Darwin’s ‘Origin’ (ch.9-10): What the (imperfect) geological record shows

Every day this week I will be publishing excerpts from my distilled version of On the Origin of SpeciesIf you never have, or never will read it, hopefully this will give you a sense of what an amazing book it is. 

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Chapter 9: On the Imperfection of the Geological Record  

The myriad forms we see today have developed gradually through infinite intermediates, branching and morphing over geological time. If this is true, then surely, examples of these linking forms should exist in the geological fossil record? They do not appear to, and the staccato story told by the fossil record has been used by some as an obstacle to my theory. I would argue, however, that this can be explained by understanding the very incomplete nature of the fossil record.

Even if they are found, we should not underestimate the difficulty of identifying ‘intermediate links’. Consider a horse and tapir, that must have developed from some distant common ancestor. This ancestor would not be a blend of tapir-like and horse-like qualities. In fact, it would probably differ more from each of them than either does to the other.

Some geologists argue that the world is far too young for the gradual changes I describe to have occurred. This cannot be true. The geological layers under our soils can be thousands of feet thick. Imagine the infinitely slow rate at which the sea breaks down rock on beaches, lapping so rarely that algae can still grow. The rock broken down in processes such as this is what forms growing geological layers elsewhere. Vast swathes of time that must have elapsed for the geological strata to build up, during which an extraordinary number of generations of organisms will have existed.

Geology; fossil remains in stone. 1833. Wellcome Collection.
Geology: fossil remains in stone. Credit: Wellcome Library, London.  Coloured engraving. 1833

The build up of geological layers, while slow and gradual, has not been simple or continuous. This has left us a fossil record that is far from complete, lacking the ‘linking’ fossils we might expect. Let us imagine how a region’s fauna might be preserved to illustrate this incompleteness. The Malay Archipelago is about the size of Europe and is one of the richest regions on Earth. Even were every organism that has ever existed in this region to have been preserved perfectly for us in fossil form, we would have but a tiny proportion of all the forms that have existed through Earth’s history. Given that we have only really explored a small portion of the fossils of Europe and America, we must have encountered a minute portion of the full geological record.

Malay Archipelago map

The richness of the flora and fauna of the Malay Archipelago will only have been very partially preserved, however. Species living anywhere except shallow waters will be unlikely to be covered by sediment. Soft-bodied creatures would not usually fossilize. Oscillations in sea and land levels would cause sedimented layers to be broken down when raised within reach of wave action.

Some geologists have argued that new forms or even whole groups of similar species have ‘suddenly’ appeared in the fossil record, and that natural selection cannot explain this. With further exploration and discovery of new fossils, these ‘sudden’ appearances often seem less abrupt. Until recently, whales were thought to have appeared suddenly in Tertiary layers, but older whale fossils have now been found elsewhere.

We have little reason to expect that there should be an unbroken chain of transitional forms presented to us in the fossil record. It seems that nature is expert in erasing the record of its work, leaving us with only a partial and broken story from which to infer the history of the organic world.


‘I look at the natural geological record as a history of the world imperfectly kept, and written in a changing dialect; of this history we possess the last volume alone, relating to only one or two countries. Of this volume, only here and there a short chapter has been preserved; and of each page, only here and there a few lines.’


Chapter 10: On the Geological Succession of Organic Beings

The incomplete nature of our geological record means that we will are unlikely to find those ‘missing links’ between ancient and modern forms that some demand as proof for my theory. Yet, the types of changes we see in this patchy record yield a great deal supporting evidence for natural selection.

Firstly, while we have only a partial geological record, we can still see that species’ forms have changed extremely gradually over geological history. Different species have also changed at very different rates. Some fossils are unrecognizable amongst modern species, while other fossils of strikingly familiar species can be found amongst the remains of long-extinct creatures. Species change in no fixed way through the fossil record, changing more slowly or more quickly, abruptly or more gradually. This change is as a result of the gradual accumulation of beneficial variations.

Geology: fossil remains in stone. Credit: Wellcome Library, London.  Coloured engraving. 1833 
Geology: fossil remains in stone. Credit: Wellcome Library, London.  Coloured engraving. 1833

It may seem surprising that fossil faunas can change simultaneously across large geographical areas. For instance, there is a great deal of similarity between the shelled faunas of European chalk formations and those in far-flung locations like Africa and India. These places’ dissimilar environments can hardly be driving the simultaneous development of similar adaptations in their marine creatures. Instead, new and competitive forms must spread from their area of origin to conquer wider ranges and replace less competitive species.

Extinction never ceases to surprise me, but it appears to be a much more gradual process than generation of a species. I once found the tooth of a horse amongst the fossils of antedeluvian creatures such as a mastodon and Megatherium in South America. When horses today have successfully spread around the globe, what could possibly have led to the demise of this ancient horse? We cannot know the complex circumstances that lead to species’ extinctions.

From the geological record, the lifespan of a species may be only a few geological periods long, or persist for truly vast swathes of time. This depends, crucially, on whether more advanced similar forms develop and replace old species. In this way, the generation of new species, and the demise of the old are inextricably intertwined.

megatherium

It is difficult to say why one form replaces another. Some naturalists like to suggest that more recent forms must be ‘higher’ or more advanced than fossil forms. To some degree this might be right, newer forms having been produced by natural selection through gradual change from older forms and having replaced less developed forms through having some highly developed characteristcs. However, we cannot really define what a ‘higher’ form is, and some ancient forms seem to persist longer than some new forms.

Another strong argument for gradual change through natural selection is the fact that, as a general rule, the older a fossil is, the more it differs from modern species.  The deeper you go in the fossil record, you find that forms increasingly become amalgamations of the characters of modern groups or species. Modern classifications entirely break down for fossil faunas, since the distinguishing characteristics we use to classify today had simply not yet developed.

Lastly the continuity of particular forms within one geographical area is surely an incontrovertible support for natural selection. The ancient mammals of Australasia are direct ancestors of the Australian marsupials, and the ancient mammals of South America are very similar to those found there today.  These ancient animals left their descendants, who were gradually modified but still inherited the characteristics of their ancestors. While the Megatherium and other giants of prehistoric South America may not have left their descendants behind them, the ancestors of our living armadillos and sloths lived alongside these monsters, and their fossils can be found preserved together.


‘each formation, on this view, does not mark a new and complete act of creation, but only an occasional scene, taken almost at hazard, in a slowly changing drama.’


Previous:

Darwin’s Origin: Introduction

Darwin’s Origin, chapters 1-3: variation and struggle

Darwin’s Origin, chapters 4-6: natural selection and some difficulties

Darwin’s Origin, chapters 7-8: instinct and hybridism

Next:

Darwin’s Origin, chapters 11-12: geographies

Darwin’s Origin, chapter 13 and conclusion

Tatu-rose

© Natalie Lawrence: The Manticore 2018
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