In his vast correspondence with other contemporary naturalists, Charles Darwin mentions an “abominable mystery”. This mystery was the origin of angiosperms or plants with flowers. The fossil record showed that flowering plants appear relatively suddenly all around the world in the mid-Cretaceous, in contrast with Darwin's belief of a gradual, slow evolution. Darwin explained the apparent sudden evolution using gaps in the fossil record.
Perhaps, he suggested, the ancestor of modern flowering plants evolved in a remote place, from where the new group quickly spread. Plants with flowers are far more likely to become fertilized, thanks to the help of insects or the wind, can quickly produce seeds and colonize new terrain. So to solve this mystery it was just necessary to find the remote place where the first flowering plants evolved. Darwin proposed an interesting explanation why this place was not found during his lifetime: "I have sometimes fancied that development might have slowly gone for an immense period in some isolated continent or large island, perhaps near the South Pole.”
Darwin speculated that the first flowers evolved on a continent, from there spread over the globe, meanwhile the continent with the transitional fossils disappeared beneath the sea, far out of reach of any fossil collector or naturalist.
Since Darwin, many plant fossils have been found, but the origin of flowering plants still remains elusive. Possible sites of origin of the angiosperms were placed in the Arctic region, Southeast or East Asia, South America and Africa. Some fossil leaves of the Triassic and Jurassic resemble leaves of modern angiosperms but there is no direct evidence to link the fossils to the group. The oldest known fossils of angiosperms, showing some typical parts of a flower, like carpels and stamens (the reproductive organs of a flowering plant) but lacking others, like petals (modified, brightly colored leaves to attract pollinators), were found in China, dating to the early Cretaceous. Archaefructus, discovered in 1998, was a plant growing in wet environments or even water, as the sediments, where the fossil is preserved, and the morphology of the leaves suggest.
The connection of Archaefructus with water supports also another idea about the evolution of flowering plants. The Cretaceous radiation probably begins somewhere in the wet tropics. The new plants then spread quickly from their place of origin and in just forty million years flowering plants make up already more than seventy-five percent of all known land plants. But according to botanists Archaefructus, despite its primitive traits, cannot be considered the first flowering plant, but just a very basal form, relocating the possible origin of angiosperms outside of China.
A modern discovery may vindicate Darwin's very speculative idea about the true origin of flowering plants. The continent of Zealandia, located east of modern Australia, disappeared in the sea in the late Cretaceous. If the first flowering plants evolved on the lost continent of Zealandia, this would explain the apparent lack of fossil forms. From Zealandia, maybe with a tropical, wet climate at the time, the new group would quickly spread over Australia and Asia, united at the time in a single landmass, coinciding with the discovery of primitive flowering plants in fossil sites of China and Mongolia. There is also some evidence to support this hypothesis observing the distribution of modern species. Research suggests that Zealandia played an important role to explain the dispersal and evolution of animals, providing a dry land bridge, in the South Pacific. It's likely that also plants used this land bridge.
This is supported by the distribution of flowering plants still sharing some traits with their primitive ancestor. Many primitive flowering plants are found clustering around the former location of Zealandia, supposed origin of the primitive ancestor. The genus Amborella is found only on the island of New Caledonia, southwest Pacific Ocean. Austrobaileya is found only in the tropical forests of Queensland, Australia. Degeneria, a genus in the family Magnoliaceae, a very old group inside the flowering plants, is only found on Fidschi, a remote island north of New Zealand.
