Apart from scorpions, the first land animals to appear in the fossil record were, until recently, thought to be millipedes, as indicated by a fossil in the Old Red Sandstone at Stonehaven, Aberdeenshire, indirectly dated to the mid-to-late Silurian. ‘The first air-breathing animal was Scottish’ ran the jubilant headline of Scotland’s Sunday Herald when the find was announced. However, in 2017 it was reported that the Old Red Sandstone bed was better dated to earliest Devonian, the next period after the Silurian. In fact, tracks of what seem to have been millipedes have been found in the Lake District dating back to the Ordovician, and even they are not the oldest evidence of terrestrial life. In Canada, wind-produced dunes preserve arthropod trackways – 25 trackways on 11 successive bedding surfaces – that go back to the late Cambrian (MacNaughton et al. 2002). Soil worms could be even older.
Crucially, and despite assertions to the contrary, there is no evidence that any terrestrial arthropods emerged from the sea. Indeed there are no fossils at all to link millipedes, scorpions, insects and so on to earlier animals of a different identity. As the discoverers of the Scottish animal noted (Wilson & Anderson 2004), ‘The oldest known millipedes were fully terrestrial.’ They had the spiracles and tracheal respiratory system of air-breathers. By the early Devonian there already existed at least four distinct orders of millipede (Shear & Edgecombe 2010). At the point when these amazing land-dwellers began to impinge on the record, they already had a considerable evolutionary history behind them.
The question of what constitutes a basic body plan was discussed in relation to the Cambrian Explosion. Now that our story has reached the point where animals were colonising the land, it is necessary to examine the reality of two purported phyla in particular: the arthropods and the chordates.
An arthropod (arthron meaning ‘joint’ in Greek, podos ‘of the leg’) is an animal that possesses an exoskeleton, a segmented body and paired, jointed appendages. These three features are not a random association. By definition, an animal is a living being and therefore capable of voluntary movement, as a larva if not in adult life (Gen 1:20-25). If an animal with an external skeleton is to move efficiently, the skeleton must be flexible as well as rigid, and therefore it must be segmented. By the same token, the skeleton must have segmented appendages attached to it to function as legs. Some arthropod legs branch into two and are termed biramous (from ramus, meaning ‘branch’ in Latin). Analysts used to think that biramous limbs evolved from uniramous limbs, but since phylogenies based on this assumption conflict with phylogenies based on other differences, they now argue that biramous limbs evolved independently. Unjointed arthropod legs are unknown. According to the theory of evolution, they should have preceded jointed legs.
Much the same logic applies to the chordates, comprising tunicates (sea squirts and salps), cephalo- chordates (lancelets) and vertebrates. These differ in so many respects that one may well question whether their few points of similarity justify regarding them as interrelated. Fossil sea squirts first appear in the early Cambrian, or possibly the preceding Ediacaran. According to the report describing the Cambrian specimen, the ‘basic tunicate body plan, once established, has changed very little’ between its first appearance and the present day (Chen et al. 2003). Lancelets comprise only around 30-35 extant species, grouped into three genera. According to a recent study (Satoh et al. 2014), their ‘morphological, physiological and genomic characteristics are unique; hence they should be recognized as a phylum’ rather than a subphylum, as should the tunicates. The oldest cephalochordate in the fossil record is Cathaymyrus, and the oldest vertebrate is Haikouichthys; both, like the oldest sea squirts, appear in the early Cambrian. All three groups are part of the Cambrian Explosion, appearing at the same time as nearly all the other phyla that make up the marine animal kingdom. There is no evidence that they originated from a common ancestor
Common to all chordates is the possession, at some stage in their life cycle, of a flexible rod called a notochord, which protects the soft nerve cord. In vertebrate embryos the notochord develops into a spine, which in addition to protecting the nerve cord supports other bones such as the cranium and rib cage. In order to facilitate movement, the spine, like the arthropod exoskeleton, must be segmented, in this case by interlocking vertebrae. A vertebrate may or may not have legs – fish move by undulation – but if it does, these will be an integral part of its skeleton and also be jointed. Unjointed vertebrate legs are unknown.
