Plant Ancestry: Where Are the Lines of Descent? – Part 2

Peculiar Characteristics of Plants Defy Common Ancestry (cont. from Part 1)

By Margaret Helder, PhD (Botany)

Note: See Part 1, where Dr Helder discusses the peculiarities of major classes of plants, including mosses, bryophytes and tracheophytes.

Alternation of generations – particularly troublesome

Alternation of generations is, of course, a major feature of land plants that is not seen in algae. Concerning the bryophyte special pattern (mosses, liverworts and hornworts), one specialist declared: “Much controversy exists over how the plant life cycle with its remarkable alternation of generations got started.”⁵  He further points out concerning these small plants that “In all cases the gametophyte is the only independent plant and in most cases it is the long-lived and most highly differentiated stage.”⁶ Nobody would disagree even today.⁷ This is all very well; however, it is the case that the vascular land plants (all other land plant groups) exhibit a pattern of alternation of generations that is exactly opposite to that of the bryophytes!

Moss life cycle (bio.libretexts.org)

Evolutionary specialists have proposed various explanations as to how the bryophyte pattern of alternation of generations was able to develop into the vascular plant pattern. As Rensing (2018) pointed out, many characteristics which land plants need to survive are observed in the haploid gametophyte generation of bryophytes. How did these features become expressed instead in the diploid sporophyte generation of vascular plants? Rensing suggests  that the key features in the gametophyte generation were later “co-opted (recruited)” into the most recent common ancestor of vascular plants for the dominant sporophyte.⁸ It is easy to speculate about ‘co-option,’ but does this actually mean anything?

All options still open

The authors of the 2019 Oct paper in Nature compare mRNA sequences to see if they can detect any patterns of similarity in order to propose lines of descent. They admit that such an effort has, in the past, proved “problematic.” That is a masterful understatement! For example, Gottlieb in 1968 declared concerning bryophyte evolution that

[B]otanists hope to compensate for an utter lack of information with an unusual amount of intuitive genius. Unfortunately the geniuses do not agree, and the liverworts are left in limbo on the evolutionary tree.”⁹

Five years later, H. C. Bold declared on the same topic: “[W]e do not know how these organisms are interrelated. Nor do we know from what earlier organisms the remote ancestors of present-day mosses and liverworts came.”¹⁰ He had already reflected that “Were we to include in our discussion only what is known with certainty, it would be brief indeed.”¹¹ Bowman et al. more recently declared: “Bryophyte phylogenetic relationships remain enigmatic, with nearly every possible topology [branching pattern of evolutionary tree] proposed.” ¹² Later, in 2018, Morris, Puttick et al. published a paper which illustrated seven different lines of descent among the bryophytes with reference to the vascular plants. At the same time, they stated that “almost every possible solution” to the question of lineage in the land plants is “currently considered viable.”¹³

The best-known members of the Tracheophytes (vascular plants) include Pteridophytes (spore-producing plants, like ferns) and seed-producing plants, like Gymnosperms (conifers) and Angiosperms (flowering plants). (DFC)

Most botanists have been doggedly seeking an evolutionary scenario which involves the bryophytes as ancestral to the other land plants. But not everyone was on board. Six botanists from University of British Columbia (in Vancouver, B.C.) in 1969 instead declared: “The Bryophytes form an isolated group; they are not closely related to any other plant group nor have they served as ancestors to other groups of plants.”¹⁴ Wow! That is quite a statement but it is not so out of line with other studies as one might first assume. Gottlieb, about the same time, similarly articulated the idea that “bryophytes and pteridophytes [ferns and fern allies among vascular plants] evolved independently from algae…. The proponents of this idea argue that similarities between bryophytes and higher plants are the result of parallelisms [convergence] in evolution – not true homologies.”¹⁵ Both the Canadians and American Gottlieb were discussing the possibility of separate origins for bryophytes and vascular plants (tracheophytes).

More recently Puttick et al. propose alternatively that the ancestor of the bryophytes was really very complex, much more like vascular plants. Working with sequences of alignments of amino acids in proteins, they introduce their argument thus:

Our results imply that the ancestral embryophyte [bryophyte] was more complex than had been envisaged based on topologies [lines of descent] recognizing liverworts as the sister lineage to all other embryophytes [bryophytes]. This requires many phenotypic character losses and transformations in the liverwort lineage.¹⁶

Jeffrey pine forest with understory of shrubs and herbaceous plants. (DFC)

