Plant Evolution

The timing of plant evolution is uncertain. Until the last few years, scientists thought that land plants evolved around 460 million years ago. Now, almost all scientists agree that there were land plants in the mid-Cambrian (515 Ma), and some think that land plants first appeared one billion to 700 million years ago. The difference is between the known fossil record (515 Ma) and the projections of molecular clock DNA analysis of plant evolution (1 Ga or 700 Ma). Regardless of the timing, scientists agree that green algae began to form 2 and 3-dimensional structures in the sea and that the first land plants were bryophytes such as moss, which had a similar structure and life cycle with aquatic plants, and which lived close to the water in moist environments. The following three videos, which you may want to watch, give various perspectives on the origin of land plants.

Prokaryotes such as cyanobacteria and other bacteria do not have any internal membranes. In contrast, eukaryotes like fungi, plants, and animals have a nucleus within a membrane and organelles surrounded by membranes. Blue-green algae are very similar to the chloroplast organelle in plants and algae. They are so similar that biologists have concluded that a eukaryote organism incorporated a blue-green alga cell through endosymbiosis, and that it then became the chloroplast organelle in eukaryotic green algae. There are several reasons to believe that endosymbiosis of blue-green algae and other prokaryotes took place. DNA analysis shows similarities between blue-green algae DNA and plant chloroplast DNA. There is also a similarity between eukaryote mitochondria DNA and heterotrophic bacteria DNA. A second reason to accept endosymbiosis is that secondary membranes around organelles in eukaryotes indicate that one cell engulfed another. Third, scientists observe endosymbiosis in the laboratory, where one cell consumes another cell, incorporates the cell into its structure, becomes dependent on the incorporated cell and cannot live without it. Fourth, the sizes of organelles in eukaryotes are similar to sizes of prokaryotic cells. Fifth, the DNA of mitochondria and chloroplast organelles in eukaryotes is structured in a ring, just as DNA is structured in a ring in prokaryotes; however, the nuclear DNA in eukaryotes does not have a ring structure. Sixth, the protein synthesizing machinery in mitochondria and chloroplasts resembles that of prokaryotes.

The first probable evidence of true eukaryote algae (algae with a nucleus) in the fossil record is the existence of fossil spores dated 1.9 Ga.[4] There are eight modern divisions of algae, but the algae of interest in the history of plants are green algae (Chlorophyta). Fossils of green algae first appear in the fossil record 750 million (Ma) years ago.[5] Green algae have the characteristic green color of plants because they have the same chlorophylls (a and b) and other pigments (for example, a and b carotene) as plants. One of the classes of green algae, the Charophyta, includes two freshwater algae orders, the Charales and the Coleochaete, that are most similar to plants.[6] In fact, the Charophyta class is so similar to land plants that it has been classified at times within the plant kingdom and vice-versa.[7] Similarly, genetic analysis of DNA places land plants within the Charophyta.[8]

Based on morphology and DNA of charophycean algae, scientists propose that charophycean algae gradually became multicellular, proceeding from single cells (Mesostigma viridae) to colonial cells (two cells growing together, Chlorokybales) to filamentous algae (Klebsormidiales) to conjugating (Zygnematales). One charophycean alga, Coleochaete, has a structure that is two-dimensional and is thought to be the precursor to three-dimensional algae and plants.

Recently, a seismic shift has taken place in the timing of plant evolution. Some scientists now think that algae moved to land in the Proterozoic, approximately 700 Ma, rather than during the Phanerozoic in 475 Ma. Figure 6-24 shows the timetree of Streptophyta (Figure 6-23) beginning in 1.6 Ga, which was during the Paleoproterozoic period (early Proterozoic). Streptophyta includes all land plants and most green algae.

Scientists use DNA analysis to construct timelines or phylogenetic trees on plant evolution. This is called a molecular clock. Not all molecular clock analyses predict a Precambrian origin of land plants. The prediction is based on the constraints applied to the molecular clock. For example, scientists can define the beginning of plant evolution as the Cambrian and then the molecular clock will start in the Cambrian. If no constraints are placed on the molecular clock, then plant molecular clocks predict the origin of plants deep in the Precambrian, such as 1 billion (1 Ga) to 700 million (700 Ma) years ago. Both approaches are reasonable. Those that do not place the constraint on the molecular clock assume that evidence of early plants has not been discovered or is lost due to lack of preservation or restricted areas of plant evolution. Those that place constraints on the molecular clock do so based on the known plant fossil record.

