 One clan organizes the first cheela agriculture , which brings predictable food supply but provokes grumbling about the repetitive work . Shortly after , a volcano emerges in the area , and the clan invents the first sledge to carry food from more distant sources . However , within a few generations the volcano pollutes the soil . One clan leads its population on a long , arduous journey to new territory that is fertile and uninhabited . Although one genius invents mathematics to calculate and measure the band 's food supply , the situation is desperate and the clan 's survival depends on the self @-@ sacrifice of the oldest members . 
