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The weald of Kent: Darwin hesitated to Ussher in a better date

2011; Wiley; Volume: 20; Issue: 4 Linguagem: Inglês

10.1002/evan.20311

1520-6505

Kenneth M. Weiss,

Genomics and Phylogenetic Studies

How old is old? This may seem like a superficial question but, when it comes to evolution, what you think you see may rest heavily on what you can't really see. Evolution is so slow that it has obviously taken a lot of time getting to where it is now. But how much time, and how can we know? If Darwin was right, or even if the Bible was right, then living species had to have arisen at some time or other. That was either when something first wiggled in the primordial soup or when Adam, ever the joker, ribbed Eve. If we're all made in God's image, nothing's changed since we were driven East of Eden or, indeed, since the emergence of the first things that creepeth on the earth. But if, instead, we've all come from plain broth, much has happened since. If evolution takes its own good time, that means a lot of time. One of the classical clowns of biology was Irish Archbishop James Ussher (1581-1656, Fig. 1), who made himself (in)famous by his use of Biblical genealogy to estimate the age of the earth. His “begat” calendar pinned New Earth's Day on Sunday, October 22, 4004 BC (at nightfall). Eve didn't carve any ‘I love Adam’ graffiti on the Tree of Knowledge (none that have been discovered, at least), but it was generally assumed that the creative force was God. Creation was seen as a series of discrete, instantaneous events. Science properly ridicules literal interpretation of this idea these days. Indeed, it may be hard for us to understand how Ussher could have been taken seriously even in his own time. But that may not be entirely fair. Without substantial evidence to the contrary, dating based on received truth was certainly reasonable in principle. But by the nineteenth century, empirical rather than narrative evidence was rapidly appearing and, perhaps more accurately, was becoming formally recognized as the appropriate basis for understanding the world in the new Age of Science. An important message clearly readable from empirical data was evidence of very slow geological changes that could explain otherwise strange discoveries, such as ocean remains like seashells found buried high in inland mountains. This was prehistoric evidence, so it had to be interpreted in some indirect way. Imagining the challenge this raised makes it understandable that religious explanations need not have been viewed as particularly fanciful. In fact, invoking vast, blind, and impersonal but unknown forces might properly have been viewed as suspect. By the late eighteenth century, geologists had begun systematically studying many aspects of the earth's surface structures: changes in coastlines, river courses, volcanic island formation, glaciers, and erosion. The leading nineteenth century figure and the icon of this change was Charles Lyell (1797-1875), a good friend of Charles Darwin.1 Lyell had developed a basically modern scientific geology, approaching diverse types of data with the same empirical, systematic, process- rather than a creation-based approach. Among the many interesting examples he used to illustrate his methodology was the evidence that even since Christian times the ground level at the Roman Temple of Juniper Serapis, outside of Naples, had twice changed about 20 feet as the result of earthquakes (Fig. 2). The evidence, which could be historically documented, included a layer of recent soil between former coastal cliffs and the current beach line, as well as marine growth on a band many feet above ground level around some historically dated pillars of the temple. Lyell stressed that this showed the error of the strongly held contemporary view that it was the sea, not the land, that had changed, a view that had led “the poets of old to select the rock as the emblem of firmness — the sea as the image of inconstancy.”2 As Lyell ends his chapter by quoting a verse by Byron,3 the evidence at Serapis shows that, of the sea “Time writes no wrinkle on thine azure brow; Such as creation's dawn beheld, thou rollest now.” James Ussher. Painting by Peter Lely (16128-1680); Wikimedia public domain. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.] The Temple of Juniper Serapis. The darker, elevated bands on the columns were made by underwater marine life. From Lyell.2 There were lively discussions of geological change in 1831 when Darwin took to the Beagle, avidly absorbing Lyell's work during his voyage.1, 2 Indeed, prominent among Darwin's first published works after his return were insightful geological observations and reconstructions. These established his credibility in the scientific community, which served him well when he later invoked that same slowness of geological processes as a pillar of support for his controversial theory of how life evolved. Though at the time there was little of the many kinds of evidence we have today, it was clear from geology, and from fossils, that the earth was not static, and that its history went back many millions of years, far beyond the Biblical Big Bang. In the first edition of his Origin of Species, Darwin4 gave considerable attention to the age of the earth because of its critical relevance to the question of whether there had been enough time for life's diversity to have arisen by natural selection from a single origin. Knowing that the facts only lead “the mind feebly to comprehend the lapse of time,” he tried to estimate the slowness of time by an example. He was curious about everything, including his local