Figure 6-23. Streptophyta with cycad in upper laft, aquatic green algae in upper right. Credit: CharaFragilis.jpg: Christian Fischer Moos_auf_Mauer.jpg 3268zauber Polypodium virginianum3.jpg: Jaknouse Cycas_circinalis03.jpg: tatograsso derivativework: Salix- CharaFragilis.jpg Moos_auf_Mauer.jpg Polypodium_virginianum3.jpg Cycas_circinalis03.jpg. Used here per CC BY-SA 3.0.

Figure 6-24 was generated by “Strategy 2” in the paper by the Davies lab.[9] It does not place constraints based on the fossil record. They also showed a Strategy 1 graph that has constrained calibration points based on the fossil record. They stated that this approach indicated an overlap of priors and posteriors, which indicates a problem with the constraints. If the model is not constrained by the fossil record (Strategy 2 and 3), it indicates a much more ancient divergence. Strategy 2 had a significant separation between priors and posteriors, which should be expected.

The Davis lab described the reason that the earliest land plants might not have been fossilized.[10] Primarily, they might not have had hard cell walls. Recently, cryptospores (only from land plants) have been discovered from the Cambrian period, which indicates a more ancient origin of land plants than in the Cambrian Period. This is earlier than the earliest previous fossils in 475 Ma. A new Chlorophyta fossil is 1 billion years old, which indicates a much more ancient origin of land plants. The Chlorophyta are the aquatic algae that are the origin of the Embryophyta, the first land plants.

Figure 6-24. Timetree of Streptophyta inferred from Strategy 2. Node ages are plotted at the posterior means and horizontal bars represent 95% credibility intervals. A total of 22 fossil calibration nodes are marked by red dots. Credit: Su et al. Used here per CC BY-SA 4.0.

According to Figure 6-24, the first algae streptophytes were algae in the water. (Streptophyte algae grade). One of the classes of green algae, the Charophyta, includes two freshwater algae orders, the Charales and the Coleochaete, that are most similar to plants.[11] In fact, the Charophyta class is so similar to land plants that it has been classified at times within the plant kingdom and vice-versa.[12] Similarly, genetic analysis of DNA places land plants within the Charophyta.[13]

Based on the extant types of charophycean algae, scientists propose that charophycean algae gradually became multicellular, proceeding from single cells (Mesostigma viridae) to colonial cells (two cells growing together, Chlorokybales) to filamentous algae (Klebsormidiales) to conjugating (Zygnematales). One charophycean alga, Coleochaete (Figure 6-25), has a structure that is two-dimensional and is thought to be the precursor to three-dimensional algae and plants and evolved approximately 1.1 Ga (Figure 6-24). The three-dimensional Charales alga shown in Figure 6‑26 and 6-27 is also called stonewort and forms plantlike structures in water.

Figure 6-25. Title: British fresh-water algae, exclusive of Desmidieae and Diatomaceae Identifier: britishfreshwate00platescook (find matches) Year: 1882-1884 (1880s) Authors: Cooke, M. C. (Mordecai Cubitt), b. 1825 Subjects: Algae -- Great Britain Publisher: London, New York, Williams and Norgate Contributing Library: MBLWHOI Library Digitizing Sponsor

Figure 6‑26. Chara foetida, var. ß. contraria. Fetid chara. Plate 1915 from Sowerby’s English Botany, 3rd Edition. Vol 12. Cryptogamia. 1886. Credit: Shrewsbury Museum Service.

Figure 6-27. Chara braunii, which is 3-D Chareles algae). 1.1 Ga (Figure 6-24). Credit: Show_ryu. Used here per CC BY-SA 3.0.

There are many similarities at the cellular level between plants and charophycean algae, four of which are plasmodesmata, cytokinetic phragmoplast cell division, meristematic stem cells, and cellulose. Plasmodesmata are narrow tubes (one million per mm²) that connect one cell to the next and allow transfer of nutrients and water through cell walls. Cytokinetic phragmoplast cell division takes place as microtubules pull the chromosomes apart and form a cellulosic cell wall between the dividing cells. Meristematic cells at the end of shoots divide and form new cells as the plants/algae grow. Unlike other algae, charophycean algae as well as plant cell walls are 20 to 25% cellulose. The strength of cellulose rivals that of steel. Because of cellulose, green algae were able to adapt to land and stand upright. The hard cell walls enable land plants to maintain their shape and geometry outside of water. Comparison of the reproduction cycles of algae and plants also shows common ancestry.

The simplest land Streptophyta are the Embryophyta (Figure 6-28), which is the clade of Streptophyta that includes all land plants. According to the timetree of the Davies lab (Figure 6-24) This clade diverged from aquatic green algae (Streptophyta) over 1 billion years ago.

Figure 6-28. Embroyphyta (land plants). Credit: Bewareofdog. Used here per CC BY-SA 3.0.

