Fossil evidence for evolution

We tend to think of Charles Darwin as a kindly-looking old man with a big beard, but let's wind the clock back to the vigorous 22-year old embarking in 1831 on the voyage of a lifetime around the world on the survey ship, HMS Beagle.

Recently down from Cambridge, his training there for the clergy had assured him that species were fixed in character, having been divinely created with perfect fit to their myriad places in nature. 'Transformist' speculation from across the Channel was right out for our Anglican establishment, keen to preserve the social and economic fabric from such revolutionary stuff!

Yet the young man's experiences over the next five years were to inspire a truly revolutionary theory that would replace the old supernatural dogma with an entirely naturalistic explanation for the diversity and adaptedness of life – evolution by natural selection.

Fossil bed in Patagonia [Image: Carlob under CC-BY-NC-ND licence]

His experiences in South America during the Beagle voyage included earthquakes and volcanic eruptions that raised land from the sea, causing him to think about how mountains grew; and fabulous fossil finds, including the remains of extinct mammals buried among the sediments eroded from those same mountains. Little wonder, then, that his passion on return was geology.

And when, after a year or so, he began his evolutionary theorizing, he naturally had high hopes for the evidence that paleontology (the study of fossils) might provide. After all, the similarities between the extinct giant sloths and armadillos whose fossils he had found in Patagonia, and their diminutive relatives still living there today, had already seeded doubts in his mind about the immutability of species.

However, as he later developed his ideas on natural selection and sought evidence for the innumerable small changes by which he expected species to have evolved, the fossil record disappointed him.

Nowhere could he find clear examples of such gradual change in fossil lineages. Instead, he had to turn to the evidence of variability seen in living species and the effects of artificial selection on domesticated animals to support his argument that such gradual change was the way of evolution.

The problem with the fossil record, he suggested, was its incompleteness – like a book from which all but a few pages had been torn, such that life's history had to be reconstructed from the few that remained.

Nevertheless, the fossil record was sufficient to reveal the broad succession of forms through time, in logical agreement with the evolutionary sequence of successively derived groups expected from comparative anatomy and embryology – for example, the first mammals preceded by reptiles, they by primitive amphibians, and the latter in turn by yet earlier fish.

Paleontology thus added significantly to Darwin's case for the branching evolutionary tree of life in 1859's On the Origin of Species and it has continued to do so. Fascinating discoveries over the last couple of decades alone include evidence for the evolution of birds from 'feathered dinosaurs' and, even more recently, fossils have offered an elucidation of the origin of bats.

But we must return to the problematical record of microevolution.

First, it must be said that, characteristically, Darwin was substantially correct in his diagnosis of the problem: the fossil record is, by and large, woefully incomplete.

However, contrary to commonly encountered creationist misinformation, many detailed fossil records of evolving lineages have been documented since Darwin's day - especially since 1972 in response to the challenge issued by Steve Gould and Niles Eldredge with their theory of punctuated equilibrium.

In contrast to the fossil 'Hollywood stars' of the macroevolutionary case histories mentioned above, such as the iconic Archaeopteryx, the microevolutionary shows are mostly played by relatively unspectacular marine and lake-dwelling organisms.

The simple reason is that while the former are exceedingly rare, the latter have the twin advantages of being monotonously abundant and living in environments where sedimentary accumulation can keep a good register of their remains.

For the ultimate key to this business is sedimentary deposition.

Of course, organisms that yield robust hard-parts, such as shells, teeth and bones, are more likely to furnish suitable material for preservation in the first place, but even these will in time be destroyed by myriad chemical and physical assaults operating at the Earth's surface - unless protected by sedimentary burial.

But you need only think, for example, of the intermittent way that flooding rivers wash sediment onto the surrounding flood-plains, to realise how sporadic must be the entrapment of skeletal remains by that means. Even so, such sedimentary 'grab-sampling' every few years or even decades would offer a pretty good record on geological time-scales if only the stuff would stay there.

More often than not, though, it doesn't, with erosion frequently removing earlier deposits.

More important to the overall accumulation of sediment in the long-term is the space made available to accommodate it by geological subsidence at the surface. This could happen, for example, by stretching of the Earth's crust, causing it to thin and sag.

Trilobite [Image: Danacea under CC-BY-NC licence]

And such settings are precisely where some of the best examples of evolving fossil lineages have been recorded. Peter Sheldon, now at The Open University, for example, painstakingly collected and studied some 15,000 trilobite fossils through approximately two million years' worth of muddy sediments that had accumulated in what was then a deep subsiding marine basin in mid-Wales.

In each of eight different lineages, he was able to demonstrate gradual transitions between different species going up through the succession. This is just one of many such studies that have been published in the peer-reviewed scientific literature.

Not all fossil lineages reveal gradual change, however, as many show instead a pattern of small-scale 'wobbling around' of variation within narrow limits (referred to as stasis) occasionally punctuated by rapid shifts to new ranges of variation.

This is the pattern described by Gould and Eldredge, mentioned earlier, though they added the additional guess that the punctuational shifts were particularly associated with speciation – the splitting of a single lineage into separate daughter species.

They based this idea on a model of speciation originally proposed by the great evolutionary biologist, Ernst Mayr, who suggested that small and geographically isolated populations were most likely to undergo rapid evolutionary change (albeit over several generations - not in a single 'hopeful monster-type' generational leap) and thereby bud off from their parent populations as new species.

Large, geographically connected populations by contrast, were, according to Mayr, subject to numerous stabilizing constraints. Hence, Gould and Eldredge argued, the patchy fossil record is unlikely to have captured most instances of the rapid (punctuational) changes in small, localized populations, and to be dominated instead by widespread species lineages showing stasis.

The ensuing debate was thus entirely over the tempo and mode of evolutionary change and never over the fact of evolution itself, contrary to some Creationist misinformation.

Investigations of population genetics and more detailed studies of fossil lineages have subsequently shown that neither Mayr's assumptions concerning the role of population size in speciation, nor Gould and Eldredge's inference about punctuational evolution being linked with speciation are necessarily valid.

Documented fossil examples of understandably elusive rapid punctuational change within single lineages - the collaborative work of Mike Bell in the US and Mark Purnell in the UK on fossil sticklebacks in North American lake deposits for example - and of stasis, together with those examples of gradual change, reveal a tremendous spectrum of evolutionary patterns and rates.

So the focus of research now is to investigate under what circumstances such differences obtain – a continuing and fascinating challenge.

It is sad, though perhaps not surprising, that creationists (including the re-branded 'Intelligent Design' lobby), resolutely ignore such carefully conducted studies, preferring instead to keep re-cycling an already falsified picture of the fossil record that is based merely on uncritical literal belief in religious myths.

As Karen Armstrong has observed in her book, A Short History of Myth, such an approach is nonsensical in view of what biblical scholarship tells us about the variegated origins and historical development of such myths:

"Creation stories have never been regarded as historically accurate; their purpose was therapeutic. But once you start reading Genesis as scientifically valid, you have bad science and bad religion".

To put it bluntly, the contributions of creationists to discussions of evolution today are on a par with what astrologers have to say about astronomy, or crystal healing folk about mineralogy. This year, indeed, we may say that they are exactly 150 years behind the times!