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History of Geology

History of Geology

What rocks tell and how we came to understand it

On the Extinction of Species

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The Dodo

by Hilaire Belloc

The Dodo used to walk around,

And take the sun and air.

The sun yet warms his native ground–

The Dodo is not there!

The voice which used to squawk and squeak

Is now for ever dumb–

Yet may you see his bones and beak

All in the Mu-se-um.

In the 17th century the origin of fossils as the bones of once living animals was fiercely discussed from author to author, the proposed explanations ranged from remains of mythical beings such as dragons, giants and unicorns, to victims of the global food of Noah and to simple inanimate forms generated spontaneously by the earth itself. One reason of the insecurity of scholars concerning this question was the surprisingly limited anatomical knowledge, even considering common animals such as domestic horses and cattle, at these times.

During the 18th century scientific progress makes it obvious that the fossil bones can be compared with bones of modern animals - which raise even more questions. Many identified fossils belong to animals unknown in Europe but found on other continents, why did these species disappear on the old continent?

Thomas Molyneux, an Irish priest, in 1695 assumes in his "A Discourse Concerning the Large Horns Frequently Found under Ground in Ireland...[]" that the giant antlers found in the soil of Ireland are related to the North American elk, extinct locally in Ireland due human hunting, but still alive in other parts of the world (instead the antlers belong, as we now know to Megaloceros, an extinct deer species).

Fig.2. Depiction of Megaloceros skull, from MOLYNEUX 1695.

However some bones are not comparable to anything known even to the greatest European explorers and anatomists – could it be that these species went outright extinct?

Impossible - the third American president and naturalist Thomas Jefferson (1743-1826) explains that if a species can become extinct in a perfect divine creation such a creation can't possibly be so perfect all along, worse - the continuous loss of species would inevitable bring this imperfect creation to a gloomy end.

"The movements of nature are in a never ending circle. The animal species which has once been put into a train of motion, is still probably moving in that train. For if one link in nature's chain might be lost, another and another might be lost, till this whole system of things should be evanish by piece-meal; a conclusion not warranted by the local disappearance of one or two species of animals, and opposed by the thousands and thousands of instances of the renovating power constantly exercised by nature for the reproduction of all her subjects, animal, vegetable, and mineral." JEFFERSON, T. (1797): A Memoir on the Discovery of Certain Bones of a Quadruped of the Clawed Kind in the Western Parts of Virginia. American Philosophical Society Transactions.

Jefferson argued with his research against a new upcoming hypothesis: in February 1796 French naturalist George Cuvier presents during a lecture about known modern and fossil elephants a new species - Elephas primigenius - an extinct creature of distant past unlike every living elephant.

Cuvier not only accepted extinction, but used the disappearance and appearance of fossils in the stratigraphic column of the Tertiary strata of France to divide the history of earth in various, successive faunas, every one destroyed by a revolution of the earth's surface. After the catastrophic event earth got soon repopulated with new species of organisms - however this spontaneous generation wasn't explained by Cuvier- there was a gap in his hypothesis filled by a possible supernatural creation.

Victorian geologist Charles Lyell, who tried to establish geology as serious science without such miraculous interference, tried first to deny and then minimize the role of these single extinction events in earth history. Lyell's hostility against extinction in general was also a consequence of his deny of organic progression (as most naturalists at these times) - implying that organism, or even entire animal classes could go lost, brings to the conclusion that new species must somehow generated and without claiming for divine creation only a transmutation of species would be possible.

Lyell accepted a local extinction of species as consequences of climatic change, concurrence and human activity (like in the case of the Dodo), however these local extinctions were reversible, surviving animals could spread again from a refuge when the conditions were favourable again (…no species may be lost…LYELL 1842).

The apparent distinct succession of fossil faunas, so Lyell, was an artefact of former distribution of land and sea, the missing preservation of land-organisms in marine deposits and the general incompleteness of the geological record. Lyell showed that various sharp boundaries between marine and terrestrial strata, as proposed in Cuviers model of the Tertiary of France, were in fact separated by sediments deposited in lakes and rivers, there was therefore no sudden change, but by slow rise the sea became first a swamp and later land.

