AT 02:56 and 15 seconds GMT on 21 July 1969, Neil Armstrong became the first person to tread the surface of the Moon. His “small step” into this alien environment was the culmination of at least a decade of planning back on Earth, and it was a giant leap for mankind. But take a look at the rocks of our own planet and you’ll find evidence that humans were not the first to conquer a hostile new world.
In the beginning, all life on Earth was aquatic. Eventually, animals crawled onto dry land. History may not record this as an impressive moment but in many ways the achievement of these pioneering terrestrials topped NASA’s Moon landing. They didn’t have a charismatic young president urging them on, they didn’t have the best brains working out ways to get them there, and they didn’t have any technological know-how to keep them alive once they landed on the beachhead. But, like Armstrong, they did leave footprints as a testimony to their conquest. What’s more, newly discovered fossil traces reveal that the first conquest of land was 40 million years sooner than we thought. By the time our own ancestors lumbered ashore as fish-like amphibians, around 370 million years ago, the Earth was already teeming with creepy-crawlies—prehistoric arthropods including millipedes, spiders, scorpions and primitive insects.
What possessed the pioneers to leave the comfort of their watery homes? There’s no simple answer, but the latest fossil findings reveal some surprises. It turns out that arthropods were not seduced by tasty green plants—the first land animals were mostly rapacious predators. Some may have been forced out of the water as the creatures they normally fed upon fought back. Others, including metre-long sea scorpions, used the beaches for mass mating rituals. And some may simply have made landfall by accident.
Advertisement
Life on Earth is around 3.8 billion years old. During most of this time it has taken the form of simple single-celled organisms. The origins of life as we know it today date back just 540 million years to an event known as the Cambrian explosion when, in a big bang of evolutionary innovation, all the major body designs we see now appeared. These animals were entirely aquatic, as lethal UV rays made the dry land uninhabitable. But things were changing. A protective layer of ozone was starting to build up around the Earth and atmospheric oxygen was increasing, making new types of life possible. Gradually the bare rocks were replaced by a green world of algae and lichen. They stabilised the land by preventing erosion and for the first time, half a billion years ago, it became possible for animals to come ashore.
The earliest visitors to terra firma may have been accidental tourists. Paul Selden from Manchester University suspects they were ancient varieties of tardigrades or water bears—bizarre little creatures that today live in moss and on seashores. They are among the toughest animals on Earth, able to survive boiling, freezing and extreme pressures. “They turn into a cyst called a tun when they dry out,” says Selden. This allows them to go into suspended animation for over 100 years. “These things could have been blowing onto very early land,” he says. “We’ve no evidence for this, but it’s a possibility.”
Tardigrades are so odd they get a phylum all of their own, but they are related to arthropods and there is plenty of evidence to show that these were among the first animals on dry land. The oldest fossilised land animals found to date come from Ludford Lane in Shropshire. There, rocks from 414 million years ago contain rare yet varied remains including a group called the trigonotarbid arachnids. “Basically they’re like spiders except they don’t have the spinnerets and wouldn’t have made silk in the same way spiders do,” says Jason Dunlop from the Humboldt Museum in Berlin. “They seem to have been the predominant group—a bit like the dinosaurs of the spiders.” And the discovery that trigonotarbids had air-breathing organs known as “book lungs” indicates that they were not simply amphibious but fully adapted to life on land.
Most of the fossils at Ludford are just tiny fragments of creatures. To get a good picture of the first terrestrial creepy-crawlies, palaeontologists need to look elsewhere. Some of the best preserved fossils come from 400-million-year-old Scottish rocks in Aberdeenshire known as the Rhynie Chert, where trigonotarbids rub shoulders with a wide range of other arthropods. “Most of them show well-developed terrestrial adaptations,” says Lyall Anderson from the National Museums of Scotland. Which suggests they took to the land before the Devonian period which began 410 million years ago. They include a recently discovered harvestman spider that is almost indistinguishable from its modern counterpart, right down to the genitals—the oldest on any land animal known to science. Other fossils look more primitive and quite distinct from modern animals. “There are unusual things coming out of Rhynie,” says Anderson. “I was quite surprised to find some euthycarcinoids.” These bizarre halfway-house animals, a link between modern insects and crustaceans, are rarely found in the fossil record.
If a recent redating of the Tumblagooda sandstone of Western Australia is confirmed, euthycarcinoids may turn out to be the oldest land animals yet. The new dating puts a euthycarcinoid known as Kalbarria at 440 million years old—40 million years older than was first thought. Even so, it’s unlikely to be the very first terrestrial species. For a dead animal to fossilise it must survive scavengers and obliteration from physical and chemical destruction, which is much less likely to happen on dry land than in marine habitats. But while the bodies may not have survived, there is another source of evidence supplying clues about the first animals to roam the Earth—footprints.
Trace fossils from Tumblagooda show there were lots of different large arthropods walking on the land at this time—an indication that terrestrial animals were becoming increasingly common. These and similar trails in rocks from the Lake District in England suggest land invasions began in earnest in the Ordovician period, over 460 million years ago. But the latest trace fossil find reveals that the earliest pioneers were piling onto beaches long before that. Robert MacNaughton from the Geological Survey of Canada and a team of researchers including one of us (Simon Braddy) have discovered fossil trackways preserved in an ancient coastal sand dune from Ontario, which date back to the end of the Cambrian period, 500 million years ago (Geology, vol 30, p 391).