There is consequently something arbitrary about treating arthropods as all related to each other, and vertebrates – let alone chordates – as all related to each other. The respective definitions are not deduced from established phylogenies; they merely assume common relationship. Legs – ‘appendages’ for walking – are also fundamental to the arthropod body plan. But this poses a dilemma, for while the chitinous exoskeletons of arthropods are radically different from the bony and cartilaginous endoskeletons of chordates, and therefore it must be correct to differentiate between them, legs in the two phyla must either have evolved independently or have been created independently. Arthropods have legs from the moment they appear. Those in the Cambrian include trilobites, bivalved arthropods, fuxianhuiids, bradoriids, megacheirans, marellomorphs and phosphatocopina. Additionally, radiodonts appeared then but their appendages were designed for swimming rather than walking. All these major groups appeared in the early Cambrian, without precursors in the fossil record. And all went extinct – i.e. they do not account for any of the arthropods in the modern world (hence their unfamiliar names). Yet, as we know from fuxianhuiids, whose soft-part preservation is quite exceptional, they had a heart-and-blood vessel system just like modern arthropods (Ma et al. 2014). Legged arthropods in the Ordovician include horseshoe crabs and eurypterids; in the Silurian, pycnogonids (sea spiders), scorpions and myriapods; in the Devonian, insects, spiders, and trigonotarbids. Again, although they were all arthropods, common ancestry is not the inference that follows. So far as the evidence is concerned, all these groups could be phyla in their own right.
Terrestrial vertebrates have four appendages: four legs, two legs and two arms, or two legs and two wings. Some terrestrial arthropods have wings, others not. Are not wings as fundamental to the respective body plan as the skeleton that supports them – seeing that flight entails differences in brain organisation, musculature, air intake and lung design? Are not hearts, eyes, brains? But even among arthropods there were different types of eye. Within the generalised plans of arthropods and chordates there are more specific plans – Richard Dawkins (2004), for instance, speaks of the ‘bird body plan’ and ‘the body plan of the adult frog’, Bryan Rogers (1997) of the ‘insect body plan’, and so on.
Of the 30 or so invertebrate phyla, only 3 are known to have made it onto land from marine beginnings: the segmented, soft-bodied annelids, represented today by (for example) earthworms; the segmented, soft-bodied onychophorans, represented by velvet worms; and molluscs, represented by slugs and snails.
Terrestrial arthropods show no indication of having had ancestral marine equivalents, and the major groups all appeared on the scene within a remarkably brief period. Centipedes are known from the late Silurian, as are the extinct spider-like animals called trigonotarbids and an extinct myriapod called Eoarthropleura. Trigonotarbids are most commonly found among the fossilised remains of the Carboniferous forests once mined as coal. Since they were carnivores, their presence in the Silurian suggests there were still smaller arthropods about that are not known to the fossil record. They represent the oldest instance of book lungs in any animal, and are unequivocally terrestrial. Despite their spider-like appearance, they share more similarities with scorpions than with spiders, so it seems safe to infer they are not related to spiders at all, or to scorpions. The oldest example had two eyes, but another species, not much later, also had a pair of lateral eyes, each comprising a row of up to ten minor lenses between a larger lower lens and two larger upper lenses. ‘Sea spiders’ are not thought to be ancestral to any terrestrial forms.
The oldest true spiders first appeared in the Late Carboniferous. In contrast to most modern forms, their spinnerets were probably under the middle rather than at the end of the abdomen. An extinct arachnid order called Uraraneida first appeared in the Middle Devonian and may be related to spiders, but if so, the divergence of the two orders must have happened earlier.
The oldest harvestman also dates to the Devonian. Harvestmen are another distinct order that superficially, with their eight legs, look similar to spiders. They comprise more than 6,600 species and four suborders, of which the mite harvestmen are analysed to be the oldest, i.e. they branched from the base of the harvestman phylogenetic tree before the others. The Devonian specimen belonged to one of the evolutionarily more advanced suborders, of which daddy longlegs are members. In the paper reporting the 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’. It was obviously terrestrial.
Like the unrelated trigonotarbids, all modern species have one pair of eyes, either at the front or, in the case of the mite harvestmen, somewhat to the side. However, a Carboniferous example surprised researchers by revealing two pairs, a median pair and a lateral pair. Vestiges of lateral eyes show up in the embryonic development of one extant species, indicating that genes for such complex features can still be present even when switched off later in development.
