Pressures Mount

Tectonic pressures mount and mountain ranges form — the Cordilleras, the Andes, the great Himalayan range. As inland seas shrink, the climate wavers through extremes of hot and cold. Ocean currents change and nutrients well up from the deep, supporting enormous growth of phytoplankton, allowing species higher in the food web to prosper as well. Seals and sea lions flourish. Species of diving birds diversify. With falling sea levels, land bridges connect Siberia to North America and England to the continent. Grand parades of intercontinental migrations take place.

Most mammals appear essentially modern in form. Fossils from the state of Nebraska include camels, deer-like animals, bear dogs, foxes, peccaries, small beavers, ground squirrels and horses. Horse evolution occurs primarily in North America. Early elephant-like creatures spread from Africa to the Eurasian continent.

Bottom: The ancestry of modern great apes is not well known. Proconsul, an early member of the hominid lineage, may have been too primitive in a number of respects to represent a link in the evolutionary chain to modern forms. (Illustration by Zdenek Burian, © Jiri Hochman and Martin Hochman)