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Dec 7th 2020

Darwin’s Abominable Mystery: The Evolutionary Enigma of Angiosperms

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In an 1879 letter to J. D. Hooker, Charles Darwin lamented the “abominable mystery” of the angiosperm explosion — the rapid diversification of multicellular life of Earth in the form of flowering plants that seemingly took a shortcut from initial appearance to ecological domination.

The evolutionary expert put it simply: Natura non facit saltum” — nature does not take a leap. While animal life took things slow and continues to evolve at a steady pace, Darwin’s abominable mystery had no easy answers. How had four major seed-bearing flower families emerged a mere 10 million years after the first appearance of angiosperms? While it confounded Darwin in his lifetime, recent scientific sleuthing has uncovered clues that could account for this explosion without the need to eviscerate evolutionary explanations.

The Angiosperm Issue

So what was Darwin’s problem with angiosperms? As noted by a PubMed piece, while the “mystery” has often been shoehorned in to address the origin and evolution of all flowering plants on Earth, “Darwin was deeply bothered by what he perceived to be an abrupt origin and highly accelerated rate of diversification of flowering plants in the mid-Cretaceous.”

The appearance and rapid expansion of these flowers appeared at odds with Darwin’s theory of evolution, leading him to propose potentially undiscovered species on lost islands or continents that could explain this uptake, or that insect/angiosperm interactions somehow increased evolutionary speed. In fact, the “mystery” itself isn’t really unfathomable — simply uncomfortable for Darwin, who firmly held that nature moved slowly and steadily — not in fits and starts.

Making the Multicellular Move

The shift from single-celled to multicellular organisms is no easy feat — scientists have compared the transition to crossing a threshold: Once cells start working in unison, there’s no going back. The momentous nature of this move naturally suggests both significant complexities and the need for substantial development over time — cells can’t simply decide to partner up and partition key functions.

According to Science Magazine, however, recent research is now painting a different picture of the rapid diversification of multicellular life on Earth. Test tube experiments have shown that single-celled life can produce the precursors of multicellular modification in only a few hundred generations — a natural “leap” by any stretch of the imagination. And according to Bob Kerr, an evolutionary biologist from the University of Washington in Seattle, ongoing analysis suggests that instead of a single, large step from uni- to multicellular life, smaller (and speedier) steps may be the more likely explanation.

What’s more, there’s now evidence to suggest that while plants and animals made the multicellular move only once, other organisms — such as fungi — jumped the cellular shark several times over the course of their evolution. For Darwin’s abominable mystery, this is the first clue: Multicellularity isn’t quite the monolith it once appeared.

Of Dates and Downsizing

Two lines of research also suggest ways to reconcile Darwin’s mystery with multicellular angiosperm acceleration. The first leans into Darwin’s theory as it stands: According to Science Direct, phylogenetic and palaeobiogeographic analysis posits necessary updates to the fossil record with a Triassic — rather than Cretaceous — origin story for stem angiosperms. Using these new dating details, there’s plenty of room for flowering plants to evolve in line with Darwin’s directives. In fact, a Triassic timeline puts angiosperm explosions on par with other large-scale diversification across bird, insect and mammal species.

There’s also new data to suggest that genome size played a role in rapid evolution. As noted by Science Daily, work by Kevin Simonin of San Francisco State University and Adam Roddy of Yale University found that flowering plants have much smaller cells than other major plant groups thanks to a significantly reduced genome scale. Because each cell in an organism must contain an entire copy of its genome, reduced genome size logically makes it possible to reduce cell size, in turn allowing these plants to cram more cells into smaller areas and improve nutrient delivery. This downsizing discovery offers a mechanism for rapid angiosperm evolution: Smaller genomes in smaller cells effectively gave these plants an evolutionary speed burst, allowing them to affect changes much more efficiently and effectively over time.

Mystery Managed

The rapid diversification of multicellular life on Earth — and specifically the angiosperm explosion — was understandably frustrating for Darwin, since his work pointed to a deliberate and calculated process that took time and effort, rather than rapid and potentially risky organism adaptations.

Like many scientific uncertainties, however, Darwin’s mystery has been managed as investigative efforts yield new results. From earlier start dates for angiosperm supremacy to evidence of downsized cellular structures, there’s no abominable issue here — just slow and steady adaptation to growing evolutionary evidence.

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