How ‘animals conquered the land’ is perhaps the most potent of the stories making up the Darwinian narrative. Life originated only once, in the sea, and for hundreds of millions of years the only animals on the planet were marine animals. Today there also exist terrestrial animals. Thus at some point evolution had to take steps onto land.
The recolonisation narrative is rather different. It identifies the ‘Late Heavy Bombardment’ at the beginning of geological history with the flood-cataclysm that, according to ancient tradition, obliterated terrestrial life. Although marine life might have survived the onslaught in lakes and seas at the poles or deep under the ground, insects, birds, reptiles and mammals could not have survived, not unaided. They had a different origin: a huge wooden capsule designed to withstand the buffeting waters and conserve a breeding pair of every terrestrial species then in existence, including one human family. Thus while all forms of life were capable of evolving in order to adapt to the ever-changing conditions of the new world, and although fish might have evolved as far as amphibians, that was the ordained limit; insects, birds, reptiles and mammals did not come from the sea.
Having long since totally recovered, life today recognises no sharp boundary between land and sea. Some animals can be perfectly at home in both environments: not because they are now evolving from one to the other but because they like to live at the boundary. Mudskipper fish use their pectoral fins to hop about on the shore and perch on mangrove roots. Turtles spend their lives in the sea but when the time comes to lay their eggs use their flippers to crawl onto the land. Seals spend much of their time in the water but haul themselves onto land to breed and raise their young.
It is equally clear that such animals were not created thus. Seals evolved from land mammals, turtles from land reptiles, and mudskippers from an order of ray-finned fish that evolved from soft-bodied forms that left no fossil record. Whether in a landward or a seaward direction, many other types of animal have undergone evolutionary transitions across the land-sea boundary. While the earliest snakes were all terrestrial, with lizard ancestors, several ingenious species have since become wholly or partly adapted to an aquatic life. Land crabs, velvet worms, ribbon worms, terrestrial ribbon worms, ostracods and snails have ancestries that began in the sea.
Nonetheless, it is striking that where the transition involved adapting from a marine to a terrestrial environment the degree of transformation was much smaller than occurred with evolutionary transitions in the opposite direction. Crabs still remained crabs, ostracods still ostracods. In cases involving the most radical transformation, such as seals and turtles, the shift is invariably in the other direction, from land to sea. Several invertebrate phyla made the transition from sea to land. It is not clear that any group of vertebrates did.
The first creatures known to be capable of living both on land and in water were the eurypterids. They seem to have inhabited shallow-water environments, such as the inter-tidal zone. Although they resembled scorpions, they were not directly related to them (if at all). Some reached lengths of over 2 metres, a record for arthropods; others were much smaller. The earliest eurypterids date back to the Early Cambrian (from the Paseky Shale, Czech Republic) and are thus part of the Cambrian Explosion. They are not preceded by any simpler organism evolving into a eurypterid, and they became extinct in the Permian.
Another group of arthropods that had an amphibian lifestyle at this time were horseshoe ‘crabs’. In fact they were not true crabs but a class (Xiphosura) without obvious ancestry to any other animal kind. They look alien, but unlike the eurypterids, they are still extant, so we know much more about their anatomy, behaviour and life cycle. One of their most remarkable features is their two kinds of eye: two simple eyes in the centre of the carapace, sensitive to ultraviolet light, and two compound eyes nearer the sides. They first appear in the fossil record in the Ordovician.
Apart from the fact that they were much more diverse in the Palaeozoic than today, the main trends in the evolution of horseshoe crabs were an overall increase in size and a loss of segmentation on the rear part of their carapace. While they have always been primarily aquatic, feeding off worms and shellfish on the seafloor, they failed to evolve a means of reproducing in water – which rather suggests a terrestrial origin. Spawning takes place on the beach where eggs are laid in nests within the sand, from which the larval crabs emerge soon after hatching and instinctively enter the sea. They moult more than 16 times before they reach adult size.
Horseshoe crabs span the greater part of the macrofossil record, and although they have evolved in the sense of diversifying into two suborders and various families – diversifying very early, in the Palaeozoic – they have not evolved in any sense corroborative of Darwin’s theory. Throughout they remain true to type, complex organisms from the beginning, with a complex life cycle, and an example of animals designed for existence at the interface between sea and land, whatever their evolutionary history was before the Ordovician. They are living proof that an ability to walk as well as swim carries no implication of being in evolutionary transit from one to the other.
Thus the first animals to make tracks on land were not amphibian ‘tetrapods’ but eurypterids – marine arthropods that had evolved some ability to live at the interface between sea and land. They were the first to make tracks on land precisely because they were aquatic animals, making their mark as they came ashore. As we shall see shortly, the first wholly terrestrial animals to appear in the fossil record were also not tetrapods, but myriapods – centipedes and millipedes. They were closely followed by insects. And contrary to the Darwinian narrative, neither myriapods nor insects show any evidence of having emerged from the sea.