Tuesday 21 June 2022

Evolution News - How Plants Colonised the Land.

The diversity of flora and fauna as we know them today and the substrate on which thrive are thanks to a single species of algae that first went ashore more than 500 million years ago. This and all other drawings in the publication are by the paper’s first author, Dr. Mona Schreiber.
Image: HHU /Dr Mona Schreiber
Universität Düsseldorf: The Greening Ashore.

A team led by evolutionary biologist Prof. Dr. Sven Gould of Heinrich Heine University Düsseldorf (HHU) has been studying the current state of research on the plant colonisation of land that occurred some 500 million years ago. The findings from this illustrated overview study published by Dr. Mona Schreiber as lead author have now appeared, open access, in the latest issue of the journal Trends in Plant Science.

It was to take several hundred million years for Earth to cool sufficiently for oceans and landmasses to form then the conditions on land were far from conducive to the life that was evolving in the oceans to begin to colonise that land. Until about 500 million years ago, seismic activity meant the atmosphere was toxic and a weaker magnetic field meant UV light was more intensive. Then things began to change, and all terrestrial plant life has developed from the first plant species, a streptophyte alga, that began to move out of the oceans onto land. This was followed by simple animal life which in turn created the opportunities for tetraploid vertebrates to exploit by coming onto the land, developing air-breathing and a tetrapodal lifestyle.

The Heinrich Heine Universität news release explains:
The blue planet with green continents that we know today did not exist as such in that era. For conditions on the continents were largely hostile to life, with a much higher volcanic activity releasing toxic gases into the atmosphere, a weaker magnetic field than exists today exposing the land more to cosmic rays, and a thinner ozone layer to filter out UV light.

This started changing approximately 500 million years ago when plants began colonizing land. The invasion catalysed a metamorphosis of the hostile environment, accelerating the transformation of the atmosphere, to lay the foundations for the development of life on land as we know it today. All this could only occur once plants, which had only lived in the oceans and inland freshwater, had conquered the continents.

Timeline of major evolutionary events leading to the conquest of land by a streptophyte alga at least 500 million years ago (Mya) (515.1 Mya to 470.0 Mya; middle Cambrian–Early Ordovician [33.]) and the rapid diversification of plant life on land.

Key steps towards the extant flora, after the origin of life and the split into Bacteria and Archaea, included the origin of oxygenic photosynthesis in the common ancestor of Cyanobacteria, the acquisition of a cyanobacterial ancestor by a heterotrophic protist, which marks the origin of the Archaeplastida, and the split and origin of streptophyte from chlorophyte algae, one of which carried the necessary adaptations to thrive on land. This ancestor subsequently evolved the land plant diversity we witness today – 3.5 billion years after the emergence of life.

Abbreviations, GOE, Great Oxidation Event; LECA, last eukaryotic common ancestor; LUCA, last universal common ancestor; MRCA, most recent common ancestor.

Now Prof. Dr. Sven Gould of the Institute of Molecular Evolution at HHU, Prof. Dr. Stefan Rensing and Dr. Mona Schreiber, a bioinformatics specialist and artist from the University of Marburg, are providing an overview of the current state of research on the plant colonization of land in the journal Trends in Plant Science. Their paper was written in connection with priority programme 2237 “MAdLand” (Molecular Adaptation to Land), funded by the German Research Foundation. The purpose of the MAdLand programme is to explore the beginnings of the evolutionary adaptation of plant organisms to life on land.

The continents only began turning green after a streptophyte alga moved from an aquatic habitat into shore zones before completely transitioning onto land over 500 million years ago, in a process involving numerous molecular and morphological adaptations. Throughout Earth’s ongoing changes, plants demonstrated tremendous adaptational capability and altered the climate in crucial fashion, chiefly by fixing carbon dioxide (CO2) on a massive scale.

