German lakes, to 10,090 BP calibrated. The tephra occurs in early Boreal contexts. So, it seems certain that the early Boreal (pollen zone V) includes the year 10,000 BP. Similarly, the late Boreal includes the Kilian/Vasset tephra of Swiss and southwest German lakes at 8200 BP, all calibrated. But the borders are less certain. Studies of bogs in northwest Russia are the basis for a division of the PreBoreal (PB) into PB-1, 10,000–9800, and PB-2, 9800–9300 BP incal. The scheme goes on to divide the Boreal (BO) into BO-1, 9300–9000, BO-2, 9000–8500, and BO-3, 8500–8000, incal. CalPal used on these dates suggests overall boundaries of 11,500 and 10,500 BP for the Pre-Boreal, and the end of the Boreal at 8900. Dates given recently are usually earlier than those given more than 10 years ago. For example, Iverson (1973) and Rud (1979) give dates of 10,000–9000 BP for the PreBoreal and 9000–8000 BP for the Boreal, which are uncalibrated C-14 dates based on Scandinavian pollen stratigraphy. Presumably, more-recent dates are more accurate, as technology improves with time, often quite rapidly. Yet, pollen and climate phases also to some degree may depend on latitude, so no date can be regarded as certainly wrong. Scientists look for the overall pattern of the dates, but that technique is not 100% reliable, either. Description Before the Pre-Boreal, Eurasia was locked in the chill of the Younger Dryas and was a mostly continuous tundra belt, with regions of taiga, covered with a blanket of grasses, shrubs and other low plants typical of open land. Large numbers of herbivores wandered in herds over vast distances. The blanket teemed with small, rapidly reproducing species, which supported food chains of larger predators. The largest predators and humans hunted the mammals of the open tundra. The Pre-Boreal began with a sudden rise in temperature that abruptly changed this ecosystem. Forest replaced the open lands in Europe, and forest-dwelling animals spread from southern refugia and replaced the ice-age tundra mammals; new climax ecosystems developed. The old fauna persisted in Central Asia, but were soon hunted out, as they were not replenished by the larger areas formerly nourishing the ecosystem. The sea brought isolation by rising rapidly and many coastal areas becoming flooded and new islands formed. Forest had closed over the former European tundra. Humans had to adapt to the encroaching forest or move east with the large mammals. Those who