Indians) consist of blood group O mixed with ten to fifteen per cent of blood group A.
    In summary, the evidence is that there is no blood group B anywhere in America, as there is in most other parts of the world.

    In Central and South America, all the original Indian population is blood group O. In North America, it is of blood groups O and A. I can see no sensible way of interpreting that but to believe that a first migration of a small, related kinship group (all of blood group O) came into America, multiplied, and spread right down to the south. Then a second migration, again of small groups, this timecontaining either A alone or both A and O, followed them only as far as North America. The American Indians of the north, then, certainly contain some of this later migration and are, comparatively speaking, latecomers.
    Agriculture in the Canyon de Chelly reflects this lateness. Although maize had long been cultivated in Central and South America, here it comes in only about the time of Christ.People are very simple, they have no houses, they live in caves. About AD 500 pottery is introduced. Pit houses are dug in the caves themselves, and covered with a roof moulded out of clay or adobe. And at that stage the Canyon is really fixed until about the year AD 1000, when the great Pueblo civilisation comes in with stone masonry.
    I am making a basic separation between architecture as mouldingand architecture as the assembly of parts. That seems a very simple distinction: the mud house, the stone masonry. But in fact it represents a fundamental intellectual difference, not just a technical one. And I believe it to be one of the most important steps that man has taken, wherever and whenever he did so: the distinction between the moulding