Darwin: Caught Between Catastrophism And Gradualism

This past Saturday was Guru Purnima and I thought I would share a short post on two of Charles Darwin's mentors who had a big influence on him particularly in the early days of his scientific career. Adam Sedgwick and Charles Lyell were both geologists whom Darwin looked to for advice and inspiration. Although Darwin had a rather broad training in natural history, he initially considered himself a geologist. 

Just before embarking on his voyage aboard the H.M.S.Beagle in 1831, Darwin had spent some time doing fieldwork in Wales with Sedgwick. Over a couple of weeks Darwin became proficient in identifying rock types, describing outcrops, and mapping and interpreting the regional geology. He first met Charles Lyell after he returned from his voyage in 1836, although he had been regularly corresponding with Lyell about geology. 

Adrian Desmond and James Moore's biography Darwin: The Life of a Tormented Evolutionist has a lovely description of Darwin's early encounters with geology as he began his travels aboard the Beagle with the shadow of Sedgwick and Lyell following him around. 

The Beagle has reached St. Jago in the Cape Verde Islands, about 300 miles off the African coast. There Darwin saw a fossil bed, rich in shells and corals, about 30 feet above sea-level. 

An extract from Desmond and Moore's book: 

" Sedgwick in North Wales had inducted him into Cambridge-style geology- a science of violent crustal movements,wrenching strata, and mountain thrusts. But how had this seashell band arrived at this height above the ocean? Lyell's Principle's of Geology could help here, even though Henslow had said to beware. Lyell pictured a world constantly and slowly changing, with the past no more violent than the present - so that today's climates,volcanic activity, and earth movements balance one another, land rises in one area as it falls in another, not cataclysmically, as Sedgwick thought, but gradually".

 Darwin studied the fossil band and reasoned that the sea itself could not have fallen over the lifetime of St Jago islands (he was wrong in this assumption, but at that time the causal link between short term climate change and sea-level fluctuations was just not appreciated). The fossil layer did not exhibit any signs of a violent geological change. He decided a gradual uplift of the volcanic island was a better explanation for the stranded fossil bed.

Darwin became and remained a gradualist throughout his lifetime. Gradualism was one of the central ideas of his theory of evolution. That biological change too proceeds slowly was something he had imbibed from Lyell's thinking about geological processes. 

His geological observations about South America were well received by his two heroes. Lyell introduced him to Britain's science elite and Darwin quickly received invitations to join various Geological Societies. He was ready to embark on a serious geology career, but a nagging question about the nature of life eventually took him on a different path. 

His work on transmutation began consuming him as he wholeheartedly devoted his research energies to solve the 'mystery of all mysteries'; how do new species originate and change?  Even after he drifted away from any serious geological work, Darwin remained a close friend with both his mentors. But neither ever embraced his theories of evolution. Sedgwick and Lyell, rooted in Anglican tradition, could not shake of their religious convictions and accept a naturalist explanation for life that Darwin had proposed. 

 Sedgwick was particularly severe in his criticism. He wrote on reading The Origin of Species (quoted from Desmond and Moore's book):

.." Parts of it I admired greatly, parts I laughed at till my sides were almost sore, other parts I read with absolute sorrow because I think them utterly false and grievously mischievous. You have deserted... the true method of induction,and started in machinery as wild, I think, as Bishop Wilkins's locomotive that was to sail with us to the moon".

Sedgwick and Lyell's cold shouldering of his theory caused Darwin immense pain. He had always felt at home with the geology community of Britain whom he thought of as more of a gentlemanly fraternity than the cantankerous zoologists. Desmond and Moore describe how physically sick  Darwin felt after a futile conversation with Lyell about his ideas on transmutation. Shivers, shakes, fever, vomiting became routine maladies throughout his later life, manifestations of the inner turmoil of working on an extremely unpopular theory.  But the dogged and outstanding scientist that he was,  he could not and did not let his mentor's rejection persuade him to stop his work or change his thinking. Patiently and 'gradually'  he put the pieces together and built a body of work that changed the world forever. 

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Fire Initiated The Anthropocene

James C. Scott in his book Against The Grain: A Deep History of the Earliest States makes the case for the transformational impact of fire on our environment.

" Hominids' use of fire is historically deep and pervasive. Evidence for human fires is at least 400,000 years old, long before our species appeared on the scene. Thanks to hominids, much of the world's flora and fauna consist of fire adapted species (pyrophytes) that have been encourage by burning. The effects of anthropogenic fire are so massive that they might be judged, in an evenhanded account of the human impact on the natural world, to overwhelm crop and livestock domestication. Why human fire as landscape architect doesn't register as it ought to in our historical accounts is perhaps that its effects were spread over hundreds of millennia and were accomplished by "precivilized" peoples also known as "savages". In our age of dynamite and bulldozers, it was a very slow-motion sort of environmental landscaping. But is aggregate effects were momentous."

