Signs of Winter 9: New Ideas on the Evolution of Land Plants

Photo by NASA, Public Domain

(Click here to listen to an audio version of this blog!)

The Earth is about 4.5 billion years old. Life began on Earth (best guess) about 3.5 billion years ago. The first organisms were bacteria: very simple entities on the eventual, massive scale of life, but incredibly complex entities on the immediate scale of physical and chemical systems of ancient Earth!

It is hypothesized that these bacteria lived by breaking down the reduced organic molecules that had accumulated via relatively slow, chemical processes over the Earth’s first billion years of existence. Living organisms, though, use energy rapidly, and the rapid breakdown of the slowly synthesizing food molecules began to exhaust the food supply and triggered the Earth’s first Mass Extinction.

Photosynthesis is a biological process that uses some wavelengths of sunlight to fix carbon dioxide into sugar molecules. It is the energy foundation for almost all present day ecosystems! The initial evolution of photosynthesis saved life from Earth’s first Mass Extinction by providing a reliable and robust source of food energy, but it was also the direct cause of the second Mass Extinction event some 500 million years later.

Stromatolites (in Hoyt Limestone, Saratoga Springs, NY). Photo by Rygel, Wikimedia Commons

The original photosynthesizing organisms were bacteria that lived in (or more precisely “on”) Earth’s ancient oceans . Fossils of these ancient “blue-green algae” (more precisely called “cyanobacteria”) can be found in layered rocks called stromatolites. Some of these stromatolites have been dated to be over 3 billion years old. The great, floating masses of cyanobacteria captured sunlight, fixed carbon dioxide and removed hydrogen  atoms from water to make sugars. They also released a waste product (oxygen) from the cleaved water molecules. Molecular oxygen, quite possibly, had never existed on Earth before this. The buildup of oxygen played havoc with the gas molecules of the volcanically derived, reducing atmosphere of the Earth and transformed it into a gaseous system that began to resemble the atmosphere we know today.

In this Second Mass Extinction any life forms on ancient Earth that were intolerant of oxygen (probably most of them) were killed off or forced to retreat into microhabitats from which the oxygen was excluded. Other organisms, though, evolved metabolic pathways by which the oxygen molecules were rendered harmless. These pathways, coincidently, conveyed great energy generating advantages to some of these species. The species that could tolerate and then utilize the photosynthetically generated oxygen in their energy generating metabolisms (the aerobic species) thrived in this Earth 2.0 (or is it 3.0?) and evolved and diversified into the large, complex life forms we see around us today!

The evolution of photosynthesizing species established a new balance between energy fixation and consumption on Earth! Life, though, was still confined to the oceans. Many problems had to be solved before living organisms could live outside of the sea, and those solutions did not come about for another 2 billion years (just 475 million years ago)!

Green algae on beach. Photo by D. Ramirez, Wikimedia Commons

Green algae are the logical evolutionary forerunners of land plants. They are, though, an extremely diverse group of organisms, and the determination of which type of green algae made the transition from aquatic to terrestrial habitats has been a topic of great speculation and contention.

Historically, the charophytic green algae have been put forward as the most likely evolutionary starting point for land plants. Their physical appearance (they are multicellular and have complex, very “plant-like” structures and shapes), their cellular features and physiologies (they have many of the same enzymes and pigments as green plants) all seem to make their connection to land plants quite logical. As one researcher put it, “they look like underwater plants!”  However, using increasingly robust genome sequencing techniques, researchers noted that the genes of charophytes do not match up very well with the genomes of early plant species. Instead, the genes of another, much simpler green algae group, the Zygnematophyceae, are the closest match to the early plants.

Spirogyra (a Zygnematophyceae green algae). Photo by Bogan, Wikimedia Commons

Ecologically, there are several species of Zygnematophyceae that are able to live outside of purely aquatic environments. These algae grow on rocks and other hard substrates alongside ponds or streams or even in wet forests. These algae, then, have some land-dwelling capabilities built into their ecological skill sets, and the gene sequences of two of these species are very similar to the genes of early green plant species. Further, several of the genes present in these algae (and also found in land plants) are not found in any other algae group!

One of these unique gene sets coded for features that allow the algae and plants to survive a variety of stresses including water deprivation. Modern plants utilize these genes to make drought resistant spores and seeds. These genes also very precisely match up with a number of genes found in soil dwelling bacteria, and it is hypothesized and discussed in a recent paper in the journal Cell (November 14, 2019) that these bacterial genes were transferred en masse from the ancient bacteria to the evolving algal cells that then became land plants.

This is an example of “horizontal” gene flow, a process that is well known in purely bacterial communities. The bacterium to bacterium transfer of genes that increase the pathogenicity of a formerly benign bacterial species or the transfer of genes that code for antibiotic resistance from one species to another are well documented. In fact, they go on in your colon all the time! The transfer of genes from bacteria to the much more complex cells of plants and animals, though, is a process that has been hypothesized but not extensively documented.

Moss (a “primitive” plant). Photo by Y. Semenenko, Wikimedia Commons

A couple of weeks ago we talked about the Tree of Life as a model of evolution (see Signs of Winter 6, January 23, 2020). In the Tree of Life genetic diversity is generated exclusively by mutations within the genomes of each species, and out of these “nodal” increases in genetic diversity new species may arise via “vertical” evolution. In the case of the evolution of land plants, though, and possibly in the evolution of many other groups or species, wholesale additions of genes from other types of living organisms (completely different branches on our Tree) may have occurred. These genes can initiate massive changes in the receiving organisms, and these changes may convey significant ecological or evolutionary advantages to the receiving species. It is as if the branches on the Tree of Life were bifurcating and then fusing with each other sharing genetic information that was thought to be isolated and unique.  The Tree, apparently, is much more fluid and dynamic than we ever imagined!

What are the implications of horizontal gene flow and horizontal evolution? First and foremost it provides a mechanism for very rapid evolutionary transformation! A species can pick up entire genetic sequences that enable them immediately to make new structures or carry out new physiological processes. This type of evolution does not have to inch forward one point mutation at a time!

And, secondly, the reality of horizontal gene flow contains a cautionary message: when we add new genes to a species and then release that species into a community of other types of organisms, there is the possibility (whose probability is not yet known) that those genes may horizontally flow into other species. The phenomenon of herbicide resistant weeds forming near fields with herbicide resistant GMO crops is a possible example (and a very dangerous one!) of this possible horizontal flow!

 

This entry was posted in Bill's Notes. Bookmark the permalink.

Leave a Reply

Your email address will not be published. Required fields are marked *