In a letter to a friend, Charles Darwin described the abrupt appearance of the flowering plants in the fossil record as "an abominable mystery". This endearing phrase has continued to be used up to the present day, although many hopefuls have been applauded in the media for having solved the mystery.
Fossil and modern maple leaves
A very helpful review of the recent research in this area is provided by Frohlich and Chase. They report that many new discoveries have sharpened thinking on the early diversity of the angiosperms. Theories that once held sway among researchers have been discarded:
"The 'anthophyte theory', the dominant concept of the 1980s and 1990s, has been eclipsed; Gnetales, previously thought to be closest to the angiosperms, are related instead to other extant gymnosperms, probably most closely to conifers."
Major new areas for research relate to molecular data, and many new ideas have been proposed. There is optimism about a solution:
"Formulation of detailed, testable theories combined with study of fossils and genes has the power to dispel the mystery surrounding the origin of both flowers and angiosperms."
One of the biggest problems is that the fossil plant material shows only crown-group specimens.
"There are no studied fossils clearly representing stem-group angiosperms, that is, of plants related to extant angiosperms but attached below the basal node of extant angiosperms in the tree. Such fossils might provide spectacular direct evidence of morphological change along this unknown stretch of evolutionary history."A few years ago, there was great excitement when one transitional form was found, but this died down when a more convincing analysis was made.
"Archaefructus, originally thought to be a stem-group angiosperm of Jurassic age, is not; it has been re-dated as mid-Early Cretaceous, and its reproductive unit has been reinterpreted as an inflorescence, not a flower."
Has the mystery been dispelled? The title of the review says it all: "the origin of angiosperms is still a great mystery". The authors expect that "evo-devo" concepts will lead to solutions:
"The appearance in the past decade of theories of flower origin, stimulated by developmental genetic data from modern plants, marks a major shift in attempts to solve Darwin's "abominable mystery". By building a model of the common aspects of floral developmental controls and comparing these with common elements of gymnosperm systems, we can build a picture of the genetic architecture underpinning floral structure in primitive angiosperms and test theories of how floral systems could have arisen."
What we have here is yet another example of abrupt appearance: an angiosperm explosion. It is "abominable" to Darwinists because of their commitment to gradualism. Their theory is in tension with the fossil record - and they have to hold this scientific data lightly by saying it is seriously imperfect. Notwithstanding all this, the reviewers retain a foot firmly in the Darwinian camp with this parting comment:
"a palaeobotanical deus ex machina is possible at any time if a fossil is discovered that illustrates intermediate steps in the evolution of critical angiosperm attributes, such as the carpel with its included ovules or the angiosperm stamen with its specialized structure."
After a dozen years of progress the origin of angiosperms is still a great mystery
Michael W. Frohlich and Mark W. Chase
Nature 450, 1184-1189 (20 December 2007) | doi:10.1038/nature06393
Here we discuss recent advances surrounding the origin of angiosperms. Putatively primitive characters are now much better understood because of a vastly improved understanding of angiosperm phylogenetics, and recent discoveries of fossil flowers have provided an increasingly detailed picture of early diversity in the angiosperms. The 'anthophyte theory', the dominant concept of the 1980s and 1990s, has been eclipsed; Gnetales, previously thought to be closest to the angiosperms, are related instead to other extant gymnosperms, probably most closely to conifers. Finally, new theories of flower origins have been proposed based on gene function, duplication and loss, as well as on morphology. Further studies of genetic mechanisms that control reproductive development in seed plants provide a most promising avenue for further research, including tests of these recent theories. Identification of fossils with morphologies that convincingly place them close to angiosperms could still revolutionize understanding of angiosperm origins.
Quote: "The rapid development as far as we can judge of all the higher plants within recent geological times is an abominable mystery."
Darwin, C.R., Letter to J.D. Hooker, July 22nd 1879, in Darwin F. and Seward A.C., eds., More Letters of Charles Darwin: A Record of His Work in a Series of Hitherto Unpublished Papers, John Murray: London, 1903, Vol. II, pp.20-21.
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