Back in the days of the dinosaurs, there were lots of ferns and other plants. (Yum, yum.)
But in the Cretaceous a new line, the angiosperms—flowers—emerged and flourished, and soon dominated the Earth. (A whole lot of new yummy!)
Just how did these new life forms emerge and achieve success so rapidly and completely? What features or circumstances enabled these plants to out compete other plants?
This month Kevin A. Simonin and Adam B. Roddy report a new theory of what is special about angiosperms .
The underlying concepts depend on deep biochemical details of photosynthesis, which depends on the availability of water inside the leaves. Leaves are exposed to air from which CO2 is absorbed, but maintaining phtosynthesis requires the leaves not dry out. So, “increasing leaf surface conductance to CO2 also requires increasing rates of leaf water transport in order to avoid desiccation” (. p. 2).
Water transport is, in turn, limited by the size of cells in the plant. “eaves with many small stomata and a high density of veins can maintain higher rates of gas exchange than leaves with fewer, larger stomata and larger, less numerous veins” (, p. 2)
The third piece of the argument is that of the many factors that influence the size of cells in a plant, the minimum size of a cell is constrained by the size of its nuclear material, which is primarily its genome. Plants vary greatly in the number of genes, and larger genomes generally have larger cells.
The idea, then, is that plants with smaller genomes can develop smaller cells, with higher density. This leads to higher water transport and higher photosynthesis.
S&R support this hypothesis with a survey of 400 (contemporary) plant species. The data show strong correlation between genome size and cell size and density, and as a consequence, with gas transport. These relationships hold across all plants.
The evolutionary story, then, is about the ‘strategic’ downsizing of genomes. Over time, plants evolve larger and smaller genomes through various mechanisms. S&R argue that in the Cretaceous, some species developed smaller genomes, and “genome downsizing expands the range of final cell size that is possible” (, p. 8). This plasticity increases the potential breath of habitats, as well as higher maximum productivity.
The bottom line is that Cretaceous angiosperms, and only the angiosperms, developed smaller genomes, which “allowed them to outcompete other plants in almost every terrestrial ecosystem” (, p. 9), even in the face of world wide declines in atmospheric CO2 levels. The rest, as they say, is evolutionary history.
Flowers are one of the signatures of planet Earth (we could as well call it “Planet Flower”), and we humans deeply love and connect with them. They are also entwined with the development of animal life. In the millennia after they emerged, they coevolved with pollinators (bees!) and herbivores (dinosaurs!), and spread to fill the land with color and scent and munchy goodness.
But this study suggests that the original success over other plants is due to very fundamental biochemical and mechanical processes. That’s pretty cool.
Our current Anthropocene Age is pressing hard on these glorious life forms. Loss of habitat and encroachment of human activities are threatening many species and whole ecosystems. Humans coevolved with plants, and it is far from clear how humans will fare in future with a dramatically changed plantscape.
- Helen Briggs, How flowering plants conquered the world, in BBC News – Science & Environment. 2018. http://www.bbc.com/news/science-environment-42656306
- Kevin A. Simonin and Adam B. Roddy, Genome downsizing, physiological novelty, and the global dominance of flowering plants. PLOS Biology, 16 (1):e2003706, 2018. https://doi.org/10.1371/journal.pbio.2003706
Genome downsizing physiological novelty and the global dominance of flowering plants
PS. Some good names for bands:
Rapid genome downsizing
Diffusivity of Water in Air