Seagoing at 1 mya to Flores, and sea gaps of >50km crossed by 47 kya to Sahul, are evidence of earlier maritime migration in the western Pacific than anywhere else. Current opinion attributes the latter to the influence of anatomically modern human cultural complexity on seagoing technology and practice, together with the impetus of serial resource depression. It is argued here that seagoing was unusually advantaged in the western Pacific by a fortuitous conjunction of the warmest seas with a ready availability of large-diameter bamboo that occurred as natural rafts, and which could also be constructed into rafts large enough to transport viable colonizing groups from island to island across Wallacea to Sahul. The geography of Wallacea allowed migration solely by drifting, and exploratory landscape learning might have produced landfalls on Sahul sooner than is implied by subsistence forcing of mobility. Seagoing by drifting raft was much harder from Sahul to the east because of the virtual absence of large-diameter bamboo and longer distance to fewer or small islands; colonization occurred much later.