Reflexions, le site de vulgarisation de l'Université de Liège
Plant life on earth earlier than originally thought
6/16/09
Just as it fish took millions of year to come out of the water, green algae, the ancestors of terrestrial plants,
clearly did not leave the sea from one day to the next. Researchers agree on this point. But exactly when did
this decisive evolution take place? The question has already been much debated. And the article published
in Science magazine (1), written principally by Philippe Steemans, palynologist at ULg, is likely to open the
debate again. In his conclusions, the researcher asserts that the terrestrial ancestors of our trees and other
vascular plants could be 30 million years older than we originally thought.
When algae came out of the water
The first forms of plant life on dry land are confirmed to have appeared at least 460 million years ago.
These were very simple plants, close to certain types of algae, that were still highly dependent on an aquatic
environment. They had no roots capable of taking water from the ground and their male sex cells had flagella
since they still needed water to move around in search of female sex cells. They are called "bryophytes".
They left very discreet yet unquestionable traces of their presence on earth in the Ordovician period (which
extended from 488 to 444 million years ago). These traces, called "cryptospores" - the cells that allowed these
primitive plants to disperse - were easily fossilized. A microscope is required to study them because they barely
measure several dozen microns, i.e. no bigger than the diameter of a hair. Nearly 500 million years later, in soil
samples taken from the four corners of the earth, these fossilized cryptospores tell the story of the beginnings
of life on dry land. But in order to find out what plants looked like during this period, scientists do not necessarily
need to journey back in time because, believe it or not, bryophytes still exist today. If you have a small pond
at the end of your garden, it may be surrounded by liverwort (a group of plants belonging to bryophytes in the
broad sense), a small greenish plant barely measuring a few millimetres. These primitive plants form a carpet
when they grow since they cannot grow upwards. But what a fantastic example of adaptation: they have been
on earth for nearly 500 million years!
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Reflexions, le site de vulgarisation de l'Université de Liège
(1) Steemans, P., Le Herisse, A., Melvin, J., Miller, M.A., Paris, F., Verniers, J. and Wellman, C.H., 2009. Origin and radiation of the earliest vascular land
plants. Science, 324: 353.
The plants' race towards the sun and the light to capture energy began later, when plants were able to form a
spine, i.e. an erect stalk through which water and nutriments could circulate. Higher than bryophytes, the first
tracheophytes (or vascular plants) only measured a few centimetres. They were able to disperse themselves
further (thanks first to trilete spores then pollen grains during the Carboniferous period) and thus ensure their
dispersal and reproduction. The oldest undisputed traces of the presence of these vascular plants on earth
go back approximately 420 million years. One of the first plants listed is called Cooksonia. It undoubtedly
grew on the sandy banks of stretches of water, barely measured 5 centimetres and had an erect structure,
with each stalk ending in a small cup (the sporange) filled with spores. Even so, the appearance of these first
vascular plants is a very important step in the history of life. Because by escaping from aqueous environments,
tracheophytes were to change the face of the earth. Over several million years, the mineral deserts were
covered with rudimentary soil, an organic environment where tiny animals, such as acarids and arachnids
could develop.
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Reflexions, le site de vulgarisation de l'Université de Liège
Contrary to bryophytes, a significant quantity of these plants became fossilized by imprinting their image
on pieces of rock, thus allowing scientists to examine the morphological diversity of terrestrial vegetation
during that era. However, tracheophytes, like bryophytes, also left numerous microfossils, essentially spores,
in the earth. The analysis of these fossilized spores is the speciality of Philippe Steemans, a qualified FNRS
researcher in the Paleobotany, Palynology and Micropaleontology Department at the University of Liège. Over
the past twenty years or so, this palynologist (a scientist who studies the fossilized residues of pollen) has
established a co-operation with several oil companies who provide him with soil samples and core boring
samples taken within the framework of prospecting. Recently, he received an express parcel from Saudi
Arabia. The box contained soil samples taken in the desert by the company Aramco. "This oil group", explains
Philippe Steemans, "signed a scientific co-operation agreement with the International Commission of the
Palaeozoic Microflora, a science company with approximately one hundred researchers. It is a fine example
of the alliance between fundamental research and applied research. We help oil companies to create precise
geological profiles of prospecting areas. And, in exchange, we can benefit from very precious geological
samples to attempt to solve far more fundamental questions." Like, for instance, discovering when the first
plants appeared on earth…
Upsetting the great puzzle of life
The small global community of palynologists shared the work between them. In these little pieces of desert
provided by Aramco, a team of researchers were given the responsibility of studying traces of acritarchs,
perhaps the ancestors of phytoplankton (dinoflagella). Another team decided to focus on chitinozoans, a group
of organisms that are still relatively unknown, belonging undoubtedly to the animal kingdom. As for Philippe
Steemans' laboratory, it decided to examine the traces of plant spores. "The spores we discovered in the
sample", explains the researcher, "strongly resemble spores that we already know about from 415 million years
ago. They are trilete spores. They have a very characteristic form, divided into three compartments. They
bear some resemblance to the logo of a famous German car brand…". But to the great surprise of Philippe
Steemans, the other teams dated the geological layer containing the fossils at 445 million years ago. "At first,
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Reflexions, le site de vulgarisation de l'Université de Liège
I thought my colleagues had made a mistake", explains the researcher from Liège. "Because from what I
could see through my microscope, they were clearly trilete spores, characteristic of vascular plants, hence,
tracheophytes, which are supposed to have appeared on earth 415 million years ago. I asked my colleagues to
recommence their manipulations, telling them that they must have made a mistake somewhere along the line!
They redid their analyses but reached exactly the same result! Likewise, we processed one of our samples in
another laboratory to find out whether there had been an accidental contamination. The test turned out to be
negative. This data quite simply suggests that vascular plants appeared on earth much earlier than we had
thought up until now, perhaps 30 million years earlier."
This means that the whole puzzle of the appearance of plant life on earth could be turned upside down.
Researchers have at least two new hypotheses: either algae came out of the water earlier - maybe 500 million
years ago - before evolving into bryophytes and then transforming themselves into tracheophytes about 450
million years ago; or the evolution between bryophytes and tracheophytes was very rapid, a few million years
instead of a few tens of thousands of years.
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Reflexions, le site de vulgarisation de l'Université de Liège
In the article published in Science magazine, researchers put forward the hypothesis that the first vascular
plants could have emerged on the supercontinent Gondwana (which grouped together what is now South
America, Africa, India, Australia and, of course, the Arabian Peninsula), before later migrating towards the
other continents where they diversified. But the conclusions of Philippe Steemans and his colleagues will
undoubtedly arouse some opposition in the scientific community. Because botanists know that certain current
bryophytes, albeit rare, also produce trilete spores. This leaves open the possibility that the spores analysed by
the researchers in Liège do not come from a vascular plant. Fascinating, uncertain, controversial and certainly
not the end, the race to discover the first terrestrial plants inevitably draws certain parallels with that of the
ancestors of modern man, where a new fossil often forces palaeontologists to turn our genealogy upside down.
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