Monthly Archives: November 2014

November 4, 1855 (a Sunday)

Frederick Orpen Bower

On this date, the botanist Frederick Orpen Bower was born in Ripon, England. His study of primitive land plants, especially the ferns, contributed greatly to a modern emphasis on the study of the origins and evolutionary development of these plants. A man who did not shy away from theorizing, one of his most productive “working hypotheses” was his application of the alternation of generations model to explaining the way the land was colonized by early plants. This subject was explored most completely in his book entitled The Origin of a Land Flora: A Theory Based upon the Facts of Alternation, published in 1908.

From his many years studying liverworts, mosses, and ferns, Bower concluded that they had evolved from algal ancestors. Bower’s hypothesis states, in essence, that the sporophyte generation (the conspicuous vegetative stage in familiar vascular plants) developed de novo from a haploid alga that lacked a diploid sporophyte generation but instead had merely a diploid zygote (a cell formed by the fusion of two gametes, such as sperm and egg). Before the evolution of embryos, this zygote would have immediately undergone meiosis (to relieve the diploid condition) and produced spores, the propagules of the next haploid generation. Growth of such a spore into a gametophyte is analogous to growth of an isolated human sperm or egg cell into a hypothetical haploid generation. Thus, the sporophyte generation first appeared as an added generation that came into existence as a result of delayed zygotic meiosis – sort of a delayed plant puberty. In other words, what might otherwise have become the new haploid cells of the next generation by chromosome reduction instead retained its diploid character and thus added, aà la Bower, a new generation to the life cycle. The final step of spore production still eventually occurred, but not until after the diploid cells had grown and developed into a new sporophyte generation, in essence an overgrown zygote.

Under Brower’s hypothesis, we suppose that, from the point of view of the gametophyte, the sporophyte generation is like a giant multicellular spore factory. For example, in Coleochaete pulvinata, a modern freshwater green alga, the surface of the mature zygote is covered by a layer of haploid cells, which form ingrowths that penetrate the zygote to provide nutrition. The protected diploid zygote in Coleochaete gives the aquatic alga advantages because many more spores can be produced from a single fertilization event than would be the case if the zygote hurried straight to meiosis and the formation of one of those four spore tetrads so common in the fossil record. Bower’s hypothesis remains to be tested, but if it is correct, the sporophyte generation (diploid cells) came to develop inside (and be protected by) the gametophyte generation (haploid cells) precisely because the arrangement ultimately benefited both generations.

An older, competing hypothesis dating back to 1874 held that the algal ancestor of embryophytes already had had alternation of two generations for a long time and was thus diplobiontic, as opposed to haplobiontic. Haplobiontic organisms, such as humans, have the gametes as the only haploid cells; diplobiontic organisms develop those haploid cells into a multicellular life stage. The diplobiontic hypothesis of 1874 is less favored now because it fails to explain how the sporophytes and gametophytes, which in modern diplobiontic green algae have no long-term physical connection, could have evolved the intimate physical connection, in both nutritional and developmental respects, shared by the haploid and diploid components of all embryophytes.

Bower’s other publications included Ferns (three volumes, published 1923-28) and Primitive Land Plants (1935). Bower was elected Fellow of the Royal Society in 1891 and was awarded the Linnean Medal in 1909, the Royal Medal in 1910, and the Darwin Medal in 1938, the latter “In recognition of his work of acknowledged distinction in the field in which Darwin himself laboured.”

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November 1, 1977 (a Tuesday)

Carl Woese

On this date, the American microbiologist and physicist Carl Woese published a report in the Proceedings of the National Academy of Sciences (PNAS) in which he defined the Archaeabacteria (a new super-kingdom of life) by phylogenetic analysis of 16S ribosomal RNA.  This technique was pioneered by Woese and is now standard practice. By 1990 Woese shortened the name Archaebacteria to Archaea and adopted the term “domains” for the three new branches of life: Bacteria, Archaea, and Eukarya. Archaea are neither Bacteria nor Eukaryotes. In other words, they are Prokaryotes that are not Bacteria. More than twenty kingdoms exist under the domains in the tree of life, far more than the five original kingdoms suggested by R.H. Whittaker in 1969. In fact, Woese found that Archaea are more closely related to Eukarya (plants, animals, fungi, etc.) than to Bacteria. This accounted for the renaming of Archaebacteria, the original name given by Woese, to Archaea.

