The evolution of flowering plants

Around 140 million years ago, flowering plants first burst into life on Earth, heralding the birth of what have become the most diverse, ecologically important plants on the planet and the major food source for humans and animals.

But it took another 40 to 50 million years for this diversification to occur, according to a new study, published in the journal Nature Ecology and Evolution, that mapped the flowering plants’ entire family tree and evolution.

It’s remarkable that the rise to dominance of these vital plants – botanically known as angiosperms – lagged by such a long time frame, says first author Santiago Ramírez-Barahona, from the National Autonomous University of Mexico.

“Understanding this is very important,” he adds, “because angiosperms are not only the [major] basis of modern terrestrial ecosystems, but they also played a massive role in the evolution of other groups of animals, such as insects.”

Although modern-day flowering plants are the youngest major group, according to fossil records, they comprise nearly 300,000 species.

To fill in gaps in their evolution, Ramírez-Barahona teamed up with Susana Magallón and Australia’s Hervé Sauquet, from the Royal Botanic Gardens in New South Wales, to construct a time tree.

First, they compiled a dataset comprising DNA sequences for more than 1200 species representing all 435 known families and time-calibrated it with a new database of 238 fossil records – a considerable addition to previous collections.

“Previous studies of this nature only used 30 to 60 fossil records, and we wanted to increase this number significantly,” says Sauquet. “We often had to translate records from different languages and do relentless detective work to get centuries old as well as the latest fossil descriptions in our hands.”

They integrated this new angiosperm tree with 16.4 million location records around the globe for nearly 250,000 species to estimate two key ages of the plants’ families – the time of their origin (stem age) and diversification into living species (crown age).

All living species on Earth have stem and crown ages, and birds are a great example, says Ramírez-Barahona.

“The stem age of birds is marked by their split from crocodiles around 240 million years ago and their crown age is marked by the most recent common ancestor of all living birds, around 100 million years ago.

It seems flowering plant families followed a similar trajectory. To their knowledge, this is the first time this time lag has been quantified, says Sauquet.

“The current idea about the origin of flowering plants is that the main phase of their explosive diversification occurred in the Cretaceous,” he says.

This was thought to correspond with the Cretaceous Terrestrial Revolution and a sweeping expansion of life that led to rich modern biodiversity.

But the team writes that although angiosperm families were diverse and ecologically important by the middle Cretaceous, the major explosion of new species appears to have happened in the Palaeocene, after dinosaurs became extinct.

This coincides with “profound changes in terrestrial ecosystems, including a well-documented trend of global warming in the Palaeocene and Eocene.”