As far as many botanists have been concerned, with fewer specialized characteristics for existence on land that liverworts demonstrate, these plants seemed suitable candidates for a sister relationship to the other bryophytes and to all the land plants as well. Unfortunately, it has proved impossible to plot evolutionary trees that make sense of liverwort traits vis a vis other groups. Thus Rensing, looking at the more obvious character traits, declares: “[L]iverworts are not the sister lineage of all other embryophytes. Hence, given their reduced complexity, they have apparently undergone a series of losses, and do not represent the earliest land plants in terms of character states.” He therefore concludes that “[L]iverwort traits should be discussed with a secondary loss scenario in mind.”¹⁷

Separate Origins

The take home lesson from all of this is that there are no obvious evolutionary relationships among these plant groups. The situation reminds me of a student in the first course I delivered some years ago in introductory botany. The student exclaimed concerning plant evolution: “I don’t think they have any idea what went on!” Well said, George M. The situation has not changed. The data certainly support separate creations of these significant plant groups. The brief overview in Nature October 31, 2019, skipped over a lot of important information. They simply assumed the reality of an evolutionary tree and went from there. Their proposal is not convincing!

“And to every beast of the earth and to every bird of the heavens and to everything that creeps on the earth, everything that has the breath of life, I have given every green plant for food.” And it was so (Genesis 1:30).      Bison herd, Black Hills, South Dakota (DFC).

References

1. 1. One Thousand Plant Transcriptomes Initiative (James H. Leeben-Mack et al. (total of 38 authors listed on-line) 2019. One thousand plant transcriptomes and the phylogenomics of green plants. Nature 574 # 7780,  679-685. See p. 681.https://doi.org/10.1038/s41586-019-1693-2
2. 2. John L. Bowman et al. (total of 113 authors). 2017. Insights into Land Plant Evolution Garnered from the Marchantia polymorpha Cell 171, 287-304, October 5. See p. 294.
3. 3. Bowman et al. 299.
4. 4. William A. Jensen and Frank B. Salisbury. 1972. Botany: An Ecological Approach. Wadsworth Publishing Company, Inc. pp. 758. See p. 387.
5. 5. Joan Eiger Gottlieb. 1968. Plants: Adaptation Through Evolution. Reinhold Book Corporation. pp. 114. See p. 14.
6. 6. Gottlieb p. 23.
7. 7. Stefan A. Rensing. 2018. Plant Evolution: Phylogenetic Relationships between the Earliest Land Plants. Current Biology 28, R210-R213. See p. R210. “Bryophytes are united by life cycles comprising dominant haploid gametophytes and nutritionally dependent, diploid sporophytes.”  https://doi.org/10.1016/jcub.2018.01.034
8. 8. Rensing p. R210.
9. 9. Gottlieb p. 28.
10. 10. Harold C. Bold. 1973. Morphology of Plants. Harper & Row, Publishers. pp. 668. See p. 287 (quoting V. Watson 1964).
11. 11. Bold p. 285.
12. 12. Bowman et al. 288.
13. 13. Jennifer L. Morris, Mark N. Puttick et al. The timescale of early land plant evolution. Proceedings of the National Academy of Sciences 115 #10, E2274-E2283. See p. E2274. www.pnas.org/cgi/doi/10.1073/pnas.1719588115
14. 14. Robert F. Scagel, Robert J. Bandoni, Glenn R. Rouse, W. B. Schofield, Janet R. Stein, T. M. C. Taylor. 1969. Plant Diversity: An Evolutionary Approach. Wadsworth Publishing Company, Inc. pp. 460. See p. 221.
15. 15. Gottlieb p. 28.
16. 16. Mark N. Puttick et al. The Interrelationships of Land Plants and the Nature of the Ancestral Embryophyte. Current Biology 28, 1-13, March 5. See p. 1.https://doi.org/10.1016/j.cub.2018.01.063
17. 17. Puttick et al. 1.
18. 18. Rensing p. R212.

Margaret Helder completed her education with a Ph.D. in Botany from Western University in London, Ontario (Canada). She was hired as Assistant Professor in Biosciences at Brock University in St. Catharines, Ontario. Coming to Alberta in 1977, Dr Helder was an expert witness for the State of Arkansas, December 1981, during the creation/evolution ‘balanced treatment’ trial. She served as member of the editorial board of Occasional Papers of the Baraminology Study Group in 2001. She also lectured once or twice a year (upon invitation) in scheduled classes at University of Alberta (St. Joseph’s College) from 1998-2012. Her technical publications include articles in the Canadian Journal of Botany, chapter 19 in Recent Advances in Aquatic Mycology (E. B. Gareth Jones. Editor. 1976), and most recently she authored No Christian Silence on Science (2016) which promotes critical evaluation of scientific claims. She is married to John Helder and they have six adult children.