Because at this time all of the earth’s land was grouped together in one continent, Pangaea, ginkgo trees spread rapidly over all the land. As Pangaea drifted apart into the continents we have now, all of the continents had ginkgo trees on them.
Ginkgo trees live for a long time. Some ginko trees are known to be more than 2500 years old, and there’s one that people think may be 3000 years old. Gingko trees can only grow in light soil near rivers, and they don’t like to be crowded together.
By the end of the Cretaceous period, about 95 million years ago, there were not very many ginkgo trees anymore. By the end of the Tertiary period, about two million years ago, ginkgo trees had died out everywhere but one small part of China. All of the world’s gingko trees today come from that small group in China. Today, people have taken gingko trees all over the world to grow them, because gingko trees grow better than other trees in polluted cities.