Chapter 9: On the Imperfection of the Geological Record

Thomas Kuhn -- the one philosopher of science that even ignorami like me have heard of -- said that during periods of 'normal science', researchers only take on problems that they know they can solve. 'Paradigm' is an overused word, but it's a measure of the paradigm-shifting nature of the Origin that in much of it, such as in chapter 9 'On the imperfection of the geological record', Darwin flies blind.

This creates almost as much trouble for the reader as it does for Darwin, as we saw in the previous chapter. The less you can explain, the more you need to describe; where Darwin has cracked a problem, he can give a couple of examples and trust generality to follow. Where he hasn't, he has to go into much more detail. But it shows that, besides the humble tone of his rhetoric, Darwin was also humble in the face of the data, and not afraid to say 'I don't know'.

I'm also reading Moneyball by Michael Lewis at the moment, and yesterday I read a line, describing the baseball statistician Bill James, which I think also applies to Darwin. James, says Lewis, has a "preference for leaving an honest mess for others to clean up rather than a tidy lie for them to admire". Darwin, of course, also left the tools to clean up a great deal of that mess. You can sense Darwin's pivotal place in science as you read the Origin and move between areas of insight and uncertainty. He's clearing biology's bottleneck.

When I began this chapter, my first thought was 'here we go again'. Darwin immediately retraces his steps to poke at something you thought he'd already poked to death -- the lack of intermediate forms in the fossil record. But the opening section actually cleared up some of the confusion I'd had while reading chapter 6, 'Difficulties on theory'. There, I wondered why Darwin didn't emphasize that the divergence of two species from a common ancestor is a process of splitting, not smearing. He makes that point here, which was a relief:

I have found it difficult, when looking at any two species, to avoid picturing to myself, forms directly intermediate between them. But this is a wholly false view; we should always look for forms intermediate between each species and a common but unknown progenitor; and the progenitor will generally have differed in some respects from all its modified descendants.

As I read on, I thought Darwin was being rather coy about the specifics of the geological record. It's a long time before he puts a date on any geological event, and even longer before he names a fossil organism. Then I remembered: no geological dating techniques, no plate tectonics, no Archaeopteryx, or fossil hominids, or Devonian tetrapods, no fossils earlier than the Silurian.

And considering how little he had to go on -- when he does hazard a date, he suggests that "in all probability a far longer period than 300 million years has elapsed since the latter part of the Secondary [Mesozoic] period", when in fact it's more like 65 million -- Darwin, it seems to me, discovered evolution at just about the earliest possible moment. Obviously, I've got no evidence for that assertion.

This massive ignorance (Darwin's, not mine) means that this chapter's subject matter actually presents a far stiffer challenge to Darwin's theory than the difficulties he took on in chapter 6. Darwin has to explain the absence of things that you'd think should be there, which is always harder to do in a satisfactory manner than explain a thing's presence. You get a greater sense of unease than at any previous point in the book:

"I do not pretend that I should ever have suspected how poor a record of the mutations of life, the best preserved geological section presented, had not the difficulty of our not discovering innumerable transitional links between the species which appeared at the commencement and close of each formation, pressed so hardly on my theory."

"[I]f my theory be true, it is indisputable that before the lowest Silurian stratum was deposited, long periods elapsed, as long as, or probably far longer than, the whole interval from the Silurian age to the present day; and that during these vast, yet quite unknown, periods of time, the world swarmed with living creatures. To the question why we do not find records of these vast primordial periods, I can give no satisfactory answer."

For the first time, Darwin seems to doubt himself. One of the main goals of 'On the Origin of Species' is to destroy the idea of the species, and make us see it as a flag of convenience pinned by humans on one region of a continuum. And yet the world, and the rocks, are full of species -- which, furthermore, seem to leap into being fully formed. What to make of that? Late in the chapter, he sounds an underdog note.

"[A]ll the most eminent palaeontologists... and all our greatest geologists ... have unanimously, often vehemently, maintained the immutability of species. ... I feel how rash it is to differ from these great authorities, to whom, with others, we owe all our knowledge."

Although, he concludes, when you think how little we know about the little that has been preserved, perhaps it's not such a worry after all. This chapter is a 26-page argument that absence of evidence does not equal evidence of absence. He puts it rather well (this is from earlier in the chapter):

"From ... our ignorance of the geology of other countries beyond the confines of Europe and the United States; and from the revolution in our palaeontological ideas on many points, which the discoveries of even the last dozen years have effected, it seems to me to be about as rash in us to dogmatize on the succession of organic beings throughout the world, as it would be for a naturalist to land for five minutes on some one barren point in Australia, and then to discuss the number and range of its productions."

How has all the geology and paleontology since Darwin's day affected our view of the quality of the fossil record? Well, there's nothing been found that casts doubt on evolution -- like the rabbit in the Precambrian that JBS Haldane suggested would disprove the theory.

Happily, palaeontologists have now reached universal agreement on the quality of the fossil record.

Only joking! Here's a quote from a recent review by Mike Benton on that very subject:

Currently, palaeontologists fall into two camps, those who are content that the fossil record is adequate to show the broad outlines of the history of life, and those who believe that sampling problems overwhelm the signal in rocks older than perhaps 20 or 30 [million years old].

The debate now centres on whether the patterns seen in the rocks -- mass extinctions, sudden burst of diversification, the timing of the origin of particular groups --reliably reflect the history life, or whether they're artefacts of preservation and sampling. Does diversity, in general, seem to rise through time, for example, because we have more new rocks than old ones?

Molecular clocks -- using DNA differences to estimate divergence times -- have thrown another factor into the mix, because they tend to suggest that most groups originated long before they show up in the rocks.

They had to have arose a bit earlier, of course, but often molecular dates of origin are twice as old as fossil dates. Some molecular studies, for example, put the origin of the animals back to about one billion years ago, hundreds of millions of years before any fossils show up. Some have argued that the improbability of soft-bodied animals being fossilized explains why they don't show up in the rocks. Other think the molecular clocks are wrong.

On Monday, more fossils: 'On the geological succession of organic beings'.