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any of several isthmuses that have connected the Earth's major landmasses at various times, with the result that many species of plants and animals have extended their ranges to new areas. A land bridge that had a profound effect on the fauna of the New World extended from Siberia to Alaska during most of the Tertiary and Quaternary periods (beginning approximately 65 million years ago), with some interruptions. Across this strip of land passed a number of organisms of Old World origin, including Homo sapiens.