Plants today are of great diversity and complexity. It was based on two decisive evolutionary leaps, a new study shows: First, the emergence of seeds in the late Devonian 375 million years ago and later the development of flowers in the middle of the Cretaceous period 125 million years ago. In the 250 million years between these development spurts, however, there was a plateau phase without major evolutionary innovations.
When it comes to reproducing, plants have developed different strategies. Some are optimally designed to allow the wind to help them reproduce, while others attract pollinators with colorful flowers. But when and how did this diversity come about? When researchers have dealt with these questions up to now, they have usually analyzed individual groups such as flowering plants, conifers and ferns separately – because the great variety made a uniform classification difficult.
Erratic development
A team led by Andrew Leslie from Stanford University in California has now used a new, simple method to make all vascular plants comparable with one another and to trace their evolution: “We have divided the reproductive structures of vascular plants into their basic components and the number of different ones Types of parts determined, ”explain the researchers. Among other things, they counted structures such as petals, stamens and spore capsules. “That tells a pretty simple story about the evolution of plant reproduction in terms of form and function: the more functions plants have and the more specific they are, the more parts they have,” says Leslie.
Using this method, the researchers classified 1,338 living and fossil plants, from ferns and conifers to flowering plants. The result: the complexity of the plants did not increase gradually, but in two significant spurts. “The first thrust occurred in the late Devonian around 375 million years ago when the vascular plants spread and eventually non-flowering seed plants appeared,” the researchers describe. “The second jump occurred in the Middle Cretaceous about 125 million years ago and was associated with the fact that the flowering plants emerged and diversified rapidly.”
Plateau phase before the decisive innovation
In the 250 million years between these evolutionary leaps, new species also emerged, but the complexity of plants did not change significantly. “This standstill is surprising,” says Leslie. “The reproductive structures look different in all of these plants, but they all have roughly the same number of parts during this plateau phase in development.” In the animal world, however, significant changes occurred during this time: land animals became larger and more diverse, the dinosaurs spread and the insect world became more diverse.
The plants, on the other hand, were able to use these developments surprisingly late. “Pollination by insects and the spreading of seeds by animals would have been possible 300 million years ago, but it was only in the last 100 million years that these really complicated interactions with pollinators have made flowering plants so extremely complex,” says Leslie. “There has been a long period of time when plants could theoretically have interacted with insects as they do with flowering plants today, but they didn’t do it to the same degree of complexity.”
Complex pollination mechanisms
The plants only reached their greatest complexity and diversity with the development of flowers. Within a short period of time, flowering plants spread all over the world and today, with around 350,000 species, make up 90 percent of all land plants. “The basis for this was a fundamental innovation, namely a carpel that encloses the seeds,” the researchers write. “This enabled the development of specific pollination mechanisms.” Among flowering plants, Leslie and his colleagues found the greatest complexity in species that are pollinated by animals. Flowering plants that rely on wind or self-pollination, on the other hand, are less complex in construction.
From the researchers’ point of view, the complexity reflects the functional diversity of the plants. Although their method of measuring complexity based only on the number of different parts neglects other aspects such as the internal anatomy of the plants, they see this as a useful tool for making very different plants comparable. “Our approach enables us to simplify the enormous range of reproductive morphology of vascular plants, to identify important temporal patterns in the development of this diversity and to review long-standing ideas about its development,” the researchers say.
Source: Andrew Leslie (Stanford University, California, USA) et al., Science, doi: 10.1126 / science.abi6984