We all love flowers but for all the attention we pay them, their origin remains elusive. Darwin called their sudden appearance in the fossil record an “abominable mystery.” Since Darwin's time, we have been able to clarify that picture a little bit. Even so, our understanding of the origin of the angiosperm lineage is dubious at best. When and why did flowers evolve?
For millions of years the land was dominated first by ferns and their allies and then by gymnosperms like cycads and gingkos. It was not until the Cretaceous that angiosperms began to rise to their current place as the dominant and most diverse group of plants. Their sudden appearance on the scene has been largely shrouded in mystery. There is scant fossil evidence to illustrate the early evolutionary steps in this development of flowers. Many paleobotanists believed that flowers had their origin in shrub-like ancestors of gymnosperms. Others felt that the origin of flowers belonged with the seed ferns (http://bit.ly/1zKfriM).
Around 2001 a fossil discovery from Yixian Formation, Liaoning, China was believed to have changed all of that. A researcher by the name of Ge Sun had stumbled upon a very primitive looking fossil plant. To his surprise, the reproductive structures seemed to show stamens in pairs below carpels and a lack of petals and sepals. The formation in which the fossil was found dated back to the Jurassic period. Could this represent the remains of the earliest flowers?
The fossil has been coined Archaefructus and since its discovery at least two species have been identified. Archaefructus was an aquatic plant, likely living on the edge of freshwater lakes. These fossils (as one would expect) are quite contentious. Some argue that it is more derived than would be expected from the first flower. Recently it has been suggested that Archaefructus is a sister lineage to early flowering plants, not unlike Nymphaeales or Amborella living today.
What Archaefructus does suggest is that flowers had their origin much earlier than the Cretaceous. Other discoveries from the same formation (ie. Archaeamphora longicervia) suggest that flowering plants were already diversifying at this time. So, if this is the case, when did flowers appear on the scene? Far from the smoking gun that a fossilized flower would represent, researchers are nonetheless finding tantalizing fossil evidence that places the origin of flowering plants all the way back to the Triassic.
By examining Triassic microfossils, some researchers believe they have found fossilized pollen grains that are distinctly angiosperm in origin. I won't go into it here but extant examples show a major distinction between pollen from gymnosperms and pollen from angiosperms. If this is true, flowers may be way older than ever expected. For now, the jury is still out on this one.
Flowers evolved for sex. We associate animals like bees, bats, and birds with flowers today but most of these lineages came much later in the game. Exactly what was around pollinating early flowers remains a bit of a mystery as well. Were the earliest flowers wind pollinated or was there some insect or even reptile that served the selection pressure necessary for their evolution? Only time and more fossil discoveries will tell.
Photo Credit: Shizhao (Wikimedia Commons)