Analysis of these tracks indicates they were made by the mysterious euthycarcinoids. Although we can’t say exactly what the animals looked like, their traces do give information about their behaviour and evolution. For example, aquatic animals tend to move pairs of legs together in a sort of swimming action, whereas land animals must evolve the ability to move limbs out of phase to walk stepwise. From the traces in Ontario, it seems these early euthycarcinoids lunged awkwardly across the sand. Indeed, they never seem to have mastered a terrestrial gait, and were perhaps never fully land-going. Nonetheless, they do seem to have hit the beaches in large numbers. The Ontario fossils show that several different-sized animals with tracks ranging between 8 and 13 centimetres wide were on the move—perhaps in a group exodus across the dunes.
It makes sense that arthropods should conquer dry land, pre-adapted as they were for life out of the water with their external skeleton, the equivalent of Armstrong’s space suit, protecting them from the alien environment. Although the exoskeleton provided support and helped prevent desiccation, other adaptations would have been required for a successful transition to terrestrial living. The colonists needed to evolve a way of breathing air, their senses must have changed to work in their new surroundings, they needed to modify their feeding behaviour, reduce the amount of water lost in excretion and adapt their sex lives. Why go to all this trouble?
One reason is simply because it was there. “If there’s a niche void, life will find a way,” says Anderson. In other words, as soon as dry land became habitable, animals moved into the newly available real estate. And arthropods are notorious ground-breakers. You can see evidence for this today in the aftermath of volcanic eruptions when creepy-crawlies are usually first to reclaim the wasteland of cooling lava flows. Half a billion years ago, the land would have been equally inhospitable, but terrestrial pioneers need not have jumped straight out of the water onto the dry, barren, irradiated interior. They may first have acclimatised themselves in the margins between sea and land.
One good reason for venturing inland might have been food. Experts have long assumed that once primitive plants started growing on dry land, animals would move in to eat them and these herbivores would be followed, in turn, by carnivores, completing the classic food chain. The reality wasn’t quite like that, though. Although the rocks at Rhynie, Ludford Lane and other well-studied sites are the remains of quite different habitats, they all tell the same story. “What we find in all these ecosystems is a lot of predators, one or two detritus feeders, but nothing eating plants whole,” says Selden. “There were no real herbivores.”
How can you have a healthy food chain when there’s a missing link? “We tend to think you’ve got to have herbivores to complete the chain—but you don’t,” says Selden. “All a herbivore does is cut up leaves, put them into its gut where it uses a whole range of bacteria and fungi to decompose those leaves.” They are effectively walking compost heaps. Detritivores, by contrast, dine on plants that have already been broken down. “You can have an ecosystem based entirely on [detritivores] and I think that’s what we’ve got at this time,” says Selden.
One clue that he might be right came when researchers took a close look at fossilised dung, coprolite, from Ludford Lane. “These were stuffed with spores and other bits of [undigested] plant,” says Selden. Evidence from the Rhynie Chert is even more compelling. Anderson describes a 400-million-year-old animal that looks like a millipede, with both fungal and plant spores as well as plant tissue preserved in its gut. “It seems to have been grubbing around in the litter,” he says, rather than chomping into fresh plants as a herbivore would. This is probably the oldest detritivore yet discovered.
Other early litter eaters include creatures that look like long, flat woodlice, but they are greatly outnumbered by predatory carnivores such as trigonotarbids and long-legged centipedes. Herbivores didn’t make their mark until much later, with the evolution of plant-eating insects after the Devonian, around 350 million years ago. “True insects appear to be missing from these early ecosystems,” says Dunlop.
The myriapods may have invaded the land to exploit new ecosystems and feed on organic litter, but some terrestrial pioneers seem to have had less benign reasons for coming ashore. One possibility is that marginal pools partially dried out, leaving creatures nowhere to hide from their predators. This could have driven some arthropods to seek refuge on land. It looks as though this is what happened to the eurypterids, ancestors of modern scorpions, around 400 million years ago.
These voracious predators, which grew to over a metre in length, emerged onto land when their aquatic prey, the fish, took revenge. “Fish started getting really big and powerful and they developed jaws,” says Selden. “The tables were turned and that seems to coincide with the eurypterids coming out of the water.” This might also explain why our own fishy ancestors later evolved to become land walkers. “If their food was fleeing onto the land, there was good reason to follow,” he says.
But there is another reason why eurypterids made their way up prehistoric beaches. Although they never became totally terrestrial—their fossilised bodies and traces of how they moved on land testify to that—they do seem to have needed to escape the water during one crucial phase of life. It looks as though these metre-long monsters came ashore in their hundreds like an invasion of amphibious landing craft, decamping in shallow lagoons where they shed their exoskeletons in safety before mating, laying their eggs and returning to the sea. These early amphibious excursions were a stepping stone towards full-scale arthropod invasions. Evidence of these mating rituals is preserved in fossil beds in New York state in the form of scores of moulted eurypterid skins.
Horseshoe crabs still perform a similar ritual today, using the Moon to coordinate their orgies. While there’s no way of knowing whether the first terrestrials looked to the night sky, it is quite possible that the Moon played a crucial role in guiding them onto the beaches. What’s certain is that this was one giant leap for creepy-crawlies.
