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AmborellaThe First Flower?

How did flowering plants (angiosperms) evolve from non-flowering seed plants (gymnosperms)? Or did they?

When did the first flowers appear on this planet?

And where on Earth did it occur?

These are some of the most hotly-debated questions among botanists today, partly because some of the fossil-based data is at odds with some of the molecular dating analyses (e.g., see Ref. 1 below).

So, what’s the story?

Goodbye Naked Seeds

Non-flowering gymnosperms, such as conifers, bear naked seeds on scales. Angiosperms have seeds encased in remnants of the flower.

Gymnosperms arose about 370 million years ago and dominated the Earth for 250 million years. Then within a few tens of millions of years, angiosperms appeared and their species greatly proliferated. (Currently almost 9 out of 10 land plant species are angiosperms.)

This abrupt origin and highly accelerated rate of diversification of flowering plants is the “abominable mystery” that confounded Charles Darwin. (see Ref. 2 below)

Today, 130 years after Darwin’s lament, this subject remains a perplexing topic among botanists.

3223aThe Missing Link?

Isn’t it exasperating when you’re trying to solve a jigsaw puzzle and discover that some of the key pieces are missing?

One of the most frustrating missing puzzle-pieces of this mystery is: Where are the intermediates between the gymnosperm to angiosperm transition?

In 2002, there was much excitement over the fossil discovery of Archaefructus (illustration on left). This aquatic seed plant fossil was initially dated to the late Jurassic, about 145 million years ago, making it the earliest example of an angiosperm.

But since 2002 this fossil has been found to be not as old as originally thought, only about 125 MYA, and some scientists think it may be a member of the water-lily family.

This would render Archaefructus less primitive than Amborella, which currently sits at the bottom of the angiosperm family tree.

Amborella (see photo at top of the post) is a small shrub with tiny greenish-yellow flowers and red fruit that grows only in the understory in the rain forests of New Caledonia.

The simple answer to the question “Where is the missing link?” is: currently, there isn’t any.

The fossil data are incomplete and difficult to interpret. The molecular (DNA analysis) data from living plants group the gymnosperms all together and the angiosperms all together, with no plant species in between.

The missing link may have gone extinct. And the first flowering plants may not have been diverse or abundant enough to leave their mark in the fossil record. So, short of a very fortuitous fossil discovery, it may never be found.

Pertinent Links:

Click here to listen to an interview with Elizabeth Pennisi regarding her essay in the 4/3/2009 issue of Science magazine on the origin of flowering plants.

11392Nova (PBS) program on the first flower

Smithsonian on Archaefructus

The Deep Time Project (featuring Archaefructus)

Recent News: It was reported in 2011 that fossils of a buttercup-like plant called Leefructus (see image on right) were found and that they are about the same age as Archaefructus .

Bottom line: Where and how flowering plants arose on Earth about 130 MYA is still very much an unsolved mystery.

References

1. Smith, S. A., J. M. Beaulieub and M. J. Donoghue (2010) “An uncorrelated relaxed-clock analysis suggests an earlier origin for flowering plants.” Proceedings of the National Academy of Sciences (USA), Vol. 107, pp. 5897-5902. (Full Text)

2. Friedman, W. E. (2009) “The meaning of Darwin’s “abominable mystery”. American Journal of Botany, vol. 96, pp. 5-21.(Full Text)

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