A new study by Dorian Q. Fuller, Nicole Boivin, Tom Hoogervorst & Robin Allaby entitled "Across the Indian Ocean: the prehistoric movement of plants and animals," examines these questions (hat tip to the Ancient Indian Ocean Corridors blog). Fuller has been the leading researcher looking at the origin of early South Asian crops outside the Indus River Valley for years now (see, e.g., his very comprehensive open access 2007 paper in the Annal of Botany,) and the latest study sums up and expands upon his work.
As the paper explains (citations omitted):
It has been known for many years that some of the major crops of the drier regions of India, such as sorghum (Sorghum bicolor), pearl millet (Pennisetum glaucum) and finger millet (Eleusine coracana), originated in Africa and arrived in India at some point in prehistory. A popular argument has been that these crops arrived in the Harappan urban period (2600–2000 BC), brought by Harappan ‘seafarers’ but there is little firm evidence to support this. Recent re-assessments, of both botanical identifications and archaeological context, leave reason to doubt the few grains reported from the Harappan urban period; in contrast, there is now a large accumulation of evidence for these crops in India from the second millennium BC, including finds from 33 sites. What the dating evidence currently suggests is that this transfer of African crops took place at the end of the Harappan era, perhaps as the urban Harappan civilisation was undergoing its transformative de-urbanisation process. Given the lack of any other material evidence for Harappan or South Asian contacts with the Red Sea or Africa before 2000 BC, we have argued that this transfer took place primarily between north-east Africa and/or Yemen and western India, probably outside of the context of the Bronze Age trade between major civilisations. It is, of course, well documented that the Harappan civilisation was involved in maritime trade with Oman, Bahrain and Mesopotamia in the second half of the third millennium BC. But this trade was between urban actors, and increasingly appears to have been built on earlier regional contacts between small-scale coastal fishing and agropastoral societies.UPDATE:
Moving in the other direction was the Asian broomcorn millet (Panicum miliaceum) ultimately of Chinese origin, which had left China on westward trade routes by 2200 BC. Broomcorn millet is known from other central Asian sites from around 2000 BC and is found in Pakistan at 1900 BC, Yemen at around 2000 BC, and in Sudanese Nubia by 1700 BC, while being absent from intervening regions such as Egypt and Mesopotamia. Zebu cattle may also have moved from India to Yemen and East Africa starting at this time, although this was presumably the first stage in an ongoing process of gene flow through introduced bulls which made the genetic landscape of south Arabian and African cattle one of hybridity between African taurine and Indian zebu stocks, with evidence for interbreeding most marked at the margins of the Indian Ocean. These zebu-hybrid cattle played an important role in the longterm success and southernmost spread of cattle pastoralists in eastern Africa.
Alas, the paper is really more of a review article (and a thin one at that) than it is a presentation of a new discovery, and aside from the material quoted above, adds only an update of an illustration from a 2009 paper by some of the same authors. The pivotal point that the paper illustrates on the South Asian side, is that oldest sites with African crops appear towards the Northwest, just to the East of the Harappan area, and then disperse over a few centuries to the Southeast. This is at odds with some earlier suggestions that the agricultural trade with Africa may have gone directly to Eastern India.
The research makes a good case for the existence of a previously unattested trade route, distinct from the Harappan-Sumerian trade, between Northeast African and/or Yemen on one hand, and India on the other, that bypassed Egypt, the Levant and Mesopotamia. It also makes a plausible case that this trade was bidirectional, with agricultural goods moving either way.
I've noted before that the one genetic marker found in India, particularly in areas where African crops and other possible cultural hallmarks of African Sahel culture appear (while the mtDNA profile and Y-DNA profile of Harappan areas indicate population genetic independence from these areas), is Y-DNA haplogroup T. This is a common, indeed predominant, Y-DNA haplogroup in the Horn of Africa, which would be a plausible launching point for maritime expeditions to India that bypassed major centers of civilization at the time like Egypt, the Levant, and Mesopotamia. But, since Y-DNA haplogroup T is also found up and down the Nile basin and at lower frequencies into the Fertile Crescent and Europe (as well as all over the Jewish diaspora), absent more specific analysis of South Asian Y-DNA haplogroup T sub-lineages of the kind that have been performed for African, Jewish and European sub-lineages, it is hard to place the source of Y-DNA haplogroup T in South Asia with any more specificity. This study strongly suggests that the evidence, when it is available, will point specifically to a Horn of Africa origin for South Asian Y-DNA haplogroup T. This genetic evidence argues, although not conclusively, for the possibility of an African origin for the Dravidian language family since its distribution coincides nicely with the geographic region from which Dravidian probably expanded.
This paper still leaves many important questions unanswered, however.
Is there archaeological evidence aside from the presence of exchanged crops, for this kind of major Indian Ocean maritime trade distinct from the Harappan-Sumerian trade, which was decidedly coastal?
An alternate reason that we might not see African or Asian crops in the intervening areas is that those areas weren't places where these crops were good alternatives to the Fertile Crescent package, and therefore weren't adopted. An Persian Gulf to Mesopotamian to Levantine to Nile basin route for these domesticates, while longer, is a better fit to trade routes we know existed at the time. The Yemen link undermines this conclusion somewhat, but not really conclusively.
Perhaps even more fundamentally, by what means did West African crops and cultural practices make their way to the Horn of Africa (basically modern day Somolia) from whence they may have sailed to India?
The transfer of African crops to India clearly predates Bantu expansion and Austronesian contact with Africa. Farming almost certain arrived later in the Horn of Africa than it did in West Africa or Egypt, and some accounts don't confirm its presence in East Africa outside the Ethiopian Highlands (whose Neolithic revolution's specifics and timing is not something that I've seen well documented) until the Bantus arrive. You can transplant crops between illiterate Bronze Age populations without bringing some farmers with you to train the recipients. But, the Horn of Africa population is not known to have had people with that skill set at that time. At least as of 1996, there was no evidence for the cultivation of these crops in this area until early antiquity. Wigboldus,J.S., "Early presence of African millets near the Indian Ocean." (In J. Reade, The Indian Ocean (pp.75-86)) (1996).
Y-DNA haplogroup T, that is predominant in the Horn of Africa, phylogenetically looks like a back migration to Africa from at least as far east as the Fertile Crescent, rather than an ancient and indigenous to East Africa Y-DNA haplogroup. It could conceivably arrived contemporaneously with mtDNA M1 and U6, the most famous of the mtDNA back migrations to Africa. It could have made its way either up the Blue Nile, or via a maritime route. Since the Horn of Africa was so far off the radar screen of written history for such a long time, and the Holocene era archaeological record from Somolia isn't widely known at this time, we have little to work upon to tell us what was going on that could have led to the current Y-DNA makeup there which is so distinct from the Y-DNA of the rest of Africa outside the Nile basin (and some Northern Cameronian Fulani). It is also rather likely that the tropical climate will have prevented ancient DNA from being preserved there.
In short, we are left hungry for evidence that could show us who the peoples were that made the trans-Indian Ocean transit of African Sahel crops possible were. Was there a village scale or larger Neolithic civilization that both farmed and traded by sea over long distances of which history is entirely ignorant? What languages did they speak? What distinguished their culture? How and when and where did it come into being? Did they replace prior peoples in their territory, and if so, what became of them? What intermediate steps did they master before learning to sail across the Ocean on commercial trips as long as theirs, or were the trips motivated by something other than commerce? Perhaps, someday, we will know more.