The first exclusively land animals to appear in the fossil record are not four-footed vertebrates but millipedes. Terrestrial Old Red Sandstone deposits in Aberdeenshire have revealed that millipedes date back to at least the mid Silurian. ‘The first air-breathing animal was Scottish’ ran the proud headline of Scotland’s Sunday Herald, when the find was announced. In fact the record-holder was English, since Ordovician trackways of what seem to have been millipedes have been found in the Lake District, even further back in time.

Although it is common to assume (and assert) otherwise, there is no fossil evidence that insects, millipedes, centipedes and spiders arrived on the land by sea. As the authors of the paper which prompted the headline confirmed (Wilson & Anderson 2004), ‘The oldest known millipedes were fully terrestrial.’ They had the spiracles and tracheal system of air-breathers, and had many ‘derived characters’; that is, they showed no signs of having evolved from some other kind of animal. They were not only advanced in morphology but highly diverse, as we know from the discovery of an entirely different order of millipede, also of mid-Silurian date (Wilson 2005).

Invertebrates can be classified into about 30 phyla – groups with fundamentally the same body plan – and of these only 3 are known to have made it onto land: annelids (represented by earthworms), onychophorans (represented by velvet worms) and molluscs (represented by slugs and snails). Arthropods may be a fourth if it turns out that scorpions emerged from the sea.

The other arthropods – insects, millipedes and so on – show no evidence of having had marine equivalents, and appeared on the scene within a remarkably brief span of time. Centipedes are known from the late Silurian, as are the extinct spider-like animals called trigonotarbids and an extinct myriapod called Eoarthropleura. The oldest true spider dates to the early Devonian. The oldest harvestman, another spider-like animal, also dates to the early Devonian. In the paper reporting this find the harvestman was said to suggest ‘an extraordinary degree of morphological stasis … , with the Devonian forms differing little in gross morphology from their modern counterparts’.

Other land-dwelling invertebrates that first appeared in the Devonian include mites, springtails, bristletails, pseudoscorpions (tiny creatures that look like scorpions except that they have no tail) and a second order of arthropleurid known as Microdecemplex. Arthropleurids looked rather like millipedes, but differed from their look-alikes in too many respects to be put in the same taxonomic class. Eoarthropleura and Microdecemplex became extinct around the end of the Devonian. A third order, Arthropleura itself, appeared in the Carboniferous and was a much larger animal than the others, with some examples attaining a length of more than 2 metres. It fed on the rotten trunks of lycopsid trees and other such detritus, and became extinct early in the Permian.

In the Carboniferous all these distinct kinds were joined by a host of other land-dwelling invertebrates: mayflies, dragonflies, cockroaches, crickets, as well as numerous now extinct groups. Around 17 orders of insects are known from the period, and most of them seem to have evolved ex nihilo. After their first appearance, they increased in number of species spectacularly.

What we see is evolution on an amazing scale, but it does not appear to be Darwinian evolution. However many new species arose, they retained the essential identity of their order. The latest cockroaches were still cockroaches, the latest mayflies still mayflies, the latest bristletails still bristletails.

Many new species did arise. Consider the scorpions, which like the millipedes first appeared in the mid Silurian. Numbering today around 1,100 species, they are by no means the most diverse of groups. Nonetheless, if each species lasted on average around 3 million years in conventional time (a conservative estimate), and if the group was once more diverse than today (as seems likely), the total number of scorpion species would be in the hundreds of thousands. According to Darwin’s theory, the differences distinctive of each new species arose by chance, as the advantage conferred by the difference enabled the population to leave more offspring and outcompete other species. But the idea is strained to breaking point by the sheer number of species. More often than not, animals appear to have increased in diversity because they were colonising new territory – avoiding competitors rather than competing with them. Certainly they needed to adapt in such circumstances and adaptation involves differentiation, but they were spreading abroad before their habitats became overcrowded. And until they had their adaptations, the new territory put them at a disadvantage, not an advantage. Had they been obliged to wait on ‘chance’ to equip them with what they needed, they would have been wiped out. The evolution must have been programmed foreordained evolution, where dispersal and adaptation went hand in hand.

Invading the land by air and sea

The fossil record shows that millipedes, centipedes, spiders and insects did not originate from marine precursors. Nor did plants. That means that some other explanation is called for, one that rests on their having been preserved rather than their having evolved into existence. We get some clues from observing how invertebrates recolonise empty territory following volcanic eruptions, such as the island of Surtsey or Mt St Helens. The first animals to invade the land are typically insects and arachnids (notably mites and spiders), and they do so by falling out of the sky – swept up by winds and borne along in the air over hundreds of miles. Similarly, after the end-Hadean cataclysm, wind-transport may have been the way in which tiny invertebrates reached new land. They were transported over the world involuntarily, like plant spores. Animals that were heavier and reproduced less prolifically would have arrived – depending on the distance travelled – later, and mostly on foot.

Water – including seawater – would have provided another means of transport. The cuticles of many springtails, for example, are water-repellant, and experiments have shown that even in agitated seawater springtails can survive for weeks. While their mode of transport is not known, within ten years of Surtsey Island’s formation no fewer than six species of springtail had reached its shores. Driftwood would have provided a slightly more comfortable means of travel.

Thus it is significant that the earliest animals in the terrestrial record are all very small – most less than 2 cm. They are also extremely sparse, becoming more common as time goes on. This is anomalous in a scenario where innovations become fixed in populations because more of the offspring with the innovations survive. Over tens of thousands of years, a mere instant in geological time, invertebrates taking advantage of the opportunities that plants provided might be expected to have overrun the earth, and ought to have been as abundant at the beginning of the multimillion-year period in which they originated as at any other time. But the actual interval between first appearance and what might be considered normal population levels is typically measured in tens, even hundreds, of millions of years. Again, this pattern fits neither the theory of evolution nor its associated timescales. It shows that, initially, populations were few and far between, but gradually global stocks recovered. The question of where the first populations came from is then something the fossil record cannot answer, since further back in time the trail peters out.

That they were preceded by a substantial history is clear from the fact that, sparse though they are, their fossils hardly ever occur in contexts which suggest they were the very first generation of pioneers – isolated, starved of sustenance and perishing where they dropped. Invertebrates tend to occur in association, and close to plant material, in other words in assemblages constituting the remains of comparatively mature ecosystems. The most spectacular such localities – for example, the Ludford Lane Bonebed or the Rhynie Chert (both, as it happens, in the UK) – are known as Lagerstätten. Evidently the plants were there before the animals, and the animals were either living off the plants or off each other: the insects using their mandibles to nibble on the spores, the spiders preying on the insects. The simultaneous appearance of diverse groups of organisms, unrelated ancestrally but related ecologically, is of course of the essence of recolonisation.

But we have not yet looked at the story of how vertebrates conquered the land. Although invertebrates fail to substantiate the Darwinian narrative – even aquatic scorpions were still scorpions after the transition – fish might present a more convincing case. The claim is that at least one lineage evolved limbs, air-breathing apparatus and a great array of other apparently miraculous changes, until it abandoned the water altogether and attained the state of reptile. Anything is possible in the natural world. A tadpole can sprout legs and evolve into a frog in a single generation. Did fish manage to do the same in the course of twenty million years?