Flowers may look delicate, but flowering plants, what scientists call angiosperms, are one of the most successful evolutionary organisms on Earth. They include more than 350,000 known species and dominate the ecological system, forming food webs and playing a crucial role in oxygen production. Plus, many of them are valuable commercial crops – think roses, grains and tomatoes.
Have you ever wondered when flowers first emerged, and what developed over millions of years of evolution to cultivate these beautiful species? Recent research brings new insights into this long-standing debate among scientists.
When you think of fossils, you probably think of dinosaurs and ammonites – creatures with bones or hard shells that have been preserved in the rocks despite massive geological changes. But flowers with weak petals and small pollen can also be preserved by sealing them in amber, crystallizing or carbonizing them.
Many people think of plants as nice-looking vegetables. Essential for clean air, yes, but for simple organisms. A step change in research is changing the way scientists think about plants: they are much more complex and more similar to us than you might think. This flourishing field of science is too delightful to do justice to in one or two stories. This article is part of the Plant Curious series, which explores scientific studies that challenge the way you look at plants.
Overall, the most convincing fossil evidence comes from the Early Cretaceous period (about 132 million years ago). However, the earliest evidence of a flower bud has a Jurassic origin (about 164 million years ago). It is called Florigerminis jurassica and was found in China. But not all scientists agree that it is an angiosperm, due to different definitions of floral organs.
Flowers can be preserved down to the smallest detail. Dripping tree sap and resin fossilized the petals and pollen of a fossil flower in a Baltic coniferous forest about 34-38 million years ago. This is the largest known fossil flower preserved in amber, measuring about 3 cm in diameter and about three times the size of most flower fossils, showing beautiful details of this ancient flower.
Jurassic or Cretaceous?
The timing of floral evolution is still a matter of debate among scientists, but most scientists are in one of two camps: Jurassic or Cretaceous.
Analyzes using molecular data (DNA or protein sequences) suggest that flowers may be much older than the fossil record shows: a Jurassic (145 million years ago) or even Triassic origin (201 million years ago).
Organisms from different major clades (a group of organisms descended from a common ancestor) can come together during their evolution. But scientists don’t always agree on which organisms belong in which evolutionary lineage. So we need to combine all the information we have to gain a more nuanced understanding of flower evolution.
Deciphering the past using molecular data
Knowing when and how flowering plants first appeared and when they diverged from other plants is important to scientists. It would help us understand how angiosperms adapted to different environments, their interactions with other organisms, and their response to major geological events, such as climate change and mass extinctions.
One technique scientists use to determine the timing of evolutionary events is the “molecular clock.” This concept arose from the insight that genetic mutations tend to accumulate at a constant rate, both over time and across the species. The rate of mutation can be compared to the steady ticking of a clock.
Read more: Explanation: what is the molecular clock?
Variations in gene sequences between different species can help scientists tell when they diverged from a common ancestor. To construct a molecular clock, researchers analyze gene segments that have been conserved throughout the evolution of a species.
For example, researchers can estimate when modern angiosperms and their close relatives, such as gymnosperms, diverged by comparing their DNA. Conifers are an example of a gymnosperm. Scientists can also estimate how close the relationship is between two angiosperm species by comparing their DNA.
Putting the puzzle together
Physical characteristics can also tell us a lot about the early evolution of flowers. Scientists examine fossil plants and observe the gradual changes in structures such as leaves, flowers and seeds over time. By comparing their anatomy, researchers can identify similarities and differences between extinct and living species, or species in different clades.
However, this approach has limitations. Biological traits that appear similar may be the result of convergent evolution, which indicates changes in traits for adaptation to the environment, rather than genetic similarity.
For example, the fins of whales and fish evolved independently for swimming. These fins may make them look alike, but they aren’t even in the same evolutionary class: whales are mammals and fish are not. Similarly, the wings of birds, bats and some insects such as butterflies evolved the ability to fly, but they have vast differences in their anatomy and genes.
Mathematical approach can also help
There is also a mathematical approach to estimating the age of angiosperms, for example using the Bayesian Brownian Bridge (BBB). This statistical model is a scientific formula that uses the distribution of fossils over time to estimate the age of a group.
Using the BBB method, an international research team found that the origin of angiosperms supports a pre-Cretaceous hypothesis. This means that flowers may have co-evolved with and survived the dinosaurs.
This finding also supports a hypothesis made by Charles Darwin about flowers – that they changed rapidly during the Cretaceous period. The project showed that the trajectory of angiosperm evolution between 125 million and 72 million years ago saw a marked increase in lineage accumulation (meaning that branches of a family showed a marked increase in new families).
Understanding the origins of angiosperms gives scientists valuable insights into the intricate web of life on our planet. It can also guide agricultural and conservation efforts. So, the next time you marvel at a vibrant flower or enjoy a juicy fruit, remember that the story of angiosperms is one of resilience, adaptation and beauty – much of which is still waiting to be discovered.
This article is republished from The Conversation under a Creative Commons license. Read the original article.
Ruolin Wu does not work for, consult with, own shares in, or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.