According to Woese:

The archaea are unique organisms. While prokaryotes in the cytological sense, they are actually more closely related to eukaryotes than to the bacteria. They are of particular interest for this reason alone-they are simple organisms whose study should provide insights into the nature and evolution of the eukaryotic cell. Their study is also central to an understanding of the nature of the ancestor common to all life. The archaea are, of course, interesting in their own right. The group contains both the methanogens and numerous organisms that grow at extremely high temperatures (in some cases above 100°C). As such, they provide potential insights into mechanisms of thermophilia and methanogenesis.

by Carl Woese

The acceptance of the validity of Woese’s classification was a slow and painful process. Famous figures, including Salvador Luria and Ernst Mayr, objected to his division of the prokaryotes. Not all criticism of him was restricted to the scientific level. Not without reason has Woese been dubbed “Microbiology’s Scarred Revolutionary” by the journal Science. The growing amount of supporting data led the scientific community in general to accept the Archaea by the mid-1980s. A shrinking minority of scientists still adhere to concepts of evolutionary radiation, but Woese appears to have been vindicated in his convictions.

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November 1, 1880 (a Monday)

Alfred Lothar Wegener

On this date in Berlin was born Alfred Lothar Wegener, a German meteorologist and geophysicist who first gave a well-developed hypothesis of continental drift. In 1912, he presented a paper entitled “Die Herausbildung der Grossformen der Erdrinde (Kontinente und Ozeane) auf geophysikalischer Grundlage” (“The geophysical basis of the evolution of large-scale features of the earth’s crust”) before the Geological Association of Frankfurt am Main. He suggested that all the present-day continents came from a single primitive land mass, the supercontinent Pangaea, which eventually broke up and gradually drifted apart about 250 million years ago. (A similar idea was proposed earlier by F.B. Taylor in 1910.) It was expanded in 1915 into Die Entstehung der Kontinente und Ozeane (The Origin of Continents and Oceans), the first comprehensive account of the theory of continental drift. Others saw the fit of coastlines of South America and Africa, but Wegener added more geologic and paleontological evidence that these two continents were once joined.

Just like the initial reaction in the scientific community to the idea of evolution in the eighteenth century (see Jean-Baptiste Lamarck), reaction to Wegener’s theory was almost uniformly hostile, and often exceptionally harsh and scathing; Dr. Rollin T. Chamberlin of the University of Chicago said, “Wegener’s hypothesis in general is of the footloose type, in that it takes considerable liberty with our globe, and is less bound by restrictions or tied down by awkward, ugly facts than most of its rival theories.” Part of the problem was that Wegener had no convincing mechanism for how the continents might move.

In the fourth edition of his book, The Origins of Continents and Oceans (1929), Wegener wrote:

Scientists still do not appear to understand sufficiently that all earth sciences must contribute evidence toward unveiling the state of our planet in earlier times, and that the truth of the matter can only be reached by combing all this evidence. . . It is only by combing the information furnished by all the earth sciences that we can hope to determine ‘truth’ here, that is to say, to find the picture that sets out all the known facts in the best arrangement and that therefore has the highest degree of probability. Further, we have to be prepared always for the possibility that each new discovery, no matter what science furnishes it, may modify the conclusions we draw.

Beginning in 1906, interested in paleoclimatology, he went on several expeditions to Greenland to study polar air circulation. Wegener made what was to be his last expedition to Greenland in 1930. While returning from a rescue expedition that brought food to a party of his colleagues camped in the middle of the Greenland icecap, he died, a day or two after his fiftieth birthday. Wegener’s theory found more scattered support after his death, but the majority of geologists continued to believe in static continents and land bridges.

Increased exploration of the Earth’s crust, notably the ocean floor, beginning in the 1950s and continuing on to the present day, has provided overwhelming evidence for the mechanism by which the continents move, called plate tectonics. By the late 1960s, plate tectonics, as well as continental drift, was well supported and accepted by almost all geologists.