All kinds of life are capable of being fossilised – even bacteria, but undoubtedly the most familiar fossils are ancient seashells, such as those seen in the stone of some buildings or in rocky outcrops along the shore. Sometimes the stone seems to contain so many shell fragments that it seems to consist of little else. Fossils of terrestrial life are much less common, primarily because deposits of terrestrial sediments are less common. Land surfaces tend to be areas of erosion, not deposition. Broken down by physical and chemical weathering, by earthquakes and gravity-driven landslides, mountains supply material to areas downslope, and ultimately most of this material is carried by rivers down to the coasts.

Geologists identify sediments as marine or terrestrial after considering a number of factors, including rock type (e.g. limestone, sandstone), sedimentary structure, and relationships with adjacent sediments. Usually the fossils in them are consistent with the picture of the ancient environment that can be built up in this way, with marine life being fossilised in marine deposits and terrestrial life in terrestrial deposits. Sometimes terrestrial organisms are found in marine deposits because they have been washed down to the sea by rivers.

In normal conditions Nature efficiently disposes of dead bodies by bacterial action, decompos- ition through fungi, scavengers and chemical dissolution. Certain forms of life – from moulds to carrion crows – seem designed specifically to carry out this role of body disposal. Even if a corpse is buried in the ground, Nature will soon find ways of breaking it down and recycling its constituents. Without exceptional conditions, all parts of an organism eventually decay and disappear. A fossil is therefore the remains of an organism that broke the normal rule.

Fossils occur in varying states of preservation. Well preserved plant and animal fossils – such as those displayed in museums – are rare. More commonly, it is clear that the organism underwent a considerable amount of decay, and we are left with only a few scattered bones or teeth or parts of a shell. Because they do not decay easily, such hard parts may be little altered from when they were part of the living animal, and extraordinary conditions are not required for their burial and preservation, just a certain rate of sedimentation. By contrast, the fossilisation of soft and delicate parts of a body is exceptional by any standard, requiring 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 the organism will be fossilised – a process which may or may not involve the replacement of organic matter by minerals. If the conditions are too vigorous, they will dismember and destroy the organism.

Fossilisation hardly ever occurs in the modern world, but it is not unknown. 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 shellfish under a permanent layer of sand. But overall, the frequency of fossils in ancient sediments suggests that the world was once a much less stable place than it is today: a world where geological deposition was comparatively rapid and on occasions catastrophic.

On the one hand, conditions were generally stable enough to allow life to flourish. Many organisms have been fossilised in life position, a few dramatically, as when encrusting organisms have formed entire beds and reefs or an animal has been petrified with its prey still in its mouth. Fossils of tracks, burrows, nests and faeces at all geological levels show that the animals once lived at those levels. Ancient delta plains and river-beds preserve in-situ traces of roots. Some plants have been preserved by fire turning them into charcoal and imparted rigidity to the fragile plant tissue. Certain marine rocks such as chalk and diatomite consist almost entirely of fossils – microscopic organisms that found conditions in the sea so congenial, so sunny and rich in nutrients, that they reproduced prolifically; collectively, they were the agents of their own burial and are certainly not to be seen as evidence of anything catastrophic.

On the other hand, the preservation of delicate structures, before they could decay or be weathered away, is an indication that they were buried quickly, in unusual conditions. For example, successive dinosaur tracks typically lie on top of sedimentary packets tens of centimetres thick. The fact that they lie on top of, rather than in, the strata indicates that each stratum was deposited quickly, perhaps in hours, between intervals that must have been much longer. Sometimes sedimentation was so rapid – but brief, as in a flash flood – that entire herds of dinosaurs were entombed. But even here it is clear that the animals must have been living close to where they were buried, and on the same stratigraphic level. The most famous catastrophe in geological history was when an asteroid struck the Gulf of Mexico and played a major part in wiping out entire categories of animals, from dinosaurs on land to ammonites in the sea. Nonetheless the sediments that left a global mark of the event are only a few centimetres thick, and one would be hard pushed to identify a single dinosaur fossil that formed as a direct result.

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.

Fossil order

There is a considerable amount of order and progression in the fossil record. The first 80%, from the earliest undisputed evidence of life near the end of the Archaean to just before the end of the Proterozoic, consists only of bacteria, algae and microscopic plankton. Then follows a brief period called the Ediacaran when more complex life forms appear, with some fauna probably possessing tissues, organs and a digestive cavity. Those features aside, few Ediacaran organisms bear any obvious relationship to the organisms that appear later.

Then come a great array of still more complex body plans, many of them as strange as the Ediacaran fauna and also failing to show evolutionary relationships with predecessors or successors. They pop onto the scene like rabbits from a magician’s hat. Palaeontologists refer to their arrival as the Cambrian Explosion, and it is

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, enriching the early Cambrian fauna.

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 sponges and starfish. 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 macroscopic plant fossils), 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)

The succession suggests ecological, not 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 about the Darwinian story. We might have expected to see organisms becoming sponges, or becoming starfish, over time. But we don’t. The suddenness with which major groups appear makes it difficult to interpret the overall sequence in Darwinian terms:

Most families, orders, classes, and phyla appear rather suddenly in the fossil record, often without anatomically intermediate forms smoothly interlinking evolutionarily derived descendant taxa with their presumed ancestors.

Niles Eldredge, Macro-Evolutionary Dynamics: Species, Niches, and Adaptive Peaks, p. 22 (1989)

Instead of intergrading, the higher categories tend to be sharply separated from the groups that are presumed to be related to them. 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. Occasionally evolution may extend as high as class level (as with trilobites) or even phylum (as with cnidarians) – there are no rules limiting the degree of transformation. But whether the transformation within a group be major or minor, systematic gaps separate the higher categories. The fundamental body plans of today’s world were there from the beginning. When we take extinctions into account, possibly more fundamental body plans existed early on than exist now.

Wonderful evolution

For the most part, the ‘enormous changes in physiology and anatomy’ occur as new groups come on the scene. Once they have appeared, these new groups may also undergo a great deal of evolution. Take sharks, for example. Although there is nothing in the fossil record to support the assumption that these animals arose from fish that were not shark-like, later they gave rise not only to many other families of shark but also to forms as diverse as swordfish, skates and rays. 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, which all descend from a species of land animal. Again, 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 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 is assumed to have been evolutionary only because there are many examples of large-scale evolution within it, commonly at the level of order and below, and these have been assumed to be evidence of Darwinian change, despite few showing a chain of ‘numerous, successive, slight modifications’ or any signs of natural selection having been at work. Some other global process was going on.

Once we escape from the sterile debate which presents 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 one of progressive colonisation 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 in this section explore this alternative scenario.

See also:
The limits of relatedness