Darwin's 'Abominable Mystery'
Plants first appeared on land approximately 425 million years ago (MYA) in the form of moss. Over the next several hundred million years, plants adapted new forms, with ferns innovating the leaf (but still spreading via spores), gymnosperms such as conifers (360 MYA), cycads (250 MYA) and ginkgoes (200 MYA) developing the seed, and finally, angiosperms giving us the flower. Our oldest angiosperm flower fossil dates to 125 MYA, and the fossil record shows an explosive growth of this evolutionary adaptation, with flowering plants becoming the dominant form of plant life on the planet in just 40 million years. You can see in this Tree of Plant Life from The US Botanic Garden just how much more diverse the flowering plants are than their plant cousins. Depending on how you count, there are anywhere from 330,000 to 400,000 species of flowering plants, forming the vast majority of plants today, and ranging from oaks, to wildflowers to water lilies.
While 40 million years may sound like a long time to us, this has actually proved to be one of the enduring mysteries of evolutionary biology. Darwin himself recognized this in a letter to his closest friend, botanist and explorer Dr Joseph Hooker in 1879:
"The rapid development as far as we can judge of all the higher plants within recent geological times is an abominable mystery."
Darwin was so perplexed by this mystery, he even speculated that flowering plants may have evolved over a longer period on some lost continent near the South Pole.
Darwin's label stuck and scientists have been trying to solve this 'abominable mystery' ever since. As is the case with peering into deep time, we are trying to infer a gargantuan jigsaw puzzle with only a handful of pieces, and so we will likely never know for sure, but here are some of science's best guesses.
First and foremost, the fossil record itself. Flowers are notoriously soft and delicate, and thus don't tend to fossilize very well - though they're stunning when they do, such as these 50 million year old sunflowers. It may just be that flowers were around much earlier, but that remnants are either nonexistent or so rare that we simply haven't found them. There is some molecular evidence that puts the advent of the flowering plants back to about 290 MYA, which would seem to back this theory.
There are also older fossils of flowers, including this Nanjinganthus dendrostyl from 174 MYA, but it is unclear whether these are related to today's flowering plants, or are from an evolutionary dead end that happened to independently evolve flower-like structures.
This may solve the mystery of how flowers appeared to take over so quickly (if they're starting date is in fact much earlier than the fossil record shows). However, ferns, conifers and the other non-flowering plants still had a head start, yet are dwarfed by the diversity of flowers. Why is that?
One factor is the role that animals play in flower pollination. As Dr Catherine Kleier, professor of biology at Regis University puts it:
"An animal moving pollen between plants could become more specialized and get pollen from only one type of plant. Now, the plants pollinated by that animal are reproductively isolated from other plants. This is the key to speciation, or the formation of new species. The other way animals can be key in speciation is through dispersal. If an animal disperses a seed to a new location where it can no longer breed with its old population, it becomes reproductively isolated and speciation occurs."
As for what the first flower may have looked like, botanists have used a variety of molecular techniques to identify the shrub Amborella trichopoda from the South Pacific island of New Caledonia as the common ancestor to all flowering plants (on the tree of life above, you'll see Amborella as the lowest branch on the flowering plant side). From this, botanists infer that the earliest flowers were also woody shrubs.