The impact of human activity on the earth's outer skin has been so considerable that atmospheric chemist Paul Crutzen proposed that we are now living in a new geologic epoch, which he called the Anthropocene. This has sparked a vigorous debate on whether a new division of our time scale is justified, and if it is, on where to place its beginning. James C. Scott make a distinction between what he terms the "thick" Anthropocene, contrasting with the idea of a  "thin" Anthropocene. 

A "thick" Anthropocene is manifest by a sudden appearance of a worldwide signal of human activity. Examples of this could be the advent of the Industrial Revolution, or even more catastrophically, the nuclear age in the 1940's which left global radioactive markers.  The "thick" Anthropocene appears to fit more closely geologic convention which demands that the beginning of a new geologic time unit need be recorded by a widespread and more or less synchronous preservation of biological and chemical changes. 

Scott calls fire, agriculture, and domestication as part of the complex that comprises a "thin" Anthropocene. These inventions changed the world patchily and slowly. Its signals appear here and there, not encoded in one geologic layer, but in many, smeared over the past few hundred thousand years. 

The term "more or less synchronous" I used to describe a new geologic boundary is relative to where in the geologic past the observer is. More recent changes are resolvable to a finer degree either as a matter of historical record or by methods like counting tree rings and annual/decadal growth layers in stalactites, cross calibrated by either carbon dating or some other radiometric dating method. The error bar increases as one goes further back in time. Take the great mass extinctions of the past. There will be a sediment layer to which a geologist can point to and state that this marks the boundary between say the Ordovician and the Silurian or the Permian and the Triassic. But that sediment layer certainly wasn't deposited instantaneously. It likely represents a few thousand years of elapsed time. If you were an observer who spent a few years in the Late Ordovician 443.8 million years ago, the situation would have been more akin to Scott's "thin" Anthropocene, with small changes occurring at different times in different places. You would have been unlikely to have anticipated the profound cumulative shift that would eventually accumulate.

Most mass extinctions which form the basis of the big divisions of geologic time unfolded over thousands of years, but their material record is collapsed into a few feet of sediment. We perceive these geologic turnovers as 'sudden' because the preceding and succeeding periods of relative stability lasted tens of  millions of years and the time the 'boundary layer' spans is unresolvable using our current dating methods.

The exception to this is that fateful day 66.03 million years ago, when a large meteorite struck what is now the Yucatan Peninsula. By the time the dust had settled (literally) in a few weeks, the world had irrevocably changed. The Cretaceous-Paleogene boundary layer is in an absolute time sense a truly instantaneous deposit. We interpret it as instantaneous, not by radiometric dating, but by using our understanding of the physical sedimentation processes that would have been triggered by the impact.

The lesson we can draw from the transformational events from deep geological time is not about the debates over the timing of Anthropocene but on its effect. Like those distant ecologic disruptions, we too have set off biological, chemical and physical processes on a different trajectory than they were several thousands of years ago. The Anthropocene will leave a permanent mark on the many future worlds to come.

Coccolithophore Life Cycles and Calcite Morphology

Our world is full of examples of biological processes leading to exquisite geological products. And none more so than the one observed in the Coccolithophores. These are single celled marine algae. They produce crystals of calcite (CaCO3), which they use to create a shell around their tissue. The shell is called a coccolith. The amount of calcium carbonate used up in these shells is enormous. About 10% of global carbon is fixed in coccolithophores, making them an important carbon sink. 

The shapes of these calcite crystals vary enormously according to species, but also, as I found out in a recently published paper, on life cycle stages of the organism.

Coccolithophores have haploid (one set of chromosomes) and diploid (two sets of chromosomes) life cycles. In a haploid life cycle stage relatively simple rhombic crystals are produced in a vesicle inside the cell. The entire shell (holococcolith) is made up of an aggregation of such rhombic crystals. The diploid life cycle stage produces more complex mineral forms. Here too, the crystals are produced inside a vesicle or a compartment inside the cell, but scientists find that the development of shape may be mediated by silicon. The resulting shell (heterococcolith) is intricately shaped, made  up of a variety of crystal shapes in different species. The functional role of the shells could be varied. They may be providing mechanical stability, helping in maintenance of buoyancy, or in scattering harmful ultraviolet light in the upper column of the ocean.

Take a look at this magnified pictures of holococcoliths (a and c) and heterococcoliths (b and d). Scale bar: a and b = 5 micrometer. c = 500 nanometers. d = 1 micrometer.