Charles Darwin became strongly influenced by the geology of Lyell. Observing at his first stop during the Voyage of the Beagle on the Cape Verde islands (January 16, 1832) sediments enclosed by lava flows and raised above the sea level, but with fossils similar to the shells in the sea nearby (implying no substantial change of acting natural forces and habitats over time), he applied the principles proposed by Lyell and became convinced of the slow, minute and gradual changes of earth surface. Darwin adopted his gradual change model of earth on the biological evolution of life; evolution did not need catastrophic events to explain extinction. He stated that one of the main factors contributing to the evolution of organisms was perpetual concurrence in an overcrowded world, catastrophic events (like a drought) could occur, killing many individuals, but nevertheless this local and rare events were outstripped by the much more significant role of long-lasting, gradual natural selection, where the less adapted organism became extinct by the concurrence and success of the modified variations. As Lyell, Darwin considered the apparent sudden transitions of fossil faunas as an artefact of the imperfection of the geological record - in principle he denied mass extinctions as we today see it in the stratigraphic record.

In 1831 the Scottish horticulturalist Patrick Matthew (1790-1874) published in an appendix of his book "Naval Timber and Arboriculture" a theory about transmutation in nature, which resembles the concept of variation, concurrence and selection adopted also by Darwin and Wallace:

"There is a natural law universal in nature, tending to render every reproductive being the best possibly suited to its condition…As the field of existence is limited and pre-occupied, it is only the hardier, more robust, better suited to circumstance individuals, who are able to struggle forward to maturity…"

Matthew however, in explaining the forces that influenced this process, gave to catastrophic events a significant role, maintaining that mass extinctions were crucial to the process of evolution by eliminating concurrence, and enabling organism to radiate in the now "free world":

" ..all living things must have reduced existence so much, that an unoccupied field would be formed for new diverging ramifications of life... these remnants, in the course of time moulding and accommodating ... to the change in circumstances."

When published, Matthew's book raised little interest, and even if both Darwin and Wallace later recognized his contribution, Matthew "evolutionary" interpretation of the geological record, including extinction events, became almost forgotten.

After "Origin of Species" the main interest of palaeontologists focused on the evolution of species, rather then their extinction. Despite proclaiming to accept Darwin's evolution, many naturalists of the second half of the 19th century struggled with the idea of "random" natural selection (intended as a process without end destination, especially not the human species). This led to the concept of a sort of guided evolution, resembling much more the transmutation of Lamarck, where single species pass trough a development process, with generation, spreading, adaption and finally overspecializiation or degeneration, leading them to extinction. The aberrant ammonites of the Cretaceous sea and the gigantic dinosaurs were seen of such examples of overdevelopment.

Fig.3. "Aberrant" ammonites from NICHOLSON 1877.

The idea of a distinct extinction event acting worldwide remained a neglected idea for the rest of the 19th century and most of the 20th century, even when large scale geological changes, as for example an ice-age, were accepted in the scientific community.

Bibliography:

BUFFETAUT, E. (2004): La misteriosa fine dei dinosauri - Come le grandi estinzioni hanno modificato la vita sulla terra. Universale Storica Newton; Newton & Compton editori, Roma: 189

DARWIN. C. (1872): On the Origin of Species by means of Natural Selection. 6th Edition. John Murray - London

LYELL, C. (1842): Principles of Geology: or, the Modern Changes of the Earth and its Inhabitants, considered as illustrative Geology. Volume III. Hillard, Gray & Co., Boston: 476

MOLYNEUX, T. (1695): A Discourse Concerning the Large Horns Frequently Found under Ground in Ireland, Concluding from Them That the Great American Deer, Call'd a Moose, Was Formerly Common in That Island: With Remarks on Some Other Things Natural to That Country. Philosophical Transactions 19 :489-512

NICHOLSON, H.A.(1877): The ancient Life-history of the Earth - A comprehensive outline of the principles and leading facts of palaeontological science. William Blackwood and Sons, Edinburgh-London: 407

RAMPINO, M.R. (2010): Darwin's error? Patrick Matthew and the catastrophic nature of the geologic record. Historical Biology, Volume 23, Numbers 2-3: 227-230

ROWLAND, S.M. (2009): Thomas Jefferson, extinction, and the evolving view of Earth history in the late eighteenth and early nineteenth centuries. In ROSENBERG, G.D., ed., The Revolution in Geology from the Renaissance to the Enlightenment: Geological Society of America Memoir 203: 225-246

ROTHSCHILD, W. (1907): Extinct Birds. Hutchinson & Co. - London: 244 + 45 plates - Figure of Didus cuccullatus taken from this book

The views expressed are those of the author and are not necessarily those of Scientific American.

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