Fossils are much more than curious objects you find on a beach or in a museum; old bones and ancient shells represent the remains of once-living, breathing animals.

Trace fossils, on the other hand, such as bite-marks and burrows, are the remains of the activities of animals, as opposed to the animals themselves. They are a particularly useful source of information for ‘breathing life’ into extinct animals, revealing a snapshot in time of their behaviour, providing direct evidence for their locomotion, and even indicating interactions between them. Trace fossils can be found in rocks that were deposited both in the sea and on the land. In fact, other evidence for life on land is rather scarce because dead bodies degrade more easily on land than in the sea.

The fossil record reveals that it was the arthropods – creatures such as spiders, insects and crabs with segmented bodies, jointed limbs, and a hard outer skeleton – who were the earliest animals on land. Even before they left the sea, arthropods were incredibly well adapted for life on land: their exoskeleton, which first evolved in the sea as protection from predators, acted as a spacesuit, shielding them from harmful ultra-violet rays and providing them with mechanical support. The oldest body fossil of a land animal is a 430-million-year-old millipede from Scotland. Body fossils are extremely rare from this time in Earth history, but there is another source of evidence that supplies clues to when ancient life first stepped out of the sea – fossil footprints.

The oldest body fossil of a land animal is a 430-million-year-old millipede

A few years ago there was a ground-breaking discovery in a quarry near Kingston, Ontario in Canada. Fossilised trackways, preserved in a coastal dune deposit, revealed that arthropods first conquered the land around 500 million years ago, some 50 million years earlier than we previously thought. The earliest fossilised footprints prior to that were millipede trails found in Cumbria. Analysis of the Ontario trackways indicates that they were produced by a mysterious group of extinct arthropods called euthycarcinoids. These bizarre-looking bugs resemble a 30-cm-long, flat woodlouse with a tail spine that would have lunged awkwardly across the sand.

They are probably not related to woodlice, however, and recent research suggests that euthycarcinoids were in fact the ancestors of the insects, the most successful and diverse group of animals on land today. The trackways from Ontario indicate that several differently sized euthycarcinoids were on the move at the same time. Who knows, but perhaps they represent the vanguard of an amphibious group exodus from the sea, trekking across the dunes into an unknown land.

Our own ancestors, fish-like amphibians, first lumbered ashore a mere 370 million years ago. There they found a world teeming with plants and giant creepy crawlies. With almost a two-hundred-million-year head start in which to populate the planet before our ancestors arrived, those early arthropods filled an empty niche, evolving into monster millipedes, super-sized scorpions and colossal cockroaches. So, the next time you swat a fly or squish a spider, spare a thought for the true terrestrial pioneers – the arthropods – who left footprints in the sand as testimony to their conquest of the land half a billion years ago. It may have been one small step for a bug, but it was a giant leap for life on Earth.

Simon Braddy’s research on ancient arthropods will feature in the forthcoming BBC series Journey of Life and Life in the Undergrowth. This work was supported by the Leverhulme Trust.

Simon Braddy/Department of Earth Sciences