Source: Role of silicon in the development of complex crystal shapes in coccolithophores: Gerald Langer et. al. 2021.

The prevailing thinking has been that the holococcoliths and heterococcoliths represent two independent origins of calcification. However, this study finds  that the calcite production sites in both life cycle stages are intracellular, and they likely use the same cellular mechanisms to transport ions, maintain calcium carbonate saturation levels, and to modulate the shape of the growing crystal by suppressing and enhancing specific growth directions. 

Based of this similarity in basic processes the researchers propose that the last common ancestor of this algal group must have had the ability to produce both holo and heterococcoliths. Holococcoliths being simpler represent the ancestral form of biomineralization in these algae. Initially, both haploid and diploid life cycle stages would have produced only holococcoliths. The haploid life stage retained this form of calcification. Subsequently, the diploid phase gained additional functionality to produce more complex crystals. Heterococcoliths thus evolved later in this ancestor,  recruiting silicon to mediate, in not yet fully understood ways, the production of varied crystal shapes. 

These algae acquired the ability to calcify around 250 million years ago. Interestingly, the simpler holococcoliths appear in the fossil record a good 37 million years later than the heterococcoliths. Scientist think that this could be an artifact of poor preservation of the simpler more fragile holococcoliths.

A parallel development in the marine realm has also had an impact on coccolithophores and other biomineralizing species. Another group of algae known as the diatoms started proliferating in the oceans in mid late Mesozoic by around 200-150 million years ago. Diatoms use silicon to produce beautiful skeletons. They progressively became efficient removers of silica from sea water. In the mid Mesozoic, large reefs built by the silica secreting sponges were common in the shallow marine settings. By late Mesozoic -Early Cenozoic times silica sponge communities shifted to deeper water and to higher latitudes, an ecologic displacement, some scientists think, forced by silica limitation in shallow tropical waters.  

By Cenozoic period diatoms had become the dominant silicon extractors from the upper layers of the ocean. So much so, that this diversion of silicon by diatoms impacted  Coccolithophores too. Many species stopped using silicon to mediate crystal growth, instead evolving alternate pathways to build their calcium carbonate shells. 

I love stories of the intricate interplay and feedbacks between evolution and geology. This is a theme I keep returning to. 

 

Dear Email Subscribers

Dear Email Subscribers-

Starting with this post, emails to you will be delivered courtesy MailChimp. Feedburner which has been sending you updates to my blog all these years is discontinuing its email subscription service from July. Your email addresses have already been transferred to MailChimp, so there is no need for any action on your part. 

I did get some feedback while I was testing out this new service. For some Gmail addresses, post were being directed to the Promotions folder. Please do keep an eye out for Rapid Uplift mails in your Promotions folder too and manually move them to your inbox so Gmail learns to treat them as Not Spam. I am still tinkering with the style aspects so you can expect subsequent emails to have differing formatting. The content however will remain strictly geology! :)

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Thank you again for your support and motivation.

Suvrat Kher

Articles: Trace Fossils, Supercontinents, Harappan Hydrology

 Some interesting geology rich readings from the past few weeks:

1) Ichnology is a branch of palaeontology that studies the traces made by organisms in soft sediment. These could be tracks and trails as animals move around on a substrate, or burrows constructed as escape structures or as dwellings, or bite marks on shells and bones. All these are indicative of behavior, which otherwise would be hard to discern from just the fossilized remains of body parts. Science writer Jeanne Timmons has written this lovely article on Ichnofossils and what they tell us about past ecology and animal behavior.

Trace fossils, the most inconspicuous bite-sized window into ancient worlds.

2) The earth has seen over its long geological history episodes of continents coming together to form a supercontinent, then breaking up and drifting apart forwhat seems an eternity, but eventually coalescing to form another giant landmass. When did this supercontinent cycle begin on earth. What are the forces that initiated and subsequently has maintained this mode of surface reconfiguration, and what are its consequences on tectonics, and the physical and chemical evolution of earth. A great review article by Ross N. Mitchell and colleagues.

The Supercontinent Cycle.

3) The rivers that sustained the Bronze Age Harappan Civilization have been the subject of lively research in recent years. Ajit Singh and colleagues have worked on the Markanda river catchment in the Sub-Himalaya dun region. Markanda joins the Ghaggar-Hakra river flowing through present day Harayana, Punjab and Rajasthan. They find that during the Mature Harappan Period (2600 B.C. to 1900 B.C.), large floods in the Himalaya foothill rivers sustained flow in downstream reaches, making  agricultural viable, even as northwestern parts of India experienced a reduction in summer monsoon strength.

Larger floods of Himalayan foothill rivers sustained flows in the Ghaggar–Hakra channel during Harappan age (behind paywall).