When did flowers originate? Solving the ‘abominable mystery’

Flowering plants likely originated between 149 and 256 million years ago according to new UCL-led research.

The study, published today in New Phytologist by researchers from the UK and China, shows that flowering plants are neither as old as suggested by previous molecular studies, nor as young as a literal interpretation of their fossil record.

The fossil record suggests that flowering plants suddenly appeared on the scene in what’s termed the Cretaceous Terrestrial Revolution. Pollinators, herbivores, and their predators, also underwent explosive co-evolution at the same time, taking advantage of the new wealth of diversity.

Image: Passion flower (credit: renê ardanuy on/off, Flickr. CC BY-NC 2.0).
Passion flower (credit: renê ardanuy on/off, Flickr. CC BY-NC 2.0).

Solving the ‘abominable mystery’

Molecular-clock dating studies suggest a much older origin for flowering plants, however, and that we may have missed a much earlier, cryptic evolution of flowers, not captured in the fossil record. The discrepancy between palaeontological and molecular estimates of the appearance of flowering plants has been the source of heated debate. The apparent explosive evolution of flowering plants in the mid-Cretaceous led Darwin to describe their origin as an ‘abominable mystery’.

“The discrepancy between estimates of flowering plant evolution from molecular data and fossil records has caused much debate. Even Darwin described the origin of this group as an ‘abominable mystery'”, explained lead author, Dr Jose Barba-Montoya (UCL Genetics, Evolution & Environment).

“To uncover the key to solving the mystery of when flowers originated, we carefully analysed the genetic make-up of flowering plants, and the rate at which mutations accumulate in their genomes.”

By carefully analysing the rate at which mutations build up in the genomes of flowering plants, the researchers have concluded that flowering plants are neither as old as suggested by molecular studies, nor as young as a literal interpretation of the fossil record.

Closing in on a mid-Cretaceous origin

Prof. Phil Donoghue, from the University of Bristol’s School of Earth Science and senior author of the study, thinks that the mismatch between fossil and molecular estimates of the origin of flowering plants is a result of false precision in both estimates. “In molecular divergence time estimation, previous studies have failed to explore the implications of experimental variables and inaccurately summarised the broad probabilistic estimates of flowering plant age with undue precision,” said Prof. Donoghue. “Similarly, interpretations of the fossil record have not fully recognised its shortcomings as an archive of evolutionary history.”

Palaeontological timescales calibrate the family tree of plants to geological time based on the oldest fossil evidence for its component branches. Molecular timescales build on this approach, using additional evidence from genomes for the genetic distances between species, aiming to overcome gaps in the fossil record.

Image: Figure 6 from Barba-Montoya et al.: The time tree of tracheophytes encompassing uncertainty of calibration strategies. Click to view full-size figure.
Figure 6 from Barba-Montoya et al.: The time tree of tracheophytes encompassing uncertainty of calibration strategies. Click to view full-size figure.

The researchers combined a large collection of genetic data for many flowering plant groups (83 genes from 644 taxa) with a comprehensive set of fossil evidence to address the timescale of flowering plant diversification.

“By using Bayesian statistical methods that borrow tools from physics and mathematics to model how the evolutionary rate changes with time, we showed that there are broad uncertainties in the estimates of flowering plant age, all compatible with early to mid-Cretaceous origin for the group,” said co-author Dr Mario dos Reis (School of Biological and Chemical Sciences at Queen Mary University of London).

Prof. Donoghue pointed out that the results of the study highlight the complementary nature of molecular and palaeontological approaches for gaining a better understanding of evolutionary dynamics in deep time.

Adapted from a press release.

Read the paper: Barba-Montoya, J., dos Reis, M., Schneider, H., Donoghue, P.C.J., Yang, Z. (2018) Constraining uncertainty in the timescale of angiosperm evolution and the veracity of a Cretaceous Terrestrial RevolutionNew Phytologist. doi: 10.1111/nph.15011


 

Mike Whitfield (@mgwhitfield)
Development Coordinator
New Phytologist