environment, so it was natural for him to pick something close to home. The Weald of Kent (Fig. 3A, B) is a geological region in southeast England, not far from where the Darwins purchased their home, Down House, in 1842. The word “weald” comes from “forest,” but the Weald in Darwin's time was a basin scooped by erosion out of a large dome that had formerly existed there. He estimated the time since the dome had eroded to its present pastoral state, revealing older strata in the process (Fig. 3C): “It is an admirable lesson to stand on the North Downs and to look at the distant South Downs [where one can]…picture [for] oneself the great dome of rocks which must have covered up the Weald within so limited a period as since the latter part of the Chalk formation.”4 Weald of Kent. A. Overview B. Terrain map. C. Geological cross section. Sources: A. Emmetts Garden, gardens-to-go.org.uk; B. Google maps; C. Kent Geology, Wikimedia. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.] Using existing estimates of the rate at which the sea had eaten into the rocky cliffs in the nearby coast, about 1 inch per century, Darwin estimated that the bowl of the Weald of Kent would have required 306,662,400 years to form. His estimate naturally gave him buoyant comfort in support of his ideas about evolution. As he wrote, “What an infinite number of generations, which the mind cannot grasp, must have succeeded each other in the long roll of years!”4 Long rolls of years were critically important, because change in species had neither been directly observed nor produced even by animal and plant breeders. To Darwin, this meant that a great amount of time was required for the unobservable slowness of the evolutionary process to have occurred. Because it was necessarily based on extrapolation rather than direct measurement, Darwin's estimate required the uniformitarian assumption that currently observable processes were inherent and had applied comparably in the past.1 But as Darwin worked on revisions to the Origin of Species, geologists were using new methods to estimate the age of the earth. Darwin quavered lest his discussion of the Weald might be so far off the mark as to be ridiculed and undermine his theory of evolution. So by the 3rd edition, like the dome of the Weald itself, his discussion of its age disappeared without a trace. Instead, he substituted rather general, hand-waving paragraphs about the length of time that must have occurred on earth and our inability to fathom something so far from our direct experience: “Unfortunately we have no means of determining, according to the standards of years, how long a period it takes to modify a species.”5 One of the threats that worried Darwin was that during these years the prominent physicist, William Thomson (Lord Kelvin, 1824-1907), was applying thermodynamic principles to estimate the age of the earth. If the earth had initially formed as a molten mass, one could calculate how long it would take it, on its way to a heat death, to have cooled to its current temperature. Kelvin's estimate was between 20 and 400 million years. His upper limit was later substantially lowered, which Kelvin thought supported a theistic rather than an evolutionary origin of life. No wonder this posed a problem for Darwin! Atomic theory was rudimentary at the time. Radioactive decay and the thermonuclear nature of the earth's core would not be discovered until long after Darwin. The use of radioactive decay in lava deposits to date archeological sites arrived on the scene only in the twentieth century. For a while, estimates based on these new technologies were in the range of 2 billion years for the age of the earth, but with progressively better understanding the date has gradually increased to about 4.5 billion. Of course the age of the earth only imposes outer bounds for the age of life. Did the increasingly old geological estimates mean that time per se was no longer an obstacle to a theory of evolution? Darwin's guess about the Weald was pretty good, given that we know now that it took about 50-140 million years to form (http://www.kgg.org.uk/kentgeo.html). Darwin reasonably backed away from his own estimate, given the state of science during his lifetime. However, he stuck to his guns about the historical process of evolution and its relation to the origin of species because there was overwhelming evidence for it on its own merits. In a sense, it just had to be true. Thus, no matter what geologists might estimate, time simply has to have been enough. Darwin didn't have the wealth and variety of evidence that we have today. We have radioisotope and other sophisticated dating methods to put fossils in proper order, as well as DNA to compare living individuals. These methods jointly contribute to estimates of the timing of species divergence and to the age of life itself. However, there are subtle issues here. First, there is no unambiguous way to determine what a species is. Morphological differences may be useful when they are substantial, though this is not necessarily so, as shown by the marked global variation within our own species. The usual definition of species, as populations that cannot interbreed, cannot be tested among fossils and/or often even among living organisms. So what does it mean to ask how if there's been enough time for them to form? Genes may be the basis of life, and DNA provides relatively rigorous methods of quantifying the amount of diversity among tested individuals, but there's no DNA-based measure that, by itself, defines species. Even if morphology were a good criterion for species, there is no necessary correspondence between the amount of morphological