Human beings, which have but a brief history compared to plants, are indeed responsible in their own right for significant changes to the planet and its climate. The extreme rapidity of those changes poses a major problem, as nature has little insufficient time to adapt. The pace of human-caused change accelerated when man developed agriculture and animal husbandry, which led to steady population growth and the clearing of ever more land for farming.

Professor Gould, senior author
Institute for Molecular Evolution
Heinrich Heine University (HHU) Düsseldorf, Germany
Terrestrial flora spread in a dominant tour de force, with flowering plants proliferating in explosive fashion; today they comprise over 90% of all known terrestrial plant species. In the history of our planet, land plants have caused several climatic changes, demonstrating tremendous adaptive capability again and again.

Researchers are studying the genomes of species of evolutionary significance with regards to terrestrilization [sic], including mosses, lycopods, ferns and certain algae, in an effort to advance our knowledge of evolutionary processes and molecular adaptation. Their work aims at identifying the mechanisms that served to mitigate hostile life conditions on land, which changed in the course of this evolution. These may indeed prove relevant with regard to climate change, including for crop modification in response to shifting environmental conditions.

In this work the collaborating authors analyse human influences on the climate, discussing the adaptability of plant life to the changes that are today unfolding.

Copyright: © MAdLand.
Published Open access (CC BY-NC-ND 4.0)

The team's findings are published open access as an opinion piece in the peer-reviewed journal, Trends in Plant Science

  • Two decisive endosymbiotic events, the emergence of eukaryotes followed by the further incorporation of a photosynthesizing cyanobacterium, laid the foundation for the development of plant life.
  • Increasing cellular complexity, the development of new body plans, new molecular adaptations, and constant colonization of novel habitats probably paved the way for plant evolution from fresh water to salt water and, at least 500 million years ago, to land.
  • The history of plant life, and particularly the greening ashore, is inseparably linked to the success of all life as we know it today.
  • Plant life enriched the atmosphere with oxygen and fixed CO2, thereby paving the way for the success of further life in this previously hostile habitat – and ultimately enabled the emergence of our own species.

More than half a billion years ago a streptophyte algal lineage began terraforming the terrestrial habitat and the Earth’s atmosphere. This pioneering step enabled the subsequent evolution of all complex life on land, and the past decade has uncovered that many traits, both morphological and genetic, once thought to be unique to land plants, are conserved across some streptophyte algae. They provided the common ancestor of land plants with a repertoire of genes, of which many were adapted to overcome the new biotic and abiotic challenges. Exploring these molecular adaptations in non-tracheophyte species may help us to better prepare all green life, including our crops, for the challenges precipitated by the climate change of the Anthropocene because the challenges mostly differ by the speed with which they are now being met.

Schreiber, Mona; Rensing, Stefan A.; Gould, Sven B.
The greening ashore
Trends in Plant Science 2022; 1360-1385 DOI: 10.1016/j.tplants.2022.05.005

Copyright: © 2022 The authors. Published by Elsevier Ltd. Open access
Reprinted under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International licence (CC BY-NC-ND 4.0)
There is then a clear evolutionary pathway, supported by fossil and genetic evidence, from the first plants, a streptophyte alga, the last common ancestor of all terrestrial plants, to the whole range of plants, including the flowering plants we see today and without which life on Earth would be impossible.

It is thanks to these first algal colonisers of land that animal life was also able to leave the confines of the oceans and establish itself on Earth.

Bible literalist creationists might like to note the time scale involved, because the conditions these first land-colonising plants would have encountered were almost identical to the conditions any survivors of the Ark would have encountered had they actually existed. Meanwhile, neither they nor the few surviving animals would have had nothing to eat save one another.

Old Earth creationists who try to fit the facts as established by science into the Bible narrative in Genesis, might also like to note this progression from single-celled algae, through the mosses and ferns to the angiosperms, whereas the Bible is perfectly clear on that matter; not only were the first plants photosynthesising, terrestrial flowering plants but they were created before there was a source of sunlight.

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