The most durable and best-preserved plant fossils are spores and pollen, and these have been a key tool in identification of plants in the fossil record. Each genera (includes one or more species) has characteristic (Figure 6-29) spore or pollen geometries. Spores are the reproductive bodies of algae, bryophytes, and ferns, and pollen is the male reproductive body (sperm) of seed plants and flowering plants. Spores and pollen fossils are a key tool in the identification of plants in the fossil record.

Figure 6‑29. Pollen and spores. Credit: USGS.

Bryophyta

The bryophytes, such as moss, are the simplest plants and are possibly the first land plants to evolve (Figure 6-24). They have no vascular system, leaves, or roots; however, they are anything but simple. All green algae and plants have interesting reproductive cycles. The bryophytes have alternation of generations (Figure 6-31). They alternate between sporophyte and gametophyte cycles. According to the Davies lab timeline (Figure 6-24), the Bryophyta diverged from the rest of the Embryophyta over 800 million years ago. Until recently, hornworts (Figure 6-30) were thought to have appeared in approximately 460 Ma. Liverworts and mosses evolved after hornworts (Figure 6-24).

Figure 6-30. Bryophyta Hornwort. Credit: Jason Hollinger. Used here per CC BY 2.0

Figure 6-31. The life cycle of a dioicous bryophyte. The gametophyte (haploid) structures are shown in green, the sporophyte (diploid) in brown. Credit: Htpaul. Used here per CC BY-SA 3.0.

Lycophyta: plants with roots and vascular systems

The other branch of the timetree from the Bryophota is the Tracheophyta (Figure 6-24), which includes all land plants except the Bryophyta. The Tracheophyta include all vascular plants. These plants have roots and a central phloem and xylem for transporting nutrients and water through the plant. The first Tracheophyta in the fossil record (Figure 4-20) and in Figure 6-24 is the Lycophyta (Figure 6-32). In Figure 6-24, the Lycophyta diverged 700 Ma from the rest of the Tracheophyta; however, the first Lycophyta in the fossil record is approximately 420 Ma; however, evidence of Lycophyta is 570 Ma (Figure 6-24).

Figure 6-32. Lycophyta Zosterophyllum (420 Ma). Credit. MUSE. Used here per CC BY-SA 4.0.

Ferns: plants with leaves

The other branch from the Lycophyta are the Euphyllophyta. There is evidence in the fossil record of the Euphillophyta at 600 Ma. Within this branch, the ferns, Pteridophyta, appeared first (Figure 6-24). The first fossil is at 390 Ma in the fossil record of the Carboniferous. Unlike the Lycophyta, the ferns had true leaves with multiple veins in the leaf (macrophylls). As with earlier plants, ferns reproduced through spores and had sporophyte and gametophyte phases. The fern sporophyte grew out of the gametophyte and then extended roots into the soil. https://youtu.be/9pLQwa6SyZc. The earliest fern in Figure 6-24 is Equisetum hyemale (Figure 6-33).

Figure 6-33. Horsetail (early fern) colony. Credit: hebdromadairies. Used here per CC BY-SA 2.0.

Gymnosperms: plants with seeds (300-250 Ma)

Figure 6‑34. Pinus sylvestris. Painting by Kohler (1883-1914). Scanned by Thomas Schoepke. www.plant-pictures.de

The other branch within the Euphyllophyta is the Spermatophyta (Figure 6-24), which include seed plants, Gymnospermae, and flowing plants, Angiospermae. There is evidence in the fossil record of Spermatophyta at 360 Ma and of Gymnospermae at 340 Ma (Figure 6-24). Gymnosperms are the first seed-bearing plants in the fossil record: wind transports the male gametophyte (pollen) to the female gametophyte. The plant nourishes the fertilized female gametophyte (embryo). The embryo grows into a seed, which falls to the ground when it matures. The seed protects and nourishes the embryo. Seed-bearing plants are all gymnosperms, which means “naked seed” in Greek. Over time, the four extant phyla of seed-bearing plants evolved: Pinophyta, Cycadophyta, Ginkgophyta, Gnetophyta. The Pinophyta (pine trees), which first began to appear 300 Ma have female gametophytes encased in hard seed cones, and the male gametophytes, pollen, grow on the softer pollen cones (Figure 6-34).

