The story of how ‘animals conquered the land’ is perhaps the most potent and iconic of all the stories composing the larger Darwinian narrative about evolution. Evolution has to take steps onto land if life is to make it to the present day. In the BBC programme Walking with Monsters the Cambrian fish Haikouichthys (thought to be related to the lamprey) is seen transforming itself into a land-walking amphibian as it slowly sprouts fleshy fins, and fins turn smoothly into limbs, developing like an embryo in the womb. Is the transformation factual, or inexcusable propaganda?

Life as a whole recognises no sharp boundary between land and sea. In today’s world certain animals can be perfectly at home in both environments: not because they are evolving from one to the other but because that is their nature. Seals, for example, spend much of their time in the water but haul themselves onto land to breed and raise their young. Mudskipper fish use their pectoral fins to walk 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.

Certain animals have ancestries which do document an evolutionary transition across the boundary, some in a seaward direction, others from sea to land. For example, while the earliest snakes were all terrestrial, several species have since become wholly or partly adapted to an aquatic life. Sea turtles are descended from turtles that lived on land. Land crabs, ostracods and snails have ancestries that began in the sea. That said, it is striking that in adapting from one environment to the other snakes still remained snakes, turtles still remained turtles, crabs still remained crabs, and their first appearance in the fossil record is invariably abrupt. Where evidence does exist of radical transformation, as in the case of ichthyosaurs, seals and whales, the shift is invariably in the other direction, from land to sea.

The first creatures known to be capable of existing both on land and in water were the eurypterids. They seem to have lived in shallow-water environments, such as the zone between low and high tide. Although they resembled scorpions in some ways, they were not related. Some species reached lengths of over 2 metres and have the distinction of being the largest arthropods ever to have lived; others were much smaller. The earliest fossil eurypterids go 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 crabs but a class (Xiphosura) without obvious ancestry to any other animal kind. Unlike the eurypterids, they are still extant, and we therefore know much more about their anatomy, behaviour and life cycle. One of their most remarkable features is that they have 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 their evolution are an overall increase in size and a loss of segmentation on the rear part of their carapace. They have always been primarily aquatic, feeding off worms and shellfish on the seafloor. Nonetheless, they failed to evolve a means of reproducing in the water. 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 while 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 just another example of animals designed for existence at the interface between sea and land. They are living proof that an ability to both swim and walk carries no implication of being in evolutionary transit from one to the other.

It may be that eurypterids, horseshoe crabs and other such creatures were the first to make tracks on land precisely because they were aquatic animals. Virtually all terrestrial life had been wiped out in the Hadean cataclysm. Although marine creatures also suffered enormous losses, sufficient numbers of them survived, and eurypterids, horseshoe crabs and the like were creatures obliged by their biology to come ashore in order to breed. They made tracks as they came ashore, and owing to the generally high rates of sedimentation at that time their tracks were sometimes preserved.