What is the thinking on how they came to be that way? Did they become shorter, or everyone else get taller? Basically, why would there be this dramatic difference between them and everyone else?

They live in a dense jungle where nothing is very large. You survive to pass on your genes if you’re small enough to escape from predators through dense underbrush.

Sounds questionable to me. The difference in height is a very significant one in terms of the human body, but seems pretty small in terms of escaping through underbrush. What percentage of those guys end up getting eaten by predators anyway? I would think it would have to be very high before diferences of this type have any meaningful advantage. (And just how dense is this underbrush anyway? And what predators are lurking there?)

Genotype + environment = phenotype. Pygmies have been around for a very long time. Homer wrote about them in 8 B.C. The average adult male height is LESS THAN 60 inches. The best known tribe is the Mbuti or Bambuti. They are also the shortest at about 51 inches tall. However, archaeologists have not found evidence of true pygmy-sized humans in pre-history which means they are a fairly recent development and not a “leftover” from earlier evolution.

This site explains the genotype/environment theory:

in the case of most african pygmies it is a genetic fault in a growth hormone receptor.
the produce GH but simple do not have the receptors to respond to it normally. hence restricted growth, but perfectly normal body shape.

Wouldn’t the development of that genetic fault be the result of genotype + environment = phenotype? I wasn’t aware of the genetic fault, but it makes sense. I wonder if smaller-sized humans from other rain forest habitats have the same or similar defect?

Per the website in my prior post, in the rain forests, which is the natural habitat of pygmies, humidity is constantly near 100%, which is dangerous for “normal” size humans. Quoting:

"But near 100 percent humidity, the body’s normal ability to cool down is hampered or stopped completely. If perspiration does not evaporate, it does not cool the body. In such critical conditions, we risk overheating, with the body temperature rising above the normal 98.6 degrees Fahrenheit or passing the survival limit of 107 to 108 degrees. Heatstroke may be fatal and we must protect ourselves in some other way.

Pygmies sweat a lot, but not enough - it is their small stature that protects them, in two ways. First, the surface area of a small body is greater in relation to its volume. It is a mathematical fact: if cube A in Fig. 1.4 is 1 centimeter along each side and cube B is 2 centimeters, then A’s surface area is one-quarter that of B, but its volume is eight times smaller. Heat is produced in the mass of the body, particularly in the liver and muscles, and is lost through the surface; if the latter is larger relative to body mass because a person is small, heat loss is easier and cooling more efficient. In a warm and humid environment, it is best to be small."

I guess over thousands of years, the genetic fault you speak of had plenty of time to evolve. It’s not dissimilar to the type of defect that causes sickle cell anemia but it protects against a life-threatening environment, rather than an endemic disease. Unlike sickle cell anemia, it does not seem to have a detrimental effect, however, the pygmy lifespan is only about 50 years, so maybe it does, although 50 years is pretty good in a primitive society.