Kingdom of Plants: A Journey Through Their Evolution. Will Benson
animal co-evolution.
© Minden Pictures/SuperStock
But for all the sense that Darwin was able to make of the lives of flowering plants and how they disperse their pollen, there was one fundamental aspect of their world that he was never truly able to fathom. He couldn’t understand how flowering plants had come to exist in such diversity in such a short period of geological history. For almost half a billion years, plants had existed without flowers, and then in a few million years they appear in the fossil record as the dominant form of plant life. In a letter sent to his friend Joseph Hooker in 1879, Darwin remarked on his puzzlement on the sudden radiation of flowering plants, stating, ‘the rapid development, as far as we can judge, of all higher plants within recent geological time is an abominable mystery.’ It went against his very theory of ‘natural selection’ that he had outlined for the rest of life on Earth.
Darwin’s work, which observed the processes by which all species struggle for survival and compete to reproduce in order to pass on their genes, helped build his theory of evolution – the process by which a species over many generations acquires novel and advantageous traits. From his acute observations of the birds, insects and reptiles of the Galapagos Islands, together with the fossils of extinct animals that he gathered during his voyage on the Beagle, Darwin discovered that the change inferred to organisms over time was a slow and gradual process. In his subsequent book On the Origin of Species by Means of Natural Selection he states that ‘natural selection acts only by taking advantage of slight successive variations; she can never take a great and sudden leap, but must advance by short and sure, though slow steps.’
However, for all the diversity that Darwin could see in the modern world of flowering plants, the fossil record revealed no trace of the expected slow and gradual transition of non-flowering plants to those with flowers. For Darwin, the seemingly sudden appearance of flowers contradicted the very rules of evolution. In the fossils of the Carboniferous period horsetails and early seed plants dominated the land. In the fossils from the time of the dinosaurs cycads, ginkgos and ferns were dominant. And then suddenly in the fossils of the Cretaceous period, 130 million years ago, an explosion of flowering life appears and takes over the land. In this short period all of the major groups of flowers that we see today emerged. Not only did each species of flower look different, but they had developed a multitude of reproductive styles – from the relatively straightforward mechanism of those flowers which released their pollen on the wind, to those possessing nectar-filled organs for the more complex task of luring insect pollinators. This sudden spurt of evolution not only had Darwin stumped but has continued to boggle botanists for the last 120 years.
Darwin knew that the fossil record was not a wholly complete snapshot of life through time, and he used this to try and explain the sudden burst of flowering plants on Earth. Due to the fact that plants do not have hardened body parts like the easily fossilised internal and external skeletons of many animals, there is a chance that the intermediate stages of the first flowers may simply have decomposed and been lost when they died. In his correspondence with Hooker, Darwin suggested that flowering plants had perhaps evolved slowly and that the fossils were yet to be found. Another suggestion was that a rapid increase in flower-frequenting insects in the Cambrian may have spurred their evolution on. In the animal world it is possible to see many of the intermediate steps that have given rise to certain animals. The embryos of snakes, dolphins and whales all sprout the buds of vestigial legs when they are embryos, echoing their evolutionary past, which then shrink and disappear before they are born. However, plants do not retain these evolutionary features in the same way that animals do, and it is far harder to trace the steps by which flowers came to be by studying their more primitive relatives. What makes things harder still is that the flowers of even moderately primitive groups of flowering plants are so different from their assumed extinct relatives among seed plants, that it is incredibly difficult to reconstruct a plausible evolutionary history for them.
However, since Darwin’s day new fossil finds and our considerable advances in genetics have helped us begin to unravel the origins of flowering plants. In the mid-1980s an international collaboration of over 40 scientists from around the world, coordinated byKew geneticist Professor Mark Chase, embarked on a mammoth project. Over a number of years the team meticulously extracted the same type of gene from over 500 different types of flowering plant, and by the early 1990s they had gathered enough information to begin to compare them. By looking at the plants which shared the most similarities for this type of shared gene, Professor Chase and his team were able to work out which groups of flowers were more closely related, and by pinpointing those in which the gene had considerable differences they could assume they had evolved separately. The findings, published in 1993, allowed them to piece together an accurate tree of life for flowering plants. Fifteen years later a team of researchers at the University of Florida built on this tree of life to create a more complex timeline for the emergance of different types of flowers. By looking at the genes of a number of living plants that could be linked to their fossil ancestors from known dates in prehistory, the team were able to work out the rate at which certain genes changed over time. The results from these calculations gave them a ticking genetic clock which could then be used to date the origins of the first flowering plants. The major revelation from their work was that previous estimates for the first flowers had been inaccurate by around 10 million years, and that the first blooms were in fact evolving as far back as 140 million years ago. But the Florida team’s findings still seemed to indicate that flowering plants did indeed rapidly radiate in as little as just five million years.
For years the debating and theorising continued as to how plants could have seemingly cheated evolution, to quickly rise from relative obscurity into a wealth of developed flowering structures during the Cretaceous period. Then scientists believed they had found an explanation, in the form of a happy coincidence, a genetic mishap discovered in some plants known as polyploidy. It has been known that when the male and female haploid sex cells of both plants and animals combine during reproduction to create the next diploid generation, some sections of genetic information from the parents can become duplicated in the new generation. In the case of humans, the accidental insertion of any additional genetic information can be extremely damaging for the child’s health; even the duplication of just one of our 46 chromosomes will cause Down’s syndrome, and the duplication of two or more chromosomes would be fatal. However, flowering plants with their comparatively simple body parts have been found to be able to live healthily with accidental duplications of genetic material, even in extreme cases where the whole of a plant’s genome (i.e. every one of its chromosomes) becomes duplicated. Not only can a plant species tolerate these polyploidy events, but it appears that they can actually thrive on them.
It had long been considered that flowering plants’ ability to duplicate large parts of their genetic information could have been a contributing factor that allowed them to increase in diversity at an abnormal rate, and a study carried out in the 1970s calculated that between 30 per cent and 80 per cent of all flowering plants have undergone a multiplication of parts or the whole of their genome at some point in their evolutionary history. Plants that have undergone polyploidy are typically more vigorous. By tracing through the lineage of many different groups of flowering plants, scientists have now found proof that a number of polyploidy events does indeed explain the fast rate at which particular types of flowers radiated, such as the prolific grasses, the nightshades, the pea family and the mustard flowers. But what of all the other flowering families that make up the 400,000 or so species on Earth today?
Nuphar lutea
These yellow water-lilies are among the oldest living relatives of the first flowering plants.
© Chris Cheadle
Up until very recently speculation still remained as to what exactly could explain the apparently sudden explosion of all flowering life 140 million years ago. Then in 2011, at a conference of the International Botanical Congress in Melbourne, a palaeontologist from the Swedish Museum of Natural History called Else Marie Friis revealed findings which outlined a previously unseen trove of exquisitely preserved primitive flowers from the charcoal fields of Catefica, Torres Vedras and Famalicao in Portugal, dating back to the Early Cretaceous. These amazing fossils, many of which were preserved