Although land plants have been around for 470 million years or longer, the earliest evidence of flowering plants, in the form of the fossil Archaefructus liaoningensis, dates to just 125 million years ago, in the early Cretaceous period. This means that flowers have only existed for about a quarter of the time of land plants in general. Fossil evidence of pollen, considered to be a strong indication of flowering plants, is a bit older, dated to about 130 million years ago.
The evolution of flowering plants was a long time in coming, but today, they are the most successful group of land plants, found on every continent but Antarctica, and on remote islands. Their abrupt appearance and success was so extreme that Charles Darwin called it an "abominable mystery." Since Darwin's time, however, more fossils have been found that reveal a series of intermediate steps before full-fledged flowers.
The evolution of plants is generally one where groups that exploit fundamental evolutionary innovations — such as vascular tissue, bark, seeds, or flowers — have the tendency to almost completely replace more primitive plants when they really get going. Furthermore, these evolutionary innovations tend to emerge in the most complex plants at the time. Accordingly, flowering plants evolved from the most sophisticated seed plants, which themselves had replaced most seedless plants about 370 million years ago, during the late Devonian period.
Flowers are a very successful evolutionary innovation because they allow a more complex range of interactions with other organisms. This opens up various symbiotic partnerships, especially with pollinating insects such as bees. The constant exchange of pollen between plants, facilitated by bees, helps these plants to stay genetically diverse and resistant to disease or other hardship.
Flowering plants diversified into the two main groups, monocots and dicots, just 5 to 10 million years after they initially evolved. By the end of the Cretaceous, 65.5 million years ago, half of today's main flowering groups had evolved, and they accounted for 70% of global plant species. The success of the plants around this time had caused scientists to speculate the the dinosaurs may have gone extinct by eating flowers. This was before scientists came to agree that the dinosaurs most likely went extinct from an asteroid impact.
Michael is a longtime WiseGEEK contributor who specializes in topics relating to paleontology, physics, biology, astronomy, chemistry, and futurism. In addition to being an avid blogger, Michael is particularly passionate about stem cell research, regenerative medicine, and life extension therapies. He has also worked for the Methuselah Foundation, the Singularity Institute for Artificial Intelligence, and the Lifeboat Foundation.