Bantu history: Big advance, although with a chronological contradiction

In their article “Bantu expansion shows that habitat alters the route and pace of human dispersals,” Grollemund et al. (1) have accomplished more than just their stated intention: to identify the role of habitat in channeling the directions of the early Bantu farming settlement of the African equatorial rainforest. What is most important is that the authors essentially bring closure to four decades of debate, at least with respect to the geography of the early stages and routes of expansion of Bantu speakers across vast portions of the African continent. On the geography of this spread, their findings confirm the validity of a particular line of linguistic and historical argument initiated more than 40 y ago (2) and developed and elaborated upon by a succession of investigations in the intervening years (3⇓–5). Grollemund et al. (1) rule out, as did those other studies, the idea that the ancestral speakers of the Eastern Bantu branch might have reached eastern Africa by a circuitous route around the north side of …

In their article "Bantu expansion shows that habitat alters the route and pace of human dispersals," Grollemund et al. (1) have accomplished more than just their stated intention: to identify the role of habitat in channeling the directions of the early Bantu farming settlement of the African equatorial rainforest. What is most important is that the authors essentially bring closure to four decades of debate, at least with respect to the geography of the early stages and routes of expansion of Bantu speakers across vast portions of the African continent. On the geography of this spread, their findings confirm the validity of a particular line of linguistic and historical argument initiated more than 40 y ago (2) and developed and elaborated upon by a succession of investigations in the intervening years (3)(4)(5). Grollemund et al.
(1) rule out, as did those other studies, the idea that the ancestral speakers of the Eastern Bantu branch might have reached eastern Africa by a circuitous route around the north side of the equatorial rainforest.
Grollemund et al.'s (1) results broadly accord in their geographical respects with the findings of the extended syntheses of Bantu history by Kairn Klieman (6) and this writer (7,8) that, beginning in the third millennium B.C.E., Bantu-speaking communities advanced deep into the rainforest belt. Their primary spread passed initially through the Sangha River regions south-southeastward out of Cameroon to the areas of the Sangha confluence with the middle Congo River, and farther south to the region of the confluence of the Kwa River with the lower Congo. From this latter region, the next major Bantu expansion proceeded eastward through the forestsavanna mosaic on the immediate southern fringes of the equatorial rainforest 1,500 km eastward to the great Western Rift of Africa (7). From that region the early Eastern Bantu then spread out all across eastern and southeastern Africa (8).
The in parts of the Sangha corridor and in the areas around the stretches of the Congo between the Sangha and Kwa confluences. So, even without climate change, Bantu settlements could have hopscotched ahead from one patch of savanna to another.

A Contradicted Chronology
Not accounting for this factor leads Grollemund et al.
(1) to propose ascribing the "initial northsouth migration of Bantu speech communities across the Equator" to the period of the "Sangha River Interval" of around 2500 B.P. (1). Unfortunately, that idea is not chronologically supported in the archaeology.
Here's why. Yes, very little archaeology as yet exists for the Sangha River region itself (1). However, large bodies of evidence do exist for other regions of early Bantu settlement. Archaeologists have known for three decades that already by the 10th century B.C.E.-500 y before 2500 B.P.-the forefront of Bantu expansion had reached the eastern side of Lake Tanganyika (9-11), more than 1,500 km beyond the Sangha and Kwa confluences with the Congo River. Offshoots of this settlement then spread farther east, into more of the Great Lakes region of East Africa by 2500 B.P. (11), and south and southeastward as far as southern Zambia before 2300 BP (12). The prior expansion of Bantu speakers eastward from the Congo-Kwa confluence to Lake Tanganyika, 1,500+ km away, had thus to have begun not just before 2500 B.P., but probably well before 3000 B.P. The even earlier, initial period of Bantu expansion through the Sangha River corridor and into the regions along the lower Congo River could only have taken place still centuries before that. Klieman's linguistic dating of this initial expansion to the third millennium B.C.E. comports well with these chronological requirements (6).

Summing Up
To sum up, Grollemund et al. (1) perform the crucial service of resolving once and for all the geographic history of Bantu dispersal, and their proposal that savanna environments helped channel early Bantu expansion through the Sangha and lower Congo River regions seems very well taken. The "Sangha Interval," however, came into being only after Bantu peoples already had passed through those regions all of the way to the edges of eastern Africa. The existing intercalary savannas of the third and second millennia, seem, therefore, the most probable enablers of the initial Bantu expansions through the western equatorial rainforest.