Did land plants evolve gradually and continuously?

Recent study has revealed land plants did not evolve gradually over hundreds of millions of years.

The evolution of land plants has been marked by an increase in the complexity of reproductive structures. A recent study has revealed land plants did not evolve gradually over hundreds of millions of years. Instead, they underwent two major bursts of diversification 250 million years apart. The first occurred early in plant history, giving rise to the development of seeds, and the second took place during the diversification of flowering plants, according to a Stanford University release.

The Stanford research uses a novel but simple metric to classify plant complexity based on the arrangement and number of basic parts in their reproductive structures. The latest study (Science) has shed light on timing and magnitude of those changes.

The researchers analysed the temporal pattern of the increase in the complexity of reproductive structures by studying fossil and extant land plants across the entire paleontological record. They found that reproductive complexity increased in two widely separated pulses that coincides with key developments in reproductive biology: the origin of seeds in the very late Devonian and the origin of flowering plants in the mid-Cretaceous almost 250 million years later. “After the origin of flowers, there was a rapid expansion in the morphological complexity of flowering plants. In contrast to many aspects of animal morphological diversity, which expanded early in evolutionary history, most complexity in plants was achieved relatively late,” the authors say in the paper.

The second burst of complexity was more dramatic than the first, emphasising the unique nature of flowering plants, according to Andrew Leslie, one of authors of the paper from Stanford. That period gave rise to plants like the passionflower, which can have 20 different types of parts, more than twice the number found in non-flowering plants.

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