Annals of Botany News in Focus

Where did the first flowers grow?

Botanists have tried to understand the conditions that led to the evolution of the first flowers by reconstructing the niches from which the earliest flowering plants diverged.

If you were to go to a real Jurassic Park, along with finding that Hollywood got the dinosaurs wrong, you’d find many of the plants odd too. There would be ferns and some trees, but you wouldn’t see a flower. Flowers didn’t evolve until the Cretaceous period if you follow the fossil record. If you follow the molecular biology, however, then the plants that became angiosperms, flowering plants, did evolve sometime in the Jurassic.

Hanging bridge in Monteverde. Image: Canva.

Ofir Katz tried to reconcile the two camps by suggesting that these early angiosperms were cryptic. Their reproductive organs maybe didn’t look much like flowers for a long while. Richard Bateman, more recently, has taken an alternative route to reconcile the two camps by comparing the hunt for a Jurassic flower to Lewis Carrol’s nonsense rhyme, The Hunting of the Snark. He argues that the molecular biologists arguing for earlier flowers are wrong and a Jurassic angiosperm is mythical.

It might help identify how angiosperms evolved if we knew where they evolved. This is what Robin Pouteau and colleagues have been investigating in their paper: Retracing the contours of the early angiosperm environmental niche. The authors cover previous conclusions and find all sorts of suggestions for the home of the first flowers.

Another method is to look at the niches of the basal angiosperms. The angiosperms have a common ancestor. However, most angiosperms share a comparatively recent common ancestor. The basal angiosperms are the plants in the lineages that diverged before the core angiosperms started evolving separately. If you were to look for common features among the basal angiosperms, you would likely to be finding features inherited from their common ancestor. Pouteau and colleagues note that the basal angiosperms tend to be found in the tropics, but they also add that the tropics are a hotspot for biodiversity, so that alone is not very convincing. However, they also to be at home above 1000m of elevation. This might also be something you’d expect from something called the mid-domain effect. The middle elevations are where high and low meet, so this boundary might also be expected to spur evolution.

So the team built a species distribution model for the most prevalent basal angiosperm species. They loaded the niche data into support vector machines (SVMs). SVMs are next-generation machine learning algorithms. Pouteau has already used SVMs to map rare and endangered native plants in Pacific islands forests. These SVMs brought tropical montane cloud forests (TMCFs) into focus as the likely origin of angiosperms.

“The global latitudinal and elevational distribution of richness in extant basal angiosperms converging in TMCFs was statistically correlated with predictions of the MDE within the ecological limits of basal angiosperms imposed by their requirements for hydraulically permissive habitats,” the authors write. “Thus, at first glance, the abundance of extant basal angiosperms in TMCFs appears not better explained by a deterministic niche perspective than by a purely null model within these boundaries. However, phylogenetic mapping of biome occupation revealed that the occupation of TMCFs is likely to have been shared with a common ancestor. This finding stems from the fact that the most basal taxa tend to occupy TMCFs more frequently than the least basal taxa, which is not predicted by the MDE.”

Because of this, Poteau and colleagues conclude that ideas that angiosperms evolved in dry habitats seem less likely, and it supports those that argue for an origin in cloud forest. They also develop this hypothesis further. “…[O]ur approach tells a slightly different story as it identified tropical temperate-like rain forests (approx. 17 °C mean annual temperature and approx. 2300–2400 mm year–1) as the most likely ancestral habitat, which is typically represented by TMCFs.”

If this research is correct, then it might suggest where the best places to go fossil hunting for the earliest angiosperm is. Unlike the characters who went hunting for the Snark, the botanists won’t have a blank map.

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