The catastrophic events of the Cretaceous-Tertiary boundary (around 65 million years ago) wiped out the dinosaurs and cleared the way for the hegemony of mammals on Earth. Following this mass extinction event, an interval of almost ten million years elapsed before faunas began to resemble those of the modern world. The early and middle Paleocene faunas of the northern continents were dominated by mammalian taxa of archaic anatomy and strange ecology, which are only distant relatives of today's lineages. Very small insectivorous mammals were abundant and diverse, but were only distant cousins of today's shrews, moles and hedgehogs. Similarly, the small and medium-sized herbivorous mammals of the early and middle Paleocene would seem strange to us, because they bore no resemblance to today's horses, rhinoceroses, deer, pigs or ungulates. Yet in the early Eocene, around 55 million years ago, the mammalian faunas of the northern half of the globe underwent a radical transformation. These Eocene faunas largely comprised the first close relatives of the lineages that populate today's ecosystems. What caused this radical transformation, and why did it happen so quickly?
From the end of the Paleocene onwards, the Earth's climate gradually warmed. This global warming reached a maximum at the Paleocene-Eocene boundary, where the speed and magnitude of warming were close to those observed today. A higher eustatic sea level than today and a slightly different position of the continental plates are responsible for a different paleogeography than today. The most important differences are: (1) the presence of an epicontinental sea, called the Obik Sea or Turgai Strait, on a north-south strip in central Eurasia, effectively separating Europe from East Asia; (2) a permanently emerged Bering Strait, connecting northeast Asia and North America; (3) a continental bridge north of the Atlantic, connecting North America, Greenland, Scotland and western Europe. The clear effect of this configuration is that the dispersal of early mammals could only take place over land bridges at high latitudes. Because of the high latitude of these land bridges, climatic warming during the Recent Paleocene and Early Eocene facilitated exchanges between Asia and North America, and between North America and Europe.
