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If life began in the ocean, how exactly did we end up living on land? Is it even possible to answer that question? Since time travel is out of the question (for now, anyway) scientists like Martin Gibling turn instead to the fossil record to piece together the process of animals colonizing land.
When you think of fossils, you might immediately think of dinosaur bones – but not every organism has bones to leave behind.
In a paper recently published in Nature Evolution & Ecology, Dalhousie’s Dr. Gibling worked with lead author Nic Minter of Portsmouth University and other collaborators to uncover the mysteries of our origins using a different type of fossil.
Reading the clues
Trace, or ichno-fossils, include things like an organism’s tracks or burrows. These unique fossils provide indirect evidence of life and are more likely to withstand the test of time.
“They’re often made by creatures that have no other method of being preserved,” says Dr. Gibling, Professor Emeritus with the Department of Earth Sciences. Although trace fossils can’t tell us exactly what creature made them, they still offer a wealth of information.
“Right away they tell you the kind of body plan that the creature has,” explains Dr. Gibling.
From tracks, you can tell if a creature had legs on either side, like a centipede, or whether it was more worm-like. Burrows can reveal a creature’s lifestyle, whether it dug deep into the ground where oxygen levels were low, or remained close to the surface, grazing on the sediment there.
“They’re behavioural fossils,” says Dr. Gibling.
Scientists can assess the rocks themselves to determine at what time the organism lived, and what kind of environment it lived in. This information, along with the interpretation of body plan and behaviour from the fossil markings, provides a piece of what happened in the past.
Combine the records from hundreds of places all over the world, and you have not only a massive database, but the chance to look for patterns in how organisms moved onto the land. It’s a hefty task, but it’s exactly what the Nature study set out to do.
Pinning down the process
“What emerged indicates that the process of animals colonizing the land surface didn’t happen all at once,” explains Dr. Gibling. “It happened in stages.”
To move into marine environments with lower salt concentrations, organisms needed to first develop a tolerance for freshwater conditions. The turbulence of tides, exposure to more intense sunlight, and a varying food supply were some of the challenges that made venturing out of the ocean a difficult task.
But eventually, organisms made their way into rivers. When they did, big things happened.
“There are bursts of diversification from the original body plan when something makes it in [to a new environment],” says Dr. Gibling. “From one species, suddenly there’s 10, 15, or 20 variations.”
Being the pioneering organism has its perks – lots of room to grow and diversify without competition.
Over 250 million years, these ‘novelty events’ of evolution led to the colonization not only of rivers and lakes but eventually of soils and sand dunes. However, Dr. Gibling says the first organisms to leave the water weren’t very impressed. One of the earliest trace fossils on land, from a quarry in Kingston, Ontario, shows where a creature scuttled up a dune out of the ocean, then immediately scuttled right back.
Nevertheless, the study determined that by the end of the Carboniferous period, approx. 300 million years ago, animals had established a widespread colonization of the earth.
In the search for answers to the biggest of questions, it seems no clue is too small – even if it’s only a trace.
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