Rewriting the History of Plant Evolution

For over a century, the prevailing view among scientists has been that plants evolved the vast majority of their anatomical diversity early in their evolutionary history, similar to a big bang of innovation (an early explosion of evolutionary change). This conventional wisdom held that the basic forms of plants—their roots, stems, leaves, seeds, flowers and so on—arose in a sudden burst after plants first moved from water onto land.

However, new research published in Nature Plants challenges this long-held assumption. A comprehensive analysis of living and extinct plant species led by scientists at the University of Bristol provides evidence that plant evolution has actually been a more gradual process, with occasional spikes of innovation in response to environmental pressures. Rather than evolving most key traits early on, plants seem to have slowly developed anatomical diversity over time, adapting forms like seeds, pollen and flowers to meet the challenges arising as they expanded into drier terrestrial settings.

This research effectively rewrites the evolutionary history of plants. While major groups like mosses, ferns and flowering plants arose at different times, the origins of their distinct body designs reflect gradual modification rather than an early big bang of innovation. The study aligns plant evolution more closely with the punctuated equilibrium pattern seen in animals and other complex life—lengthy periods of incremental change punctuated by bursts of novelty in the face of environmental shifts. By overturning such a fundamental assumption, these findings promise to advance our understanding of how plant life has diversified over billions of years.

Previous Research into Plant Evolution

For over a century, the dominant viewpoint within evolutionary biology has been that plants underwent an explosive diversification early in their history. This idea was first proposed by paleobotanists in the late 19th century who were uncovering new types of ancient plants in the fossil record. They theorized that the major plant groups—mosses, ferns, conifers, flowering plants—each evolved their distinct anatomical designs during the Devonian period around 400 million years ago.

This “big bang” model of plant evolution persisted as the conventional wisdom, supported by phylogenetic analyses in the 1990s that seemed to place the initial divergence of key plant lineages deep in the past. The notion fit with ideas about evolution proceeding in sudden bursts followed by long periods of stability. However, the limited fossil evidence made it difficult to conclusively determine if plant anatomy had originated across a short span of Devonian time or evolved gradually over a longer timescale.

• Late 19th century: Paleobotanists uncover ancient fern, conifer, and seed-bearing fossils from Devonian period (~400 million years ago).
• 1990s: Phylogenetic analyses appear to place divergence of major plant groups deep in the past, further supporting explosive diversification model.
• However, fossil evidence from Devonian still limited, making it hard to determine if plant anatomy evolved rapidly or gradually.
• Many analyses rely on living plants only, lacking extinct transitional species.
• Assumption persists that mosses, ferns, conifers, flowering plants, etc evolved distinct body designs during short span in Devonian.
• Explosive diversification model aligns with prevailing “punctuated equilibrium” ideas about evolution proceeding in bursts surrounding major environmental changes.
• But gradual evolution of plant diversity over longer timespan can’t be ruled out based on available evidence.
• Key questions remain about pace of early plant evolution and how anatomy originated across plant kingdom.

Darwin’s Views on Plant Evolution

Charles Darwin himself expressed uncertainty about the origins of plant diversity. In The Origin of Species, he acknowledged that the abrupt appearance of varied plant forms in the fossil record seemingly provided evidence for distinct plant groups evolving their unique anatomies in a compressed period of time. However, Darwin proposed an alternative viewpoint:

• Darwin suggested plant evolution may have been more gradual, but sparse plant fossils obscured the transitional forms.
• He theorized natural selection could slowly accumulate adaptations over immense timespans, making major changes possible without abrupt bursts.
• Darwin noted that living plants vary widely within groups, hinting at how gradual modification over time could bridge major anatomical differences.
• He argued factors like changing climate conditions could drive adaptation and variation among plant lineages.
• Darwin maintained an open mind, stating that new paleobotanical evidence could shed light on the evolutionary steps linking major plant groups.

Though a “big bang” view later gained wide support, Darwin recognized that the fossil record was incomplete and proposed plausible mechanisms for gradual plant evolution. His ideas left open the possibility that plant diversity arose incrementally rather than suddenly.

Bridging Darwin’s Knowledge Gaps

While Darwin recognized gaps in the fossil record that obscured insights into plant evolution, major discoveries over the last century have helped fill in some of those missing pieces. Advanced fossil analysis techniques and phylogenetic methods have allowed scientists to trace anatomical changes across extinct and living plants in much finer detail. Now, cutting-edge modeling approaches leveraging massive data sets are clarifying patterns and generating novel findings, including new perspectives on the emergence of plant diversity. The pioneering study highlighted at the outset of this article represents one such breakthrough. By comprehensively analyzing variation across living and fossil plants, the researchers have gained new clarity on the incremental evolution of key plant traits, effectively rewriting the narrative on the origins of plants.

New Techniques

In recent decades, evolutionary biologists have developed powerful new statistical approaches for reconstructing the relationships between organisms and modeling how their traits have changed over time. These phylogenetic techniques can incorporate both living and extinct species to create more complete evolutionary trees. Researchers can code observable traits across many taxa and use computational tools to infer ancestral characteristics and build models of how novel features emerged.

For example, maximum likelihood modeling can determine the most probable order of evolutionary events by calculating which tree structure and trait changes have the highest likelihood based on the input data. Molecular clock analyses can then estimate when different anatomical adaptations first appeared by using genetic mutation rates as a molecular clock to trace divergences back to their origins. Anatomical data from the plant fossil record can provide minimum age estimates for the antiquity of various plant groups.

By combining phylogenetic reconstructions with fossil evidence, researchers today can often pinpoint the periods when major evolutionary innovations arose with greater precision than Darwin could in the 19th century. However, reliably unraveling the tree of life still depends on high quality and complete data spanning both living and extinct organisms. Our understanding of plant evolution continues to be refined as more fossils are discovered and analytical methods progress.

Here is an overview of what the press release from the University of Bristol:

• The researchers analyzed morphological traits across 248 plant groups, including 160 extinct species known from fossils.
• They generated a data set with over 130,000 observations by systematically scoring the presence or absence of various anatomical features in each plant.
• The plants analyzed ranged from single-celled algae to flowering plants, spanning early land plants to angiosperms.
• They quantified the similarities and differences between the plant groups over evolutionary time.
• The press release states they used “computerized statistical techniques” to measure changes in plant anatomy, but does not specify the phylogenetic methods.
• They tried to associate major evolutionary innovations with potential drivers like environmental changes or whole genome duplication events.
• The article mentions they were able to trace the order in which key traits like seeds, flowers, and pollen evolved by analyzing patterns across living and fossil plants.

Paper

‘Evolution of phenotypic disparity in the plant kingdom’ by James W. Clark et al in Nature Plants

The study provides new clues into how plants navigated the pivotal transition from water to land. Rather than a sudden explosion of diversity, adapting to terrestrial living likely occurred gradually over millions of years. Analyzing both living species and long-extinct transitional forms points to step-wise anatomical changes in response to new environmental pressures. The laborious process of modifying cell walls, preventing water loss, absorbing nutrients from soil, and reproducing in air may have proceeded through extended trials of many imperfect solutions before arriving at the anatomies of modern land plants.

This paints a narrative of plant ancestors slowly leaving aquatic environments, adapting parts like stems and leaves to support themselves on land and generate energy from the sun. Reproductive structures like spores evolved to spread progeny without water, culminating in protected seeds and pollen. While the march towards today’s flowering plants was lengthy, genome duplications may have accelerated key innovations like vascular tissue, fruits, and flowers. The scale and detail of this study provides a robust framework for continuing to unravel plant evolution’s seemingly quiet revolutions across the eons.