Baja California's pioneering prehistorian, William C. Massey (1961a, 1966), propounded what has been termed the “layer cake” model for the region’s past. Elaborating on similar ideas developed earlier by Malcolm J. Rogers (1939, 1945) and Paul Kirchhoff (1942), Massey sketched a view of culture history characterized by waves of migration and cultural diffusion entering the peninsula through its northern continental gateway. Successive waves had penetrated shorter distances to the south, overlaying progressively smaller portions of the region. The image was in reality less culinary than geological, with the northern areas, as they were encountered at the time of Spanish contact, being underlain by earlier prehistoric cultural strata, while those same earlier strata still outcropped on the ethnohistoric surface farther to the south.
Arguably, Massey's model was a legitimate first approximation toward an explanation for Baja California's archaeological and ethnohistoric records. The peninsula did receive waves of cultural diffusion and perhaps migration that came south from mainland North America, and areas lying farther down the peninsula were isolated from the effects of some of those waves. However, several decades of archaeological investigations now require us to adopt a more complex picture of the region's past.
Key elements in the "layer cake" model concern the peninsula's extreme south. One of the sources for the idea that an older stratum had survived in the south was the image of that area's people as being extremely "primitive" in their lifeways at the time of colonial contact. The main source for this idea was an entertaining but highly acerbic account written by an eighteenth-century Jesuit missionary, Jakob Baegert (1952, 1982). Baegert found little to praise in either Baja California or its inhabitants. However, it's notable that Baegert's main experiences were not with the Pericú in the Cape Region of the extreme south but with the Guaycura, living a little farther north on the relatively sterile Magdalena Plains. He was also not observing Guaycura lifeways as they had existed at the time of initial European contact; he knew these people only after two centuries of intermittent contacts and after more than a decade of incorporation within the local mission system.
The cultures of the extreme south were distinctive in some details of their technology (Massey (1961a). Most notably, they had retained the use of the atlatl and dart in place of or alongside the bow and arrow down to early contact times (Massey 1961b; but see also Laylander 2007a). In general, however, it's not clear that these cultures were more "primitive" or more conservative than those elsewhere in Baja California or the Desert West.
Another apparent anomaly in the extreme south of Baja California was the extremely long (hyperdolichocephalc) skull shape of people represented in Cape Region burials. An early suggestion, which is still occasionally heard, was that a population had reached the Cape Region directly from across the Pacific (e.g., Rivet 1909). No persuasive evidence has ever been advanced in support of this idea, and it appears unlikely to be true. The "layer cake" model suggests instead that the anomaly reflected an extreme genetic conservatism among an isolated remnant of the continent's first occupants (e.g., Rogers 1939:71; González-José et al. 2003). However, an alternative explanation may be that it represented genetic drift or other mechanisms of localized divergence arising from the region's relative isolation, rather than the conservation of an early pattern.
Movements in prehistoric Baja California were not exclusively unidirectional, leading south from the northern gateway. An alternative external link has been plausibly suggested as bridging the central Gulf of California, across the chain of the Midriff Islands that lie between the coasts of Sonora and Baja California (Kowta 1984). Evidence for the prehistoric occupation of the islands is now well-established, although the timing, character, extent, and direction of trans-Gulf interactions are still not clearly known (Bowen 2009).
An element of north-to-south movement that was perhaps not encompassed within the original "layer cake" model was a terminal Pleistocene maritime or littoral migration down the Pacific shores. Early radiocarbon dates from Isla Cedros, off the central peninsula's western coast, and from Isla Espíritu Santo, in the southern Gulf, lend plausibility to the idea that the region's first settlers may have had a maritime orientation (Des Lauriers 2010; Fujita 2010). Such coastal movements were probably not exclusively north-to-south. If the initial settlers followed the coast south from Alta California, it's likely that they reversed direction upon reaching Cabo San Lucas and continued to follow the coast from south to north up the Gulf, at least as far as the Midriff Islands, before they would have recognized the possibility of crossing the Gulf eastward to Sonora (Bowen 2009:84).
A linguistic hypothesis that still enjoys support is that speakers of the Cochimí, Kiliwa, Delta-California Yuman (Tipai and Cocopa), and Paipai languages each entered the peninsula in a separate migration through the northern gateway (Mixco 2006). However, a contrary scenario, which is at least equally plausible according to the available evidence, is that the original homeland of the Cochimí and Yuman linguistic families lay within the northern part of the peninsula, and that south-to-north movements carried Yuman groups northward, out of the peninsula, during the last 2,000 years of prehistory (Laylander 2010). The Pai branch of Yuman, which is represented by two languages, the widely separated Paipai in northern Baja California and Upland Yumans (Yavapai, Walapai, and Havasupai) in western Arizona, is particularly intriguing. The Pai distribution evidently attests to a very late migration, either southward into the peninsula, or northward out of it, or in both directions from an intermediate location in the Colorado River delta or on the shores of now-extinct Lake Cahuilla (Laylander 2007b).
Goods moved in multiple directions both within and beyond Baja California. The most readily traced material is obsidian, the volcanic glass so highly valued for flaking into refined tools such as projectile points. At least a dozen chemically distinct geological obsidian sources have been identified along the eastern margins of the peninsula. Artifacts derived from these sources are found in archaeological sites lying to the north of their source locations, extending into southern Alta California, at least as often as they're found at sites to the south of the sources (Laylander 2012; Panich and Porcayo 2012). Pottery has generally been seen as representing one of the final prehistoric waves of cultural diffusion from the north, reaching only a short distance down the peninsula before Spanish contact. However, an alternative hypothesis has been proposed, according to which the manufacturing of the brownware pottery characteristic of the mountains and west coast was initiated in northern Baja California and then spread north into southern Alta California (Griset 1996:273).
In Massey's day, migration and diffusion loomed large as preferred explanations of cultural patterning. In subsequent decades, archaeology in general, and studies of Baja California's prehistory in particular, have tended to give more credence to local innovation as an important mechanism of cultural change. The peninsula's best known prehistoric remains, the life-size or large-than-life Great Mural paintings of humans, deer, and other animals found in the rockshelters of the central sierras, can't be explained as something copied from elsewhere; they're evidently an original, local creation (e.g., Gutiérrez and Hyland 2002). The elaborate and diverse burial practices of the Cape Region are evidently not explicable as holdovers from an initial colonization, but instead represent a set of practices that evolved locally through time (see, for example, Rosales-López et al. 2007). The social complexity of the archaeologically documented maritime adaptations on Isla Cedros and Isla Espíritu Santo, for instance, belie the image of cultural impoverishment, simplicity, or conservatism in these remote areas (Des Lauriers 2010; Fujita 2010).
There's a lesson to be drawn from the layer cake model. Useful as such a model may be as a first approximation, the real stories of the past are almost certainly going to be much more complex, more multidimensional, more regionally variegated, and more interesting.
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