Ever since Darwin’’s theory of evolution, biologists have assumed that environments teeming with complex forms of life served as the nurseries of evolution. But two recent papers in Science magazine have turned that notion on its head. Last month some biologists reported that in the ocean it is the relatively barren areas that serve as "evolutionary crucibles(熔炉) ," not regions with great diversity of species. Other researchers announced this summer that the Arctic, not the rain forest, spawned many plants and animals that later migrated to North America. Says John Sepkoski of the University of Chicago, "Harsh environments may be producing the major changes in the history of life." These "changes" do not result merely in a longer tail or a bigger claw for an existing species but, rather, in dramatic leaps up the evolutionary ladder----a rare innovation that comes along once in a million years. In the Arctic, reports Leo Hickey of Yale University, the innovations ran to forms never before seen on earth. By dating fossils from many geologic layers, he concluded that large grazing animals first appeared in the Arctic and migrated to temperate places a couple of million years or so later. Among plants, species of redwood and birch originated in polar regions some 18 millions years before they showed up in the south. Examining fossils as old as 570 million years, Chicago’’s Sepkoski found that shell-less, soft-bodied creatures were suddenly replaced by trilobites (三叶虫), then by the more advanced clam-like animals. These changes, he notes, "first become common near shore." That surprised him--an environment with as few species as exist in the near shore, and with such a poor record of producing new species, seems an unlikely place for biological innovation. But when Jablonski dated fossils of 100 million years ago, he found that during this era, too, the near shore spawned biological breakthroughs--more sophisticated sea creatures that move and find food in ocean sediments instead of passively filtering whatever floats by. The findings are too new to apply to human evolution, but at first glance they seem to fit the facts. Anthropologists believe that our ancestors became fully human only after they left their secure life in the trees for the harsh world of savanna( plain without trees). There, the demanding conditions triggered that most human of traits, the large brain, and the most profound evolutionary step of all was taken. Two recent papers in Science magazine claim to have found evidence which contradicts the traditional notion that _________.