Angiosperms: fruiting and flowering plants

Figure 6‑35. Rice. Oryza sativa L. Painting by Kohler. Scanned by Thomas Schoepke. www.plant-pictures.de

Angiosperms are the fruiting and flowering plants. They began to evolve 250 Ma (Figure 6-24). Angiosperms have become the dominant phylum in the plant kingdom with 260,000 species in 453 families. Unlike the naked gymnosperm seed, the carpel encases angiosperm ovules and seeds, which then develop into fruit. The angiosperm closed carpel evolved in response to two needs: to protect the ovule from beetles and to protect the female ovule from self-pollenization by male pollen on the same plant, which promoted cross-fertilization with other plants.[14] Animals are one of the primary seed dispersal mechanisms for angiosperms. Thus, fruits evolved to have a taste and texture that attracted feeding by animals, which consumed the best-tasting fruits, and they dispersed these seeds in feces. Thus, plants with better tasting fruit were more competitive than plants with less appealing fruit.

One of the two major groups of angiosperms are the monocots, which include flowers such as lilies and orchids, palm trees with a fossil record dating back 80 Ma, and the grasses, which include rice (Figure 6‑35), wheat, barley, bamboo, sugar cane, and other grain crops. Unlike most insect-pollinated angiosperms, the grasses are wind pollinated and include small flowers (florets) that do not have petals and are not attractive to insects. Dried grain is actually a dried fruit and is not a naked seed like gymnosperms.

https://youtu.be/13aUo5fEjNY

Figure 6‑36. Apple tree. Painting by Kohler. Scanned by Thomas Schoepke. www.plant-pictures.de

The second major group of angiosperms includes the eudicots, which have over 200,000 species. This group includes nuts, vegetables, fruit trees, and legumes. Most eudicots are woody plants with woody stems or trunks. Bee-pollinated angiosperms have a beautiful diversity of flowers. Plants that are better attractors of insects (color and shape of flowers), better at attaching pollen to outgoing insects (higher stamens), and better at removing pollen from incoming insects (higher and stickier stigma) would naturally become dominant through natural selection. In addition to colors that are visible to humans, many plants have attractive patterns in the UV range visible to insects. In this way, evolution resulted in the amazing diversity of flowers observed today in nature. Fruit trees (Figure 6‑36) were the last of angiosperms, and thus the last of the plants, to appear in the fossil record.[15]

References

[1] Patricia Sanchez-Baracaldo, P. Hayes, and C. Blank. Morphological and Habitat Evolution in the Cyanobacteria using a Compartmentalization Approach. Geobiology. 3 (2005) no. 3: 145-165.

[2] U. Lüttge, B. Büdel, E. Ball, F. Strube and P. Weber. Photosynthesis of Terrestrial Cyanobacteria under Light and Desiccation Stress as expressed by Chlorophyll Fluorescence and Gas Exchange. Journal of Experimental Botany, 46 (1995) no. 3:.309-319.

[3] Nora Schultz, Photosynthetic Viruses keep World’s Oxygen Levels Up. New Scientist. August 30, 2009. http://www.newscientist.com/article/mg20327235.000-photosynthetic-viruses-keep-worlds-oxygen-levels-up.html

[4] Andrew Knoll, The Early Evolution of Eukaryotes: a Geological Perspective, Sci. 256 (1992): 622–627.

[5] N. King, Review: The Unicellular Ancestry of Animal Development. Developmental Cell, 7 (2004): 313–325.

[6] Jeremy Pickett-Heaps, Green Algae, (Sunderland, MA: Sinauer Associates, 1975).

[7] Rolf Dahlgrenand Kare Bremer, Major Clades of the Angiosperms, Cladistics 1 (1985): 349-368.

[8] Kenneth Karol, Richard McCourt, Matthew Cimino, and Charles Delwiche. The Closest Living Relatives of Land Plants, Science. 294 (2001) no. 5550: 2351-3.

[9] Su, Danyan, Lingxiao Yang, Xuan Shi, Xiaoya Ma, Xiaofan Zhou, S. Blair Hedges, and Bojian Zhong. "Large-scale phylogenomic analyses reveal the monophyly of bryophytes and Neoproterozoic origin of land plants." Molecular Biology and Evolution (2021).

[10] Su, Phylogenomic.

[11] Jeremy Pickett-Heaps, Green Algae, (Sunderland, MA: Sinauer Associates, 1975).

[12] Rolf Dahlgrenand Kare Bremer, Major Clades of the Angiosperms, Cladistics 1 (1985): 349-368.

[13] Kenneth Karol, Richard McCourt, Matthew Cimino, and Charles Delwiche. The Closest Living Relatives of Land Plants, Science. 294 (2001) no. 5550: 2351-3.

[14] David Dilcher, Toward a New Synthesis: Major Evolutionary Trends in the Angiosperm Fossil Record, 97 (2000) no. 13: 7030-6.

[15] Dilcher, New Synthesis, 7030-6.

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Moss, one of the first land plants. Credit: James919. Used here per CC BY-SA 3.0

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