Rather than evolving over hundreds of millions of years, land plants underwent considerable diversity in two 250 million-year bursts, according to a Stanford-led study. The first occurred early in plant history, leading to the creation of seeds, and the second occurred during flowering plant diversity.
Plant complexity is classified using an unique yet simple metric based on the arrangement and number of basic elements in their reproductive mechanisms. Scientists have long assumed that plants got more complex with the introduction of seeds and flowers, but the latest findings, published in Science on Sept. 17, shed light on the timing and scale of those changes.
“The most surprising thing is this kind of stasis, this plateau in complexity after the initial evolution of seeds and then the total change that happened when flowering plants began diversifying,” said Andrew Leslie, lead study author and assistant professor of geological sciences at Stanford’s School of Earth, Energy & Environmental Sciences (Stanford Earth). “All of these plants have distinct reproductive structures, but they all have roughly the same amount of parts during that stasis.”
An interesting analogy
Flowers are unlike any other plant group in that they produce a wide range of colors, scents, and shapes that nourish animals and thrill the senses. They’re also complex: petals, anthers, and pistils are intertwined in exact patterns to entice pollinators and deceive them into distributing pollen from one flower to the next.
Scientists find it difficult to compare flowering plants to plants with simpler reproductive systems, such as ferns and some conifers, because of their intricacy. As a result, botanists have long focused on traits within family groups, and non-flowering plants are often studied apart from their more complex flowering relatives.
Leslie and his co-authors overcome these disparities by devising a system that categorizes the number of different types of pieces in reproductive systems solely based on observation. Each species was given a score based on the number of different pieces it possesses and the degree to which those parts clustered. From roughly 420 million years ago to the present, they classified about 1,300 land plant species.
“In terms of form and function, this offers a relatively clear tale about plant reproductive evolution: the more functions and specificity the plants have, the more components they have,” Leslie explained. “It’s a useful method of thinking about large-scale changes that span the entire history of plants.”
From shrubs to blossoms, there’s something for everyone
Earth was a warmer environment with no trees or terrestrial vertebrate animals when land plants first evolved in the early Devonian, about 420 million to 360 million years ago. The tallest terrestrial organism was a 20-foot fungus that looked like a tree trunk, while arachnids like scorpions and mites roamed the land among small, patchy vegetation. Huge changes occurred in the animal kingdom after the Devonian: land animals gained larger bodies and more varied diets, insects diversified, and dinosaurs arrived — yet plants didn’t witness a significant rise in reproductive complexity until they produced flowers.
“Insect pollination and animal seed dispersion may have appeared as early as 300 million years ago, but these really complicated interactions with pollinators are driving this extraordinarily high complexity in blooming plants only in the last 100 million years,” Leslie said. “Plants might have engaged with insects in the same manner that blooming plants do today for such a long time, but they didn’t to the same degree of intricacy.”
Around 100 to 66 million years ago, during the Late Cretaceous period, Earth looked more like the globe we know today, similar to Yosemite National Park but without the flowering trees and plants. According to Leslie, the second burst of complexity was more striking than the first, underscoring the unique nature of flowering plants. Plants like the passionflower, which can have 20 different types of components, more than twice as many as non-flowering plants, arose during this time period.
Leslie conducted part of the research on and around Stanford’s campus by simply taking apart local plants and counting their reproductive organs, which resulted in the classification of 472 living species. Except for mosses and a few early plants that lack supporting tissue for carrying water and minerals, vascular land plants are included in the study.
Leslie explained, “One thing we suggest in this research is that this classification merely reflects their functional variety.” “Basically, they split up their labor to be more efficient at what they needed to do.”