The catalogue from the Devonian is thus a mix of the familiar and the unfamiliar. Other terrestrial invertebrates appearing at the time include mites, springtails, pseudoscorpions (tiny, still extant 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 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 one example attaining a length of more than 2.5 metres. It was restricted to equatorial latitudes and probably fed on seeds and nuts among the leaf litter, perhaps also other invertebrates. It became extinct in the early Permian. The only insects known from the Devonian are the bristletails, the least evolved of the insect orders.
All these distinct kinds were joined in the Carboniferous by a host of other land-dwelling invertebrates, most of them with wings: mayflies, dragonflies, cockroaches, crickets, as well as numerous extinct groups. Wings were themselves a mysterious innovation. “The two very first winged insects that we have in the fossil record, they’re about as different from each other as you could imagine,” admits entomologist Sandra Schachat (Sci. Am. 318, 18 (2018)). And they are wonderfully engineered. Dragonfly wings, for instance (Sun & Bhushan 2012),
are mainly composed of veins and membranes, a typical nanocomposite material. The veins and membranes have a complex design that … result in dragonflies being supremely versatile, maneuverable fliers. The wing structure, especially corrugation, on dragonflies is believed to enhance aerodynamic performance.
Wings need to be stiff enough to withstand aerodynamic loads yet flexible enough to deform without failure. The hollow tubes of the veins provide the supporting framework, their thickness increasing from tip to root. While the leading edge consists primarily of rectangular frames, the main surface consists of hexagons and other polygons, which optimise stability under stress. Protuberances along the edges reduce drag. Corrugation increases the stiffness and strength of the wing and creates vortices that enhance aerodynamic performance.
Altogether, around 17 orders of insects are known from the period, most of which seem to have evolved ex nihilo. After their first appearance, they increased in number of species spectacularly.
What we witness is evolution on an amazing scale. Only, it is not 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.
There were, eventually, many new species. Consider the scorpions, which like the millipedes first appeared in the mid Silurian. Numbering today around 2,400 species, the group is diverse, and was probably more diverse in the past. Nonetheless, if each species, on a conservative estimate, lasted on average 3 million years in geological time, the total number of scorpion species would be some three hundred thousand. In the case of spiders, more than 45,000 species have been described.
That is problematic. According to Darwin, the difference that distinguished each new species had to have arisen by chance, as the advantage conferred by the difference enabled the population to leave more offspring and outcompete other species. But the sheer number of species strains the idea to breaking point. As often as not, animals appear to have increased in diversity because they were colonising new territory and niches – avoiding competitors rather than competing with them. While it is true that they needed to adapt in such circumstances and adaptation involved differentiation, they were spreading abroad before their habitats became overcrowded. Moreover, until they had their adaptations, their new surroundings put them at a disadvantage, not an advantage. The environments to which spiders have adapted include the most hostile imaginable, such as the Arctic tundra (the Arctic wolf spider), or the extreme heights of the Himalayas (the Himalayan jumping spider). Had species been obliged to wait on ‘chance’ to equip them with what they needed, they would have been wiped out. Their evolution in the sense of diversification must therefore have been foreordained. Dispersal and adaptation went hand in hand.
If the shift from one terrestrial environment to another put not already adapted species at a disadvantage, how much more must the hypothesised shift from marine to terrestrial. Yet the various classes of terrestrial arthropod – myriapods, scorpions, insects and so on – were distinct from the moment they first appeared in the fossil record. Thus, since even their earliest representatives were fully terrestrial, those committed to the Darwinian story are forced to conclude that the acquisition of terrestrial adaptations occurred, unbeknown to the record, independently in these separate groups. The marine ancestors from which they supposedly evolved left no record. This is just one of countless examples where limitless evolution proves not to be a hypothesis deduced from the scientific evidence but a dogma imposed on it.. Worse still, the various classes of terrestrial arthropod – myriapods, scorpions, insects and so on – were distinct from the moment they first appeared in the fossil record. Thus, since even their earliest representatives were fully terrestrial, those committed to the Darwinian story are forced to conclude that the acquisition of terrestrial adaptations had to have occurred, unbeknown to the record, independently in these separate groups. Even the marine ancestors from which they supposedly evolved left no fossil record. This is just one of countless examples where evolution in this sense proves not to be a hypothesis deduced from the scientific evidence but a dogma imposed on it.
