All kinds of life are capable of being fossilised – even bacteria – but undoubtedly the most familiar fossils are seashells, such as those seen in the masonry of some buildings or in outcrops along the shore. Sometimes building stones contain so many shell fragments they seem to consist of little else. Fossils of land life are much less common because terrestrial sediments are less common. Land surfaces tend to be areas of erosion, not deposition. Broken down by gravity slides, earthquakes and weathering, mountains supply material to lower-lying areas, and ultimately most of this material is carried by rivers down to the sea.
Sediments may be identified as terrestrial or marine by considering a number of factors, including rock type (e.g. limestone, sandstone), bedding style, and the relation to neighbouring sediments. Fossils of terrestrial organisms usually occur in terrestrial deposits but, if plants and animals get washed down by rivers, they may end up in a marine setting. Marine fossils nearly always occur in marine deposits.
In normal conditions, Nature disposes of dead bodies by means of bacteria, fungi, scavengers and chemical dissolution, and does so very efficiently. Moulds and carrion crows, amongst others, seem specifically designed to carry out this role of body disposal. Even when a corpse is buried, Nature soon finds ways of breaking it down and recycling its constituents. In the absence of exceptional conditions, eventually every part of an organism decays and disappears. A fossil is therefore something that has broken the normal rule.
Museum-quality specimens are nonetheless rare. Most fossils are the remnants of organisms that underwent considerable decay and dismemberment, to leave only a few scattered bones or teeth or parts of a shell. These hard parts may be little altered from when they were part of the living animal. Extraordinary conditions are not required for their preservation, only a certain rate of sedimentation. By contrast, fossilisation of delicate tissues requires both rapid burial and surroundings devoid of oxygen, so that the organism is completely sealed off from natural agents of decay. Up to a point, the faster the rate of sedimentation, the more likely fossilisation will occur – a process which may or may not involve the replacement of organic matter by minerals. If the conditions are too vigorous, they will destroy the organism.
Fossilisation rarely occurs in the modern world, but it is not unknown. Modern coral reefs are essentially living organisms on top of progressively older dead ones. Some of Pompeii’s inhabitants were fossilised two thousand years ago in the eruption of Vesuvius, and some shellfish were fossilised last century in the eruption of Surtsey Island. A severe storm may be enough to bury shells under a permanent layer of sand. But overall, the frequency of fossils in ancient sediments suggests that the world was once a less stable place than it is today: a world where geological deposition was comparatively rapid and on occasions catastrophic.
We stress two points. One is that conditions were generally stable enough for life to flourish. Organisms were fossilised in life position, sometimes dram- atically, as when encrusting organisms formed entire reefs, or an animal was petrified with its prey still trapped in its mouth. Fossils of tracks, burrows, nests and faeces at all geological levels show that animals once lived at those levels. Ancient delta plains and river-beds preserve in-situ traces of roots. Some plants were preserved as fire turned them into charcoal and imparted rigidity to the fragile tissue. Marine rocks such as chalk and diatomite consist almost entirely of the remains of plankton that found conditions in the sea so congenial, so warm and rich in nutrients, that they reproduced prolifically. Accumulating little by little, they were the agents of their own burial and do not speak of anything catastrophic.
The other point is that the preservation of delicate structures is evidence that they were buried quickly, in unusual conditions. Successive dinosaur tracks, for example, typically lie on top of sedimentary packets tens of centimetres thick. The fact that they lie only on top of, rather than in, the strata shows that each packet of sediment was deposited quickly, perhaps in hours. Occasionally a flash flood entombed an entire herd of dinosaurs. In one of the most catastrophic events in history, an asteroid struck the Gulf of Mexico and contributed to the wiping out of entire categories of animals, from dinosaurs on land to ammonites in the sea. But this was no Noah’s Flood. Apart from tsunami deposits, the sediments are only centimetres thick, and one would be hard pushed to find in the fossil record any dinosaur killed and buried on that day.
Thus, when we consider the fossil record as a whole, the impression is of a world that was neither tranquil nor caught in the midst of a raging deluge. It was somewhere inbetween.
