's 1491 is a very interesting read. The main point is that the natives to the Americas were a lot more numerous and a lot more advanced before the arrival of the European adventurers than anyone realized until recently. Whatever you learned in school vastly underestimates the accomplishments and the scale of their civilizations. There are a lot of details still to be worked out, but that much seems clear.
is very good at taking the discoveries and recent analysis and making the possibilities clear: there is disagreement in many cases about how many people were here, how much contact there was between groups, when and how they died off, and just how sophisticated they were, but at the end there is so much evidence that there was large-scale engineering in so many places (the North American Midwest, the Amazon basin, the Andes and along South America's west coast, and Central America) that the conclusion stands above the minor disagreements.
More than anything, it's an enjoyable read. Even though Mann is heaping up evidence, he tells good stories of how the people must have lived and struggled. He seems to make it clear when he is speculating, when the experts are in agreement on the basics, and when he is reporting on aspects of the history that are still in dispute. The North American natives didn't write anything down, so he can't tell many stories about individuals, but the South Americans had a variety of writing systems, so he's able to report on political struggles that demonstrate the extent of the civilizations and gives some hints about how the internal wars may have contributed to their demise in the face of the conquistadors and their diseases.
In all these areas, there are constructions that were large enough, and that have endured well enough that they are still visible if you know what to look for. Until recently, anthropologists and archaeologists didn't realize they were there, and so hadn't studied them. In the Andes and the western plains, there are dikes and large scale water works that have only been identified as such in the last twenty years. In the Midwest, there are humongous mounds that are clearly artificial. New pyramids have been discovered in the Amazon in the last twenty years using LIDAR that can penetrate the tree cover.
The mounds are a fascinating chapter all to themselves. My favorite is the Cahokia mound. Cahokia, near present day Saint Louis, was the largest (maybe the only?) city north of the Rio Grande for 300 years starting about 1000 years ago. They built a series of mounds, of which Monks Mound is the most interesting.
Its core is a slab of clay about 900 feet long, 650 feet wide, and more than 20 fee tall. From an engineering standpoint, clay should never be selected as the bearing material for a big earthen monument. Clay readily absorbs water, expanding as it does. The American Bottom clay can increase in volume by a factor of eight. Drying, it shrinks back to its original dimensions. Over time the heaving will destroy whatever is built on top of it.
To minimize instability, the Cahokians kept the slab at a constant moisture level: wet but not too wet. Moistening the clay was easy—capillary action will draw up water from the floodplain, which has a high water table. The trick is to stop evaporation from drying out the top. In an impressive display of engineering savvy, the Cahokians encapsulated the slab, sealing it off from the air by wrapping it in thin, alternating layers of sand and clay. [...] The final result covered almost fifteen acres and was the largest earthen structure in the Western Hemisphere: though built out of unsuitable material in a floodplain, it has stood for a thousand years.
The time of arrival of the pre-columbian inhabitants is more contentious than I realized. I've been using the figure of 13000 years before the present as the time that the first wave came over the Bering Strait, and wiped out all the mega-fauna. It appears that there were probably two waves of immigration before that, and possibly more, though the timing is completely unclear. There are indications that early humans were here as early as 30,000 years ago, and that there was another wave around 20000 years ago based on genealogical evidence and tantalizing archaeology. Those people didn't leave many artifacts, so there is still plenty of disagreement.
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