One hundred thousand years ago mankind entered a new stage of development . At this time the climate of the Northern Hemisphere changed drastically . Glaciers from the north began moving southward . The greater part of Europe was covered by ice . In the warmer countries the rains began . The present - day Sahara Desert was once an area covered with lakes , rivers and grassy plains interspersed with dense trop ical forests . All heat - loving plants and animals either died out...