TOEFL Academic Passage Practice 028The Mystery of Flowering Plant Evolution

Many aspects of the history of flowering plants (angiosperms) remain mysterious. Evidence of the earliest angiosperms comes from fossilized leaves, stems, fruits, pollen, and, very rarely, flowers. Scientists have also studied the structure and genetics of modern plants to determine which living species might be most closely related to ancient angiosperm ancestors. Despite more than 200 years of research, scientists have not reached consensus on which type of plant was the ancestor of the angiosperms or when and where they first evolved. Charles Darwin called the origin of flowering plants an “abominable mystery.”

What type of plant was the ancestor to the angiosperms? Most botanists now agree that flowering plants are monophyletic in origin, meaning that they evolved from a common ancestor. Some paleontologists have suggested that this ancestor may have been a type of cycad, a palmlike tropical plant. Others maintain that angiosperms may have evolved from seed-bearing ferns. Analysis of morphological traits in some primitive living plants suggests instead that the ancestor may have been related to modern pines. The question of angiosperm ancestry remains unresolved.

The time and place of the first appearance of flowering plants have long been topics of interest. Fossils show that early angiosperms, including forms resembling modern magnolias, were present in the Early Cretaceous, more than 100 million years ago. Angiosperms became increasingly abundant during this period. Between 100 million and 65 million years ago, the Late Cretaceous, they increased from less than 1 percent of plant life to well over 50 percent. Many modern plant families appeared during this time. In the following Early Tertiary period, angiosperms increased to 90 percent or more of Earth’s total flora. Where did these successful plants first originate and spread from?

Analysis of fossil leaf structure and the geographic distribution of the earliest Cretaceous angiosperms has led many biogeographers to conclude that they evolved in the tropics and then migrated poleward. Angiosperms did not become dominant at high latitudes until the Late Cretaceous. Paleontologists have recovered fossil angiosperm leaves, stems, and pollen from Early Cretaceous deposits in eastern South America and western Africa. These continents were joined as part of Gondwanaland, one of two supercontinents that existed at that time. During the Early Cretaceous, these sites would have been close to the equator, consistent with a model in which angiosperms spread from the tropics toward the poles.

Not all botanists agree with an African-South American center for angiosperm evolution and dispersal. They point out that many of the most primitive living flowering plants are found in the South Pacific, including portions of Fiji, New Caledonia, New Guinea, eastern Australia, and the Malay Archipelago. Recent genetic research has identified the rare tropical shrub Amborella as the living plant most closely related to the ancient ancestor of all angiosperms. This shrub, which has tiny yellow-white flowers and red fruit, is found only on New Caledonia. Many botanists conclude that the large numbers of primitive living angiosperms in the South Pacific indicate that flowering plants first evolved there and that these modern species are relics of that early evolution. Comparisons of Amborella DNA with that of hundreds of flowering-plant species suggest that the first angiosperm arose and separate species developed about 135 million years ago.

Recently discovered fossils complicate understanding of angiosperm origins. Paleontologists from China found beautifully preserved angiosperm fossils, including flowers and seeds, in Jurassic deposits near modern Beijing. The site is about 130 million years old, and the fossil plant is one of the oldest known angiosperms. The fossils’ age and the primitive features of their flowers have led the discoverers to suggest that the earliest flowering plants may have evolved in northern Asia.