Plants evolved in complexity in two bursts, with a hiatus of 250 million years

A Stanford study reveals that instead of gradually evolving over hundreds of millions of years, land plants underwent major diversification in two spectacular bursts, 250 million years apart. The first occurred early in the history of plants, giving rise to the development of seeds, and the second occurred during the diversification of flowering plants.

The research uses a new but simple metric to classify the complexity of plants based on the arrangement and number of basic parts in their breeding structures. While scientists have long assumed that plants became more complex with the advent of seeds and flowers, the new findings, published on September 17 in Science, provide an overview of the timing and extent of these changes.

“The most surprising thing is this kind of stasis, this plateau of complexity after the initial evolution of the seeds, and then the total change that occurred when the flowering plants started to diversify,” said the author. study principal Andrew Leslie, assistant professor of geological sciences at Stanford’s. School of Earth, Energy and Environmental Sciences (Stanford Earth). “The reproductive structures look different in all of these plants, but they all have roughly the same number of parts during this stasis.”

An unusual comparison

Flowers are more diverse than any other group of plants, producing colors, smells and shapes that nourish animals and delight the senses. They are also complex: petals, anthers and pistils intertwine in precise arrangements to attract pollinators and encourage them to spread pollen from flower to flower.

This complexity makes it difficult for scientists to compare flowering plants to plants with simpler reproductive systems, such as ferns or certain conifers. As a result, botanists have long focused on characteristics within family groups and generally study the evolution of non-flowering plants separately from their more complex flowering parents.

Leslie and her co-authors overcame these differences by designing a system that ranks the number of different types of parts in reproduction structures based solely on observation. Each species was rated based on the number of part types it has and the degree to which these parts are grouped together. They classified around 1,300 species of terrestrial plants from around 420 million years ago to the present day.

“It tells a pretty simple story about how plants have evolved in terms of form and function: the more functions plants have and the more specific they are, the more parts they have,” Leslie said. “It’s a useful way to think about large-scale changes encompassing the entire history of plants.”

From shrubs to flowers

When land plants first diversified in the early Devonian around 420 to 360 million years ago, Earth was a warmer world, devoid of trees or land vertebrate animals. Arachnids like scorpions and mites roamed the earth among short, uneven plants, and the tallest earth organism was a 20-foot fungus resembling a tree trunk. After the Devonian, enormous changes occurred in the animal kingdom: land animals evolved to have large size and more varied diets, insects diversified, dinosaurs appeared, but plants did not. not seen a major change in the complexity of reproduction until they develop flowers.

“Insect pollination and animal seed dispersal may have appeared 300 million years ago, but it is only in the last 100 million years that these really complex interactions with pollinators are at the origin of this very great complexity of flowering plants, ”said Leslie. “There has been such a long time that plants could have interacted with insects the same way flowering plants do now, but they don’t have the same level of complexity.”

At the end of the Cretaceous, around 100 to 66 million years ago, Earth looked more like the planet we know today, much like Yosemite National Park without the trees and flowering bushes. The second explosion of complexity was more dramatic than the first, highlighting the unique nature of flowering plants, according to Leslie. This period gave birth to plants like passionflower, which can have 20 different types of parts, more than double the number found in non-flowering plants.

The researchers classified 472 living species, some of which Leslie performed on and around the Stanford campus by simply separating out local plants and counting their reproductive organs. The scan includes vascular land plants, everything except mosses and a few early plants that lack supporting tissue to conduct water and minerals.

“One thing we are arguing in this article is that this classification simply reflects their functional diversity,” Leslie said. “They basically divided up their work in order to be more efficient at doing what they needed to do.”