variety of sources reviewed by Diamond has been the object of both praise and criticism: on one hand, the multidisciplinary nature of the work is part of its appeal. He synthesizes an enormous amount of scholarly work into a coherent narrative in service of a fairly straightforward argument. On the other hand, he dabbles in disciplines, especially history and anthropology, that he has no formal training in, which has led to claims that he oversimplified and even mischaracterized historical arguments. Others have argued that his approach to history elides many of the themes—contingency, agency, and human decision-making—that are fundamental to the discipline. In short, Diamond, like any author attempting to write such a grand synthesis, leans on decades of original research by scholars in a number of fields.

Jared Diamond’s landmark work Guns, Germs and Steel employs a vast array of different kinds of evidence to support his overarching theory that geographical factors, rather than any inherent superiority between peoples, account for power disparities on a global scale throughout human history. His use of primary sources, secondary sources, scientific evidence, archaeological evidence, and citing the existence of different flora and fauna and their impact on different peoples all work in conjunction to make a very compelling argument. The strength of Diamond’s work, however, is not so much in the evidence he uses but in how he puts them to task together—a quilt made out of already known factors, seen through a new light and used for an original purpose.

One overlooked argument Diamond uses in favor of his thesis is logic. Logical thinking cannot be counted as evidence per se, but his writing and theories follow such a smooth, logical track, they become difficult to disagree with. Some things are true because they make sense, even when evidence is lacking. Rational, logical thinking acts as its own argument. Guns, Germs and Steel employs different systems of evidence, but the logic with which they are put together is the true strength of Diamond’s work.

Over the course of the book, Diamond uses about every kind of evidence you can think of.  He uses scientific evidence, linguistic evidence, and historical evidence, among others.

For example, in the early parts of the book, Diamond uses a great deal of scientific evidence.  He uses things like evidence about the size of the seeds of prehistoric grasses.  He uses evidence about the ways in which diseases evolve.  These are very scientific types of evidence.

In other chapters, Diamond uses "softer" kinds of evidence.  He uses linguistic evidence to make claims about how the Bantu came to dominate Africa and about how Austronesians spread through the Pacific.  He uses historical (and geographical) evidence to talk about the ways in which different kinds of Polynesian societies evolved.

In these ways, Diamond uses many kinds of evidence over the course of this book.

Unless readers are expert in all of the fields Diamond discusses, they must take essentially all of his arguments on faith.

For example, most readers of this book do not have any independent knowledge of the number of domesticable plants that were native to any of the regions of the world in prehistory.  We have to take Diamond's claims about this on faith.  Most readers do not have any independent knowledge of the various Polynesian cultures that Diamond discusses in Chapter 2.  We have to take his claims about them on faith.

Of course, Diamond has sources for all of these facts, but he does not cite specific sources for specific claims.  He does not write this book as an academic work with footnotes and specific citations.  This requires us to take his arguments on faith unless we want to do research on our own.

Beyond that, Diamond's arguments about causation must be taken completely on faith because there can be no firm evidence to prove or disprove them.  When Diamond says that culture plays no part in whether some cultures are willing to adopt technology, we have to take his argument on faith.  There is no way to prove why some cultures adopt technology and why some do not.  Diamond's arguments are plausible, but they cannot be proven.

So, lay readers must take essentially all of the book on faith because they do not many (if any) of the facts in the book independently.  However, some claims that Diamond makes (those about seed size of ancient plants) are provable with research.  Other claims that Diamond makes (that China rejected technology because they had no countries competing with them for dominance) can never be proven and will always have to be taken on faith.

The answer to this can be found in Chapter 5.  There, Diamond tells us that scientists typically use a method called radiocarbon dating to determine how old an early agricultural community is.  All living things contain carbon.  One part of carbon is carbon 14, which is radioactive. From the time that a living thing (plant or animal) dies, its carbon 14 content decays into carbon 12.  It does this at a steady and predictable rate.  Therefore, scientists can look at the ratio of carbon 14 to carbon 12 in the remains of plants and animals to determine how long ago they died.