So, this claim comes from the paper "On the Iron Front" in the Journal of African Archaeology, by Augustin Holl and Etienne Zangato. The specific argument is this: Holl and Zangato conducted archaeological digs at several sites at the Adamawa plateau, discovering several Iron artifacts as well as slag and iron bloom nodules from smelting. They also reached layers of charcoal deposits closely associated with the slag and nodules. They then conducted Carbon 14 analysis on the charcoal, arriving at a date circa 3,000 BCE, and stated that the charcoal provides a date for early operation of smelting furnaces for the production of iron.
The trouble with with this assertion is, it is not in itself a firm dating of the slag or the iron. It relies on the assumption that wood was turned into charcoal near the same time that the smelting furnaces were in use.
Indeed, the claims of Holl and Zangato were met by a number of essays in response which are worth reading. Shadreck Chirikure's essay (and others) raise the question of how securely sites can be dated based on a single C14 test, and raises the possibility that the site soil was disturbed by past human activity, meaning perhaps incorrect C14 results. Chirikure argues that Holl and Zangato's dating would be more secure if they had been able to find charred animal bones (used for carbonization of iron ore in smelting) and tested those to arrive at a more secure date which would confirm or disprove the C14 date.
I will also mention that there have been many other examples of iron production sites claimed to be much older than 550 BCE. In 1983 Marie-Claude Van Grunderbeek asserted a date circa 1200 BCE for charcoal at smelting sites in Rwanda. Similarly, Peter Schmidt asserted that he had sites which had been dated before 1000 BCE in the Kagera region of Tanzania (directly adjacent to Rwanda and Burundi). However, in the face of criticism, both Van Grunderbeek and Schmidt have backed away from those claims, because they could not clearly associate the C14 test sample and the furnaces. They now date the beginning of iron working in the Great Lakes region to circa 800 BCE, based on other sites where charcoal samples are very securely associated with furnaces.^1
If true, would this definitively mean that African ironmaking was developed independently from other traditions.
I hope from above explanation it should be clear just how challenging it is to provide solid, unassailable dates for earliest production. Archaeologists interested in African metallurgy will try and suggest different scenarios in order to "stress test" claims like those of Schmidt, Van Grunderbeek, Zangato and Holl.
But, even if we put the issue of dating aside, there are other arguments both for and against independent African invention of iron metallurgy.
On the independent invention side of the argument: they point to the fact that accepted dates circa 550 BCE in Nigeria, Lake Chad Basin, and in the Great Lakes Region point to widespread production and use of iron implements at the same time or slightly earlier than iron becomes widespread in Egypt and North Africa. In a model assuming diffusion of iron working technology from Mesopotamia and Anatolia to Sub-Saharan Africa, it seems to "skip" Egypt and North Africa.
Secondly, the archaeological evidence of iron smelting in Sub-Saharan Africa does not appear to match that coming from Anatolia and Mesopotamia. African smelting furnaces leave behind a slag "plug" at the bottom of the furnace. There have not yet been any finds of similar slag plugs in Anatolian or Mesopotamian smelting sites. This implies very different smelting techniques, further suggesting independent invention.
On the other hand, detractors of the "independent invention" argument point to the technical challenge of producing sufficient heat to melt iron ore to produce smelted iron (circa 1250 C). They argue that pottery firing techniques do not come anywhere close to reaching those temperatures. In West Asia there is a long tradition of copper and bronze or brass casting which predates widespread iron smelting, and scholars argue that copper metallurgy provided an avenue for experimentation in producing hotter ovens and furnaces which allowed iron smelting to be discovered.
In contrast, Sub Saharan African metallurgy mostly sees iron smelting introduced at the same time or earlier than copper and gold working. Skeptics argue that development of iron smelting furnaces shows up as a "quantum leap", without any copper or brass "intermediate step". They then propose that the solution to this conundrum is that the idea of furnace came from outside, but that there was substantial local invention that accounts for the dilemma of the slag plugs.
In response to the copper argument, supporters point out that there was early copper production in Mauritania and Niger. Folks like Scott MacEachern have also said that the notion that Sub Saharan Africa exhibits "missing intermediate steps" between pottery firing and iron production is basically a failure of imagination. We cannot say for sure what ancient African peoples were thinking, and we can't state definitely they could not hit upon idea of furnaces through independent invention.
Paul Craddock introduces somewhat circumstantial evidence of West African systems of bronze making (e.g. the Benin Bronzes). In the early 20th century, colonial ethnographers believed bronzemaking was learned from ancient Greeks or Phonecians or perhaps the early Portuguese. But, more recent consensus is that West African copper and brass casting techniques and recipes are distinct and likely represent local invention.
Similar circumstantial evidence comes from Babatunde Babalola's digs at Ile Ife circa 2015 which show that there was a West African tradition of glass bead making which predates European contact in the 1400s.
So, my sense of the field of African Metallurgy in archaeology is that there is greater acceptance of the proposition that African iron production was developed independently. The specific claims of Augustin Holl and Etienne Zangato about earliest date of production in Adamawa have been greeted with interest and perhaps some skepticism. If new evidence came out that strongly supported the date, that would tend toward strengthening the case for independent invention. But an early date isn't absolutely essential to proving independent invention.
1 Metals in Past Societies by Shadreck Chirikure, pp 27.
