The non-random order in which fossils appear over time can be interpreted in one of four ways. This article summarises these various interpretations.
(1) To begin with the most familiar: Charles Darwin pioneered the idea that life arose naturally, with successively more complex forms evolving as a result of chance mutations. In graphic representation the interpretation looks like this:
Though the point is often overlooked, Darwin did not come up with his theory by looking at the fossil record. The record was too poorly known, and he relied on other evidence, such as variation amongst domestically bred dogs and pigeons, and geographical variation in species. Recognising that plant and animal forms are not static, he predicted that as knowledge increased, investigators would see one great branching pattern. The struggle for existence would have caused variants to be selected much as the decisions of dog and pigeon breeders did, and in adapting to new environments organisms would have become more diverse and complex, one form passing into another along a ‘finely graduated organic chain’. Gradualism was of the essence:
Natural selection acts only by the preservation and accumulation of small inherited modifications… If it could be demonstrated that any complex organ existed which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down.
The Origin of Species, 1859.
Darwin felt he could imagine a finely graduated series of forms meandering from, say, no eye at all to the complex eye of an octopus or eagle, and though it might have been paradoxical to suggest that chance processes could produce exquisite design, he believed such progressions were plausible. However, the ultimate test still lay with the fossil record. The question was whether life had so evolved, not merely whether it could have.
Although he laid the burden of proof on those who doubted his theory, the fossils did not, admittedly, reveal any such pattern:
Why then is not every geological formation and every strata full of such intermediate links? Geology assuredly does not reveal any such finely-graduated organic chain; and this, perhaps, is the most obvious and serious objection which can be urged against my theory.
He put the lack of corroboration down to the record’s being inadequately known. One hundred and fifty years later, that explanation no longer serves. The record is now well sampled and known but still does not reveal the expected pattern. Some lineages show gradual evolutionary change, others jerky change, including a number of quite amazing transformations. What palaeontologists have not been able to trace is a single tree beginning with simple organisms in the sea and ending with an array of complex organisms on land. Organisms appear to have had multiple ancestries. Having had ample opportunity to prove itself, Darwin’s audacious idea has not delivered.
(2) The diluvialist theory – defunct for over a century – was revived in 1961 by John Whitcomb and Henry Morris in their book The Genesis Flood. Their idea was that the entire sequence, from the lowermost fossiliferous strata to all but the uppermost strata, was laid down in Noah’s Flood. They expected to be able to detect a pre-Flood surface (preserved, eroded or completely destroyed) that was overlain by catastrophic Flood deposits. Fossilised terrestrial plants and animals would all have been transported from that original surface.
This theory is also not borne out. From the moment fossils first appear, the geological record is stacked with living surfaces, both in marine and terrestrial settings, as is apparent from the reefs, animal tracks, burrows, droppings, nests and roots that are sometimes fossilised on or in them. Each such surface was a fresh colonisation and the whole succession clearly occupied a substantial length of time.
How, then, should one explain the sequence of fossils, seeing that (whatever diluvialists might say) there certainly are evolutionary lineages within the Phanerozoic sequence? If it is not a record of life gradually creating itself, nor a catastrophic record of organisms that perished in the Flood, there are only two possibilities:
(3) The start of every apparently new lineage in the record represents a fresh act of creation – the view taken, for example, by 19th-century geologist Charles Lyell and 20th-century founder president of ‘Reasons to Believe’, Hugh Ross. Commonly called progressive creationism, this is a theistic attempt to explain the fossil record. Whenever it proves difficult to connect a new form of life to a form that went before, the inference is that God created the organism shortly before its appearance. The days of creation in Genesis are interpreted as aeons hundreds of millions of years long, with numerous creative interventions occurring at different moments in the course of each day. To quote an expositor of Ross, ‘God may have supernaturally intervened millions, possibly even billions, of times throughout the history of the universe.’ As the total number of species ever to have existed does not exceed billions, this comes very close to the fixity of species concept.
The explanation infers creation from biological design, but the acts of creation are now spread across the whole of history and have no chronological logic. If (as sometimes happens) a new discovery pushes back the first appearance of a species by tens or hundreds of millions of years, God is assumed to have created the new form that much earlier. Moreover, whole groups of fossil organisms appear in an order different from the order in which Genesis presents them, notably marine animals before seed plants and trees, and new species of marine animal continue to appear right through to the end of the fossil record. There was never a time when God rested. Darwin himself had little respect for this evolution-avoiding ad-lib invocation of the supernatural. ‘Do they really believe that at innumerable periods in the earth’s history certain elemental atoms have been commanded suddenly to flash into living tissues?’
It is also difficult to see how one should make sense of the creation of the Sun one aeon after photosynthesising plants and four aeons after the Earth. Reasons to Believe suggests that the Genesis passage should be re-translated. Others will see such contradictions as reasons not to believe, and as an indication that the six days of creation cannot be interpreted as six distinct time-divisions of Earth history. It would be surprising if they could. Creation ought to have occurred ‘in the beginning’, not episodically throughout history.
The fact is that bacteria, algae and marine forms appear first, but there is no universal tree of life. Rather, there is stupendous diversification over time from a small number of basic designs. That leaves one final possibility:
(4) The fossil sequence represents the recovery of marine and terrestrial populations after a cataclysm at the base of the geological record. The original landmass was destroyed by water rupturing it from below and asteroids from above, as attested geologically at the end of the period called the Hadean.
Preserved Earth history begins immediately after the catastrophe, as magma surged towards the surface behind the released subterranean waters and renewed its crust. Only now was there an ocean on the surface; there was almost no dry land. The geological record documents the planet’s gradual recovery, beginning with oceanic bacteria and algae, in conditions that were to remain unstable for tens of thousands of years. Continental crust grew by volcanic processes, in the course of which it was repeatedly torn apart and reworked, layer upon layer.
The Hadean cataclysm was not the only global catastrophe in Earth history, albeit the first and by far the most deadly. A second occurred at the end of the Permian period and a third, only slightly less devastating, at the end of the Cretaceous. These mark respectively the end of the Palaeozoic and Mesozoic eras. The macrofossil record is divided into the Palaeozoic, Mesozoic and Cenozoic precisely because of these catastrophes and the extinctions they caused. They had a profound effect on the history of life.
Thus at various points along the way (there were also lesser extinction events) Earth’s recolonisation was severely set back. At the end of the Permian, more than 90% of species are estimated to have gone extinct. The succeeding period was more than a story of continuing recovery from the Hadean cataclysm; it was a recovery from an ecological disaster that had only just happened. The Hadean provides the context and some of the explanation for these later events. Earth has not always been as tranquil as in the last few thousand years, when man, taking advantage of its quietness, has completed the recolonisation process and built cities on it as if it would last forever.