Evolutionary taxonomy does not test the idea that all organisms are interrelated; it assumes it. Similarities are interpreted as evidence of close common ancestry, and differences interpreted as evidence that the common ancestry was more distant. Differences are never seen as evidence of non-relationship. The hypothesis of special creation is therefore methodologically and philosophically excluded. This is not how science is supposed to work.
As we try to understand how terrestrial invertebrates might have spread abroad, and done so ahead of vertebrates, perhaps there are clues to be found in observing how invertebrates recolonise empty territory today, after volcanic eruptions such as at Mount St Helens or the island of Surtsey. Typically the first animals to invade are insects, mites and spiders, and they do so by falling out of the sky – swept up by winds and borne in the air over hundreds of miles. Wind-transport may have been one of the ways in which invertebrates reached new land after being released from the ark. Most of them were tiny: less than 2 cm in length. They were transported over the world involuntarily, like plant spores. In general, animals that were heavier and reproduced less prolifically arrived later – how late, depending on the distance travelled – and mostly on foot.
The appearance of winged insects after the first invertebrates – and after the first wingless insects – suggests that, whether or not they evolved from a common ancestor, these insects did not acquire wings until some time later. For of course, if they had had them, they would have been more mobile and have impinged the fossil record that much sooner. As we know from studies of modern insects, insect wings are controlled by regulatory gene networks that can switch their development on or off (Whiting et al. 2003, Tomoyasu 2017). The sudden and virtually simultaneous appearance of wings in diverse insect orders in the Carboniferous makes it clear that wings did not evolve by a chance process of gradually accumulating beneficial mutations. If insects did all descend from a common ancestor, they acquired wings at the same time as they acquired all the other features that distinguished one order from another.
How millipedes and scorpions got to their find spots is anybody’s guess. It is interesting that the find spots of the earliest scorpions – Wisconsin, SE Ontario, New York state – were not far apart. Nor was Stonehaven, which at that time was part of Laurentia (i.e. North America). Even the Lake District and Powys, Wales, separated from Laurentia by the Iapetus Ocean, were relatively close.
Water could have provided another means of transport. The cuticles of many springtails 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.
The first animals were extremely sparse, becoming more common as time went on. This is anomalous in a scenario where innovations became fixed in populations because more of the offspring survived. Over tens of thousands of years, a mere instant in geological time, invertebrates taking advantage of the opportunities provided by plants might be expected to have overrun the earth, and to have been as abundant at the beginning of the multimillion-year period in which they originated as at any other time. Typically, however, the interval between their first appearance and what might be considered normal population levels (frequency of fossilization) is measured in tens, if not hundreds, of millions of years. The pattern fits neither the theory of evolution nor its associated timescales. It shows that populations were initially few and far between, but gradually increased. Where the first populations came from no one can say, since further back in time the trail peters out.
That they were preceded by a substantial history is clear. The fossils, sparse though they were, rarely occur in contexts suggesting that the invertebrates were the very first generation of pioneers – isolated, starved of sustenance and perishing where they dropped. They tend to occur together and close to plant material, as part of assemblages constituting the remains of some sort of ecosystem. The assemblages in the late-Silurian Ludford Lane Bonebed or the early-Devonian Rhynie Chert (again, both in the UK) are spectacular. Scorpions, pseudoscorpions, harvestmen, trigonotarbids, mites, myriapods and springtails have all been recorded from the Rhynie Chert. Evidently the plants were there before the animals, and the animals were either living off the plants or off each other. Signs of herbivory in spores (within coprolites), stems, roots and leaves go back almost as far as the plant elements themselves. The simultaneous appearance of diverse groups of organisms, unrelated ancestrally but related ecologically, is of the essence of recolonisation.
In the next article we look at the story of how vertebrates conquered the land. Although invertebrates fail to substantiate the Darwinian story, perhaps fish 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 innovations, 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?