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06 May 2020

Zealandia and the Mystery of the Origin of Flowers

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With the publication of his seminal work, The Origin of Species, Darwin solved what the astronomer and one of the greatest scientists of the time, John Herschel, called the 鈥渕ystery of mysteries鈥: extinct species of living beings had been replaced by others through a process of natural selection. But Darwin left other very important mysteries unsolved. Specifically, the 鈥渁bominable mystery鈥 鈥 the enigmatic origin and initial evolution of flowering plants 鈥 has remained without a clear answer through the present day. The recent discovery of a submerged continent east of Australia, Zealandia, could play a part in solving the puzzle that first faced Darwin 140 years ago.

Portrait of Charles Darwin in 1881. Credit: Elliott & Fry

DARWIN鈥橲 ABOMINABLE MYSTERY

In The Origin of Species, published in 1859, Darwin argued that evolutionary changes take place gradually. After the publication of his theory, he tried to explain different scenarios in which evolution apparently occurred abruptly.

He particularly pondered the 鈥渁bominable mystery鈥 (as he referred to it in a letter to Joseph Hooker in 1879) of flowering plants, called angiosperms, which include everything from lowly weeds like wild daisies to trees like apple and pear trees. They seemed to have a sudden origin and explosive diversification starting from the Cretaceous Period, 100 million years ago. Apparently, prior to this period, the only plants that existed were flowerless, what are called gymnosperms. Examples include pines, cypresses, araucaria, etc. which together with ferns, comprised the large forests typical of the time of dinosaurs, the Jurassic period just preceding the Cretaceous.

A scan of the first page of Darwin’s notebook on the transmutation of species. Source: John van Wyhe, ed., 2002- The Complete Work of Charles Darwin Online (http://darwin-online.org.uk/)

While attempting to solve this mystery, he corresponded with Hooker and other writers, and proposed that flowering plants in fact originated prior to the Cretaceous period, evolving slowly rather than in a rapid burst. But this gradual evolution must have occurred in a remote, isolated place 鈥 possibly even submerged in the Southern Hemisphere, an inaccessible location during Darwin鈥檚 time. He postulated that after having evolved and diversified, flowering plants spread to the parts of the Earth where fossil evidence was available.

THE MYSTERY POST-DARWIN

Various research projects conducted since Darwin’s time have shed light on some of the aspects of this mystery, but they haven’t unraveled it fully.

Perhaps the clearest clue comes from various molecular studies, which have revealed that angiosperms must have diverged from other plants (gymnosperms, ferns, mosses, etc.) before the Cretaceous and well into the Jurassic period.

But fossil records don’t go as far back as the molecular studies. What’s more, since Darwin’s time, discoveries that have occurred in the fossil register for angiosperms 鈥 pollen, leaves, fragments of trunks, etc. 鈥 are cause for speculation about the alleged angiosperm fossils from the Jurassic period.

Neither is it very clear which gymnosperm plants gave rise to the angiosperms, with various hypotheses none of which have been fully accepted. One the most recent suggestions is that they could have originated from ferns with seeds after the presentation of a genome duplication followed by selective divergence of some genes, which must have endowed them with considerable adaptability and significant diversification potential. In fact, angiosperms currently constitute the most successful group of plants with between 300,000 and 400,000 species counted, as compared to just a few thousand gymnosperm species.

Today angiosperms are the most successful group of plants, in contrast with the gymnosperm species. Credit: Creative Commons

With regard to the origin of the uniquely special trait of the angiosperms 鈥 the flower 鈥 there are also various hypotheses and no clear consensus. In fact, there currently are no angiosperms with floral traits that are intermediate with gymnosperms. However, there is ongoing investigation into the origin of genetic factors 鈥 specifically, what are called the 聽homeotic genes A, B, and C, which were responsible for the development of the gymnosperm 鈥減rimitive flower鈥 (basically naked flowers lacking a varied structure marked by the calyx and corolla with loose seeds that are not enclosed in a fruit ) into the typical and more complex angiosperm flower (with various whorls, sepals, petals, stamens, and ovaries; with seeds enclosed in a fruit, etc.). The present conclusion is that some genes and genetic factors that determine these structures are new, others were already present in the gymnosperms and continue with the same function, and still others are being reused for different purposes.

It can at last be said that a part of Darwin鈥檚 mystery has been cleared up, insofar as the angiosperms indeed are the evident result of evolutionary processes that 鈥渂ury their roots鈥 (pun intended) in the ages prior to their emergence in the Cretaceous fossil record. But a large part of the mystery persists: what were the initial phases of these processes and where did they occur if they did not leave hints of themselves in the known fossil record.

This is where it is thought that the study of the submerged continent east of Australia, Zealandia, might help identify these pieces of the puzzle.

AND NOW THE ARRIVAL OF ZEALANDIA

There are two fundamental reasons why researchers suggest that Zealandia may be the region of Earth where angiosperms originated and initially diversified 鈥 their disembarkation point to the rest of the world.

First of all because it is on islands like those in New Zealand and New Caledonia (which would be the peaks of the submerged continent) where the greatest number of the oldest groups of angiosperms are found, the arqueo angiosperms, in some cases being endemic, as is the case with what is believed to be the most primitive of angiosperms, the amborella trichopoda, endemic to New Caledonia.

Amborella trichopoda. Source: Wertheim Conservatory, Florida International University, Miami, Florida, United States.

And then, due to its geological history. Before the Cretaceous period, Zealandia formed part of a landmass along with Australia and Antarctica. Zealandia broke away from the mass between 80 and 105 million years ago. Subsequently, the crustal thinning process caused by various tectonic phenomena led Zealandia to sink into the ocean, such that virtually the entire continent was submerged 55 million years ago. After a period of rejuvenation, now only the highest points of the continent rise above the ocean, the islands of New Caledonia and New Zealand, which represent less than 10 percent of the total continent.

If angiosperms originated and initially diversified in Zealandia before it sank, that would explain why the initial phases of the process are not evident in today鈥檚 fossil register; they would be deep underwater. Nonetheless, there would have been enough time prior to the sinking of the continent for the already evolved angiosperms to spread to nearby regions (and then to the rest of the world). In fact, arqueo angiosperms are also abundant in eastern Australia and New Guinea, both regions adjoining the already mentioned island peaks of Zealandia.

Zealandia and New Zealand 90 ma Gondwana, current location of New Zealand in white. Source: Wikimedia
Zealandia and New Zealand 90 ma Gondwana, current location of New Zealand in white. Source: Wikimedia

Ultimately, if all of this is true, Darwin would have been correct with his idea that the angiosperms did not suddenly appear. The studies that are underway in Zealandia might fill in the last pieces of the puzzle.

FOR MORE INFORMATION

While there is a lot written on the subject, the most comprehensive coverage is in a recent article by Sgorbati S. et. Al. 2018. Was Charles Darwin right in his explanation of the 鈥渁bominable mystery鈥. Italian Botanist, 5:25-30.

Manuel Ruiz Rej贸n

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