The debut of flowers on the Earth’s stage transformed terrestrial life to the point that 90 percent of land plants reproduce through them. Using a vast dataset of flowers and estimations of their family trees, a team of researchers from five continents has created a three-dimensional reconstruction of what the first flowers probably looked like, along with some early steps in diversification.
According to a paper in Nature Communications, the ancestral flower was “bisexual and radially symmetric, with more than two whorls of three separate perianth organs each.” Bisexual to botanists means they have both male and female sexual organs in one flower. Perianths are the non-reproductive part of the flower, including the petals and layers that protect the flower when it is budding.
To reach this conclusion, the authors took a sample of 792 species, spanning 63 orders and 372 families of flowering plants. Relationships between these families were established through molecular clocks calibrated with fossil records. From these, they made probability estimates of the points in time where 27 floral traits emerged. A number of reconstruction methods were used, which gave consistent results for some traits and divergent ones for others, giving the authors a high level of confidence in about half of their conclusions, while others remain merely the most likely possibility.
Among the long list of features the authors describe are a spiral arrangement for the female organs and the theory that pollen was shed towards the center of the flower. “In spite of similarities with some extant flowers, there is no living species that shares this exact combination of characters,” they write.
Creating this picture is a remarkable achievement considering how little we know about floral origins. Estimates of their first appearance range from 140 to 250 million years ago, an enormous span of uncertainty, and we have no fossil flowers older than 130 million years.
The ancestral flower referred to here is the last common ancestor of all flowers surviving today. It is possible, the authors note, that previous flowers existed, but that these formed the stem of a family tree with no earlier branches.
Previous attempts have been made to reconstruct the ancestral flower through various routes. The new findings differ most sharply from these attempts in the idea that it had a whorled, rather than spiral, arrangment of petals. There has also been extensive debate as to whether flowers were originally unisexual or bisexual, something the paper’s authors are fairly confident they have settled.
The paper also proposes that early flower diversification involved the appearance of many varieties with fewer whorls, which the authors think increased pollination efficiency. They are puzzled, however, as to why flowers began with so many perianths in the first place.