difference and genetic difference. Moreover, even change in a single gene can produce mating incompatibility,6 yet such a small difference could never be detected by sequence comparisons alone unless you already knew which gene was responsible, which is rarely the case. That's because there is always variation in millions of places across the genome even among individuals within a species. Mutations occur very, very rarely. We can estimate the rate experimentally or observationally by, for example, comparing a child's DNA to that of its parents. Fortunately for the child, but unfortunately for the molecular clock-reader, only two to three nucleotides are mutated per ten million nucleotide bases transmitted between a human parent and one of its children. Since an offspring has two parents, and the genome has about 3 billion base pairs, that's roughly 150 new mutational events in each new person. These changes then accumulate over generations so that, using modern sequencing technology, we can obtain an enormous amount of data. In samples of individuals or comparisons between putative species, this large amount of data allows us to average over the highly probabilistic nature of individual mutation events and treat the number of nucleotide differences we find as approximate ticks of time since common ancestry, when the differences began to accumulate. Indeed, we can even directly tally sequence differences between modern species like ourselves and relatively recent fossils, such as some Neandertal specimens in which DNA is still preserved. The argument does become at least a bit circular because we use fossil ages estimated from the rocks they were found in to calibrate the molecular genetic clock to estimate separation times. Nonetheless, there is a rough correspondence, and genetic estimates for the origin of life are consistently converging on a very old date, around 4.0 billion years, which fits snugly within the geologically estimated time for the earth itself. That is, we can be pretty confident about the general picture, even if many species-timing details remain unclear. Think what might have been: The arrival of molecular data didn't guarantee that there would be smooth sailing for the theory of evolution. If genetic and geological estimates had turned out to be wildly discordant, we would have had a big problem. For example, if geologically calibrated fossil dates showed the separation of a group of primates to have occurred only, say, a hundred thousand years ago, but the amount of nucleotide difference seemed to require a hundred million years, then we would be searching desperately for a better explanation of life than Darwin's. It would mean there was some aspect of the evolutionary process that we profoundly did not understand. Something major in our ideas would have to give. What a boon for textbook authors, who could write new editions that really were different from older ones! Ironically, Darwin needn't have stewed over whether there had been enough time for evolution. The reason is not related to geological dates. He could have drawn comfort from his own theory of inheritance. His hypothesized units of inheritance, which he called gemmules, were not carriers of information, as DNA is, but rather were some unspecified kind of direct miniatures of an organism's traits. The gemmules from each body part would end up in the gonads and the collection of them would then transmitted to the organism's offspring. What an organism did during life would mold its gemmules accordingly, and its children would inherit the improved versions. That's adaptation in vivo, and it doesn't require the crude screen of natural selection. If that were how life worked, there would not need to be any particular time constraint. As rapidly as organisms changed their ways of living in response to their circumstances, so would their gemmules be modified. No waiting for Godot. Whatever the age of the earth, it would not have been an issue. Perhaps Darwin didn't realize how his own theory of inheritance would relieve him of any of the tricks the time-trap might play. Still, time can still play tricks on our common sense about evolution. The earliest known fossils provide an example. When we ponder how long life took to evolve into the forms we know, it is sobering to realize that complex, ecologically responsive, social decision-making bacteria had already evolved about 3.8 billion years ago.8 Fossil structures of that age, called stromatolites, bear remarkably detailed resemblances their present-day descendants. They are aggregates of bacteria, not always even of the same species, that form in response to environmental conditions. The first known stromatolites were formed, at most, only a half-billion years from when there was no life at all. And by then they had essentially already reached their modern level of complexity. This shows that the amount of functional difference is not, by itself, an indicator of how hard it is to evolve nor of how much time is needed. If anything, the ancientness of complex life raises a somewhat novel question that is the opposite of Darwin's. If, relatively speaking, from the primal soup to social bacteria, most of life's heavy lifting — the evolution of cells with their highly organized insides and complex monitoring of their outside world — was achieved in a period only a few times longer than it took to form the Weald of Kent, then complex life does not pose a serious question about whether the earth is old enough. Instead, the most interesting question might be why it would take fully seven times longer for those already not-so-primitive cells to generate creatures like the vertebrates, including