Humans have been around for a blink of Earth’s history. In that time we’ve come through ice ages, plagues, and every kind of disaster the planet could throw at us, which sounds impressive until you drop someone 400 million years into the past.This is the Devonian Period, and it is genuinely one of the strangest chapters in the story of life here. Oceans covered most of the world, armored fish the size of buses ruled the water, and the land looked like nothing you’d recognize.So say the time machine breaks down. How long do you last?Your first bit of luck would be landing on dry ground at all, since roughly 85 percent of the planet was underwater. What land existed sat mostly in two enormous supercontinents, Gondwana and Laurussia.It wouldn’t look like home. Most of it would be bare and rocky, with mountain ranges slowly wearing down over millions of years and feeding nutrients into the seas. That runoff is a big part of why ocean life went wild here, which is how the era earned its nickname, the Age of Fishes.Your first real problem would be shelter.Early in the Devonian there were no forests. Trees hadn’t evolved yet, so there was no timber, no shade, and very little vegetation of any kind. Mostly primitive mosses and small plants clinging to rock.You’d have to improvise everything. No wood, no fabric, nothing to build a decent shelter with. Clothing would be the same problem, and if humans had somehow evolved here, the wardrobe would have run to moss, seashells, and fish bone. Not stylish, but survival rarely is.Then there’s food, and this is where it gets genuinely dangerous.The oceans were packed with life, which sounds convenient until you meet the neighbors. Placoderms lived here, heavily armored fish with powerful jaws, and some species reached around ten meters long.Go fishing in that water and you can quite easily end up on the wrong side of the transaction.Suppose you managed to grab a fish or collect shellfish from the shoreline anyway. Cooking becomes the next puzzle, because with no forests there’s almost no firewood. Dry moss and scraps of plant material are about all you’d have to work with.The climate at least cooperates. Much of the Devonian was fairly warm, so freezing isn’t your main worry. Eating without being eaten is.Realistically you’d spend your days working the coastline, gathering mollusks and early arthropods while keeping one eye permanently on the water.Now skip ahead about forty million years, into the Late Devonian.This is when one of Earth’s great extinction events hit, wiping out somewhere between 79 and 87 percent of species. Strangely, that might work in your favor.Researchers think a major cause was the spread of vascular plants. Trees and ferns arrived and started reshaping the land, and their roots changed soil chemistry and washed nutrients into lakes and oceans.All those nutrients fed enormous algae blooms. Bacteria feeding on the algae stripped oxygen out of the water, and huge stretches of ocean became uninhabitable, killing off marine life on a massive scale.Fewer giant predators in the sea is good news for you. The catch is that life was starting to climb out of the water.The interesting ones were lobefins, ancient fish that eventually led to amphibians and every four legged animal after them. The famous example is Tiktaalik, found in the Canadian Arctic.It had a crocodile like head, sharp teeth, and strong fins containing primitive wrist bones that let it prop itself up on land. Nearly three meters long, and one of the clearest transitional forms evolution has left us.If one turns up, give it room.The water hadn’t gotten safe either. Dunkleosteus was still out there, an enormous armored fish with thick bony plates across its head and a bite powerful enough to crush almost anything.Some reached about ten meters, and as food grew scarce, they may have started eating each other. Swimming was never going to be relaxing here.On land, though, things were finally improving. All that new plant life meant materials at last, for tools, for real shelter, maybe even a home built among the first forests on Earth. One of those early trees, Archaeopteris, grew around thirty meters tall with a trunk several meters across.The Devonian was beautiful and constantly changing and completely indifferent to whether you survived it. Getting through it would take adaptability, caution, and a permanent, healthy suspicion of anything moving under the surface of the water.