Darwinius masillae and the fallacy of a “missing link”

See thro’ this air, this ocean, and this earth
All matter quick, and bursting into birth:
Above, how high progressive life may go!
Around, how wide! how deep extend below!
Vast chain of being! which from God began;
Natures ethereal, human, angel, man,
Beast, bird, fish, insect, who no eye can see,
No glass can reach; from infinite to thee;
From thee to nothing.—On superior powers
Were we to press, inferior might on ours;
Or in the full creation leave a void,
Where, one step broken, the great scale’s destroyed:
From Nature’s chain whatever link you like,
Tenth, or ten thousandth, breaks the chain alike.

- Alexander Pope, Essay on Man (1734)

Alexander Pope, in this portion from his Essay on Man, demonstrated in lucid prose the social significance that the great chain of being, or scala naturae (“ladder of nature”) had for centuries of philosophers and naturalists. Now, the analysis of a Middle Eocene primate dubbed Darwinius masillae, published in PLoS ONE by Jens Franzen and colleagues, has elicited a firestorm of media commentary as journalists scramble over one another in their rush to label this exquisite fossil discovery as yet another “missing link” in Pope’s great chain. However, as Nature’s editor Henry Gee has succinctly pointed out, the concept of the missing link is an antiquated concept that has no place in modern paleontology.

[Read on below the fold.]

A missing link implies an otherwise smooth gradation in the chain of life, with every space in the cosmos filled and each organism in its proper place. For Pope’s 18th century worldview, and for centuries of thinkers beforehand, it meant that any alteration in the links that bound the system together, whether tenth or ten thousandth, would result in an upheaval of the divine plan. This view of the world was more than simply a way to order nature from highest to lowest; it was also a social philosophy that demanded all economic classes know their place as part of the natural order. It was a vision that permeated all aspects of life and, like Aristotle’s physics or Galen’s medicine, was “known” to be an obvious truth for a millennium.

Arthur O. Lovejoy, in his monumental work The Great Chain of Being, demonstrated that this concept rests on the combination of two important ideas that first appear in the works of Plato and Aristotle: “plenitude”, or the idea that nature is complete and coherent in design, and that of “continuity” between one form of life and another. As Lovejoy stated:

From the Platonic principle of plenitude the principle of continuity could be directly deduced. If there is between two given natural species a theoretically possible intermediate type, that type must be realized – and so on ad indefinitum.

Any perceived “gaps” in this design would suggest that the universe was not filled to its fullest extent and was therefore not considered “good” by the assumed creator. Plato argued that it is wrong for us to think “the world was made in the likeness of any Idea that is merely partial; for nothing incomplete is beautiful.” According to this view, every plant and animal must be in perfect balance with this divine order and all aspects of nature must work together as though they each were organs in the body firmament. Aristotle’s “footnote” to Plato was that nature:

passes so gradually from the inanimate to the animate that their continuity renders the boundary between them indistinguishable . . . For plants come immediately after inanimate things . . . And the transition from plants to animals is continuous.

In this way the divine schema consisted of every part of the world filled to its fullest capacity and every gradation of being represented in its proper place. The balance of nature was complete and, from such perfect order, humans could understand their place in the grand design.

This was clearly understood by the Royal Society of London, according to their first historian in 1667, when they combined this Platonic view with the Baconian formulation of empirical data collection to understand and control the great chain of being:

Such is the dependence amongst all the orders of creatures; the animate, the sensitive, the rational, the natural, the artificial; that the apprehension of one of them, is a good step towards the understanding of the rest. And this is the highest pitch of humane reason: to follow all the links of this chain, till all their secrets are open to our minds; and their works advanc’d or imitated by our hands. This is truly to command the world; to rank all the varieties and degrees of things so orderly upon one another; that standing on the top of them, we may perfectly behold all that are below, and make them serviceable to the quiet and peace and plenty of Man’s life.

Plate from Darwin’s On the Origin of Species illustrating natural selection as a branching tree.

However, this vision of divinely ordered perfection was dramatically ripped apart, link-by-link, on November 24, 1859. The publication of Darwin’s On the Origin of Species revealed the inherent flaw in the great chain of being: species were not created and placed into a stable hierarchy following Pope’s “Beast, bird, fish, insect," rather, each species independently evolved from ancestral forms in a step-wise fashion and were no more “lower” or “higher” than any other. Whereas the great chain saw life arranged horizontally from simpler to complex along a two-dimensional axis, natural selection revealed that life was not a chain but a tree, producing species vertically along a three dimensional matrix from the past to the present. Every species alive today, no matter how “primitive” from our perspective, are the current products of more than 3 billion years of evolution. Horses and horny toads, pandas and people, each species is just as “evolved” as any other, each are the new spring leaves adorning the tree of life.

As a result of this Darwinian revolution, referring to Darwinius masillae as a “missing link” is using an antiquated metaphor completely outside of the worldview that gave rise to it. It would be like referring to a medical breakthrough in the treatment of lung disease by using Galen’s view that it caused a “reduction of phlegmatic humours” in the chest. This new Eocene primate is not a missing link any more than any other fossil find is, whether they be ancestral to humans or ancestral to turkey buzzards. What the term reveals is nothing more than our human chauvinism implying that we were the one and sole purpose of creation.

The more appropriate way to view Darwinius is as the authors do: by referring to it as a “primate of modern aspect,” emphasizing the find as the earliest known specimen of an animal that looked more like us. Whether Darwinius was a “transitional form,” an ancestral species of all living primates, or is simply a twig off the main branch 47 million years lower on the tree, will remain to be seen. Media frenzy aside, good science operates through a slow and methodical analysis. In this way, Pope’s metaphor still has relevance to our worldview today, even if the philosophy he was referring to has long since gone extinct. Evolution may not operate horizontally, by adding links to a linear chain, but science certainly does. The great chain of being may have no relevance to modern biology, but any links that are missed in the scientific process may fall prey to Pope’s warning.

Where, one step broken, the great scale’s destroyed:
From Nature’s chain whatever link you like,
Tenth, or ten thousandth, breaks the chain alike.

References:

Franzen, J., Gingerich, P., Habersetzer, J., Hurum, J., von Koenigswald, W., & Smith, B. (2009). Complete Primate Skeleton from the Middle Eocene of Messel in Germany: Morphology and Paleobiology PLoS ONE, 4 (5) DOI: 10.1371/journal.pone.0005723

Lovejoy, Arthur O. (1936). The Great Chain of Being: A Study of the History of an Idea. Harper and Row, New York.