ourselves, which we spend our lives studying and were of central concern to Darwin. A possible answer that has been widely suggested is that the build-up of developmental and physiological complexity involved the interlocking interactions of hundreds of genes, so that the more life achieved, the harder it became to achieve anything new. There were simply too many constraints for easy radical change. Rather than the abstract question about the absolute need for time, the focus would be placed on the degree to which complexity affects the relative rates of life's plodding pace. Are there any implications for understanding nearly brand-new evolution, such as of the divergence among primates or the divergence of humans from common ancestry with other apes? One might feel relieved that the basics were laid down so long ago that we don't have to dig too deeply to understand this divergence. Perhaps only a few genes would be required to make these minor adjustments in body plan or physiology. That may sometimes be the case, but it doesn't seem to be the general answer. Instead, it seems more typical that minor changes in countless genes contribute to the adaptations we spend so much of our time trying to reconstruct. Such change can be relatively fast, since selection works on the net result, the trait itself. But small tinkering around a more rigid framework means that most of the grains in the evolutionary sand piles would be very hard to find. That has been the usual experience to date. In any case, whatever we find today is what there was enough time for under the particular circumstances. Despite his unnerving doubts, Darwin was compelled by the power of his theory to continue digging around, to unearth the truth about slow change. His final book concerned the slow but transforming action of “vegetable mould,”7 or how earthworms undermine the soil and lead to the slow sinking of buildings, burying them so they provide employment for archeologists. Earthworms, in their inexhaustible millions, not only feed the trout in British streams (at least they feed the ones that get away) but, in passing what they eat through the length of their bodies, turn and churn it into “mould,” bit by bit, which then becomes part of the soil. The worms break up the structure of the soil so that heavy objects settle slowly down and much of the worms' detritus moves to the surface. Darwin found an interesting example in England, in a trench that a farmer he knew had dug in his field. The trench revealed the remains of the Roman ruins of Abinger, which had been abandoned about 1,450 years earlier. The farmer had unearthed the red-tiled atrium floor (Fig. 4 G, H), of what had probably been the reception room of a Roman villa. Worms had buried it so completely that the farmer had discovered it only by accident. At that point, about a foot of worm-derived mold covered the land and two deeper, previously existing surface beds were identified (Fig. 4 E, F). Darwin describes how the worms had excavated into the building and undermined it. He was surprised that the ruin had lain so long undiscovered. He observed worms quickly working their way up through the tile-over-concrete floor that the trench had just uncovered. He wrote that he had been told that farmers had never ploughed deeper than 4 inches, so he reasoned there must have been at least that much coverage by the time the land had first been ploughed. A section of the foundations at the Roman ruins at Abinger. Darwin's legend: A, vegetable mould; B, dark earth full of stones, 13 in. in thickness; C, black mould; D, broken mortar; E, black mould; F, undisturbed subsoil; G, tesserae; H, concrete; I, nature unknown; W, buried wall. From Darwin's work on vegetable “mould.'.7 Wherever we look, we see a consistent picture suggesting that, ever since Darwin's time, life, from weald to mould, creeps along ever so slowly. The panoply of organisms we see around us is remarkable, especially since the details of their origins are so difficult to observe. Leave it to someone with deeply creative powers of observation to stress the importance of the accumulation of slow events for us, and to see the evidence in the hills around him and even in the humble worms in his garden. It was a stunningly convincing way to account at once for global resemblance with diversity, the adaptations of organisms to their circumstances, the nature of fossils, and biogeography. Just as slow geological forces can sculpt the earth, evolution sculpted a lot of deeply held but nonscientific ideas about living variation. Yet ironically, once we realized that evolution is a fact, Darwin's question of the adequacy of time became unimportant. It's a remarkable property of a theoretical understanding like that of evolution that today, when we attempt to understand a particular aspect of diversity among or between species, we do not do not ask whether there was enough time for it but, given the estimated time, do ask what combination of mutation, genetic mechanism, chance, selection, population dynamic, and environmental ecology was responsible. Time provides the constraint, but no longer the justification: When our proposed scenario doesn't seem to fit, we may recheck the time estimate, but mainly we recheck our scenario, to make things fit. In that sense, time may rule, but cannot rule out, evolution. I welcome comments on this column: kenweiss@psu.edu. I co-author a blog on relevant topics at EcoDevoEvo.blogspot.com. I thank Anne Buchanan, Holly Dunsworth, and John Fleagle for critically reading this manuscript. This column is written with financial assistance from funds provided to Penn State Evan Pugh professors.