A new study led by Stanford University has found that rather than evolving gradually, land plants evolved in two dramatic bursts which occurred over 250 million years apart. The first burst occurred early in plant evolution (420 million to 360 million years ago), giving rise to the development of seeds. The second burst was characterized by the emergence of flowers, and started only about 100 million years ago.
“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 started diversifying,” explained lead study author Andrew Leslie, an assistant professor of Geological Sciences in the School of Earth, Energy & Environmental Sciences at Stanford. “The reproductive structures look different in all these plants, but they all have about the same number of parts during that stasis.”
Based solely on observation, Professor Leslie and his co-authors designed a system which classifies the number of different kinds of parts in plants’ reproductive structures. They scored each species according to how many types of parts it had and the degree to which they exhibited clustering of those parts. With this method, they managed to categorize about 1,300 land plant species from 420 million years ago until the present.
“This tells a pretty simple story about plant reproductive evolution in terms of form and function: The more functions the plants have and the more specific they are, the more parts they have,” said Professor Leslie. “It’s a useful way of thinking about broad-scale changes encompassing the whole of plant history.”
Through this method, the team discovered an initial burst in complexity, with the evolution of seeds in the early Devonian period, when the Earth was warmer and devoid of trees and terrestrial vertebrate animals. Afterwards, although massive changes occurred in the animal kingdom with the emergence and diversification of land animals, plants did not see any major evolutionary burst until the development of flowers in the late Cretaceous period.
“Insect pollination and animal seed dispersal may have appeared as early as 300 million years ago, but it’s not until the last 100 million years that these really intricate interactions with pollinators are driving this super high complexity in flowering plants. There was such a long period of time where plants could have interacted with insects in the way that flowering plants do now, but they didn’t to the same degree of intricacy,” concluded Professor Leslie.
The study is published in the journal Science.