There is a considerable amount of order and progression in the fossil record. The first 80%, from the earliest evidence of life near the end of the Archaean to just before the Phanerozoic, consists only of bacteria, algae and microscopic plankton. Then, in a brief period called the Ediacaran, more complex life suddenly appears, of a kind that has defeated attempts to categorise it in relation to modern forms. After them come a great array of still more complex body plans, many as strange as the Ediacaran and also showing no evolutionary relationships with predecessors or successors. They pop onto the scene like rabbits from a magician’s hat. Geologists refer to their arrival as the ‘Cambrian Explosion’ –
perhaps the most striking single event documented by the fossil record. In the strict sense, the explosion refers to a geologically abrupt appearance of fossils representing all except two of the living [animal] phyla that had durable (easily fossilizable) skeletons. … A number of enigmatic organisms of obscure relationships also appear during the explosion.
S. A. Bowring et al., Science 261:1293-98 (1993)
All these radically different body designs (‘phyla’) belong to marine life, and almost all to invertebrates, animals without a backbone, such as crustaceans, anemones and molluscs. Fishes appear in significant numbers only later, and terrestrial animals later still, beginning with invertebrates such as centipedes and millipedes (about the same time as the earliest plant fragments), then insects, amphibians and reptiles, reptile-like mammals, mammals (mostly small and nocturnal), dinosaurs and birds, and finally large mammals, including man.
The invasions of the land by plants and animals were accomplished by enormous changes in physiology and anatomy, which enabled exploitation of new ecospace. The subsequent radiations of land plants, terrestrial tetrapods [four-limbed animals] and insects were explosive.
S. B. Carroll, Nature 409:1105 (2001)
Because the phyla maintain their distinct identity from the moment they appear, the succession suggests ecological rather than evolutionary succession: invasions of new environments by rapidly increasing populations of pre-existing organisms, diversifying as they spread and multiplied.
Just the fact that we can use familiar terms such as sponges and starfish at all to describe organisms as far back as the Cambrian indicates that there is something not quite right with the Darwinian story. We might have expected to see organisms becoming sponges and starfish over time. But we don’t. The only major groups in the fossil record that cannot be classified in terms of the living are those that went extinct, without becoming anything else.
If anything, the discontinuities become sharper as collection continues and the database increases. Evolution tends to be limited to the multiplication of species within the lower categories: within genera, families and orders. Often it may extend as high as class or phylum level – there are no rules limiting the degree of transformation. But whether the transformation was major or minor, wide gaps separate the higher categories. The fundamental body plans of today were there from the beginning. When we take extinctions into account, more fundamental body plans existed early on than exist now.
The ‘enormous changes in physiology and anatomy’ occurred as new groups came on the scene rather than as existing groups evolved. One can point to lineages where a great deal of evolution occurred. Take sharks, for example. Although there is nothing to indicate that these animals arose from creatures that were unshark-like, they subsequently gave rise to swordfish, skates and rays as well as other families of shark. This was ‘evolution’ on a large scale, including (in the electric and torpedo rays) the emergence of new organs. Another example is the various whale, dolphin and porpoise families, all of which descend from some land animal species. The transformation involved was enormous, and should not be downplayed. The coming-into-being of new families and orders can be as wondrous as the sudden appearance of the very first in their line.
But as we have seen, the overall pattern – the progression from marine to terrestrial life, and from various kinds of invertebrate to various kinds of vertebrate – is not an evolutionary one. It has been assumed to make up a single genealogical tree only because there are many examples of large-scale evolution within it, commonly at the level of order and below. Few show, as Darwin expected, a chain of ‘numerous, successive, slight modifications’ or any signs of natural selection being important. Some other global process was going on.
Once we escape from the sterile debate which sets creation and evolution as opposites, it becomes possible to recognise that there is evidence for both: creation of many ancestral kinds, followed by increasing diversity within each kind. Acknowledgement of the one does not imply denial of the other. The process was the progressive colonisation of the world by pre-existing plants and animals. At the beginning there was creation; subsequently, there was a catastrophe that left land and sea so barren that populations, food chains and ecological communities had to be built up again from scratch, stage by stage. Organisms of diverse physiology and anatomy exploited new ecospace, and the radiations were explosive.
The pages that follow explore this alternative scenario, beginning with the catastrophe.