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When I visited him in Aspen in the summer on 1988 , just weeks before his fell to his death on Pyramid Peak , he seemed to be finally recovering from the tragic death of his son Mark , at age 6 , little more than a year before , and was once again sparkling with humor and joie de vivre . Susskind is the author of The Cosmic Landscape : String Theory and the Illusion of Intelligent Design . Leakey Foundation gratefully acknowledges the generous support of Wells Fargo Bank , the series ' principal sponsor , as well as the Leakey Foundation Fellows and Abercrombie & Kent . The Leakey Series on Human Origins , addressing the subject of human evolution from the perspective of news-making scientists and their field research . Mr. Papaj has worked at Treasury for more than 32 years , beginning his career in 1973 with the Bureau of Government Financial Operations , which was the prior name of FMS . When these gateways are closed , as they have been for most of geological time , it is because North America has become joined to Asia and / or Europe by land bridges that cut off migrations of marine organisms , but allow land animals to migrate between continents .