Four hundred million years ago, Earth was in the early stages of the Devonian period, a stretch of time running from 419 to 359 million years ago that would fundamentally reshape life on this planet. The oceans teemed with fish so diverse that paleontologists call it the “Age of Fishes.” On land, the first vascular plants were just beginning to colonize bare rock and soil, and the earliest arthropods were crawling out of the water. The world looked almost nothing like it does today, yet the evolutionary groundwork for every land-dwelling vertebrate, including us, was being laid.
A Warmer World With More CO2
The climate 400 million years ago was significantly warmer than today. Global mean surface temperatures over the past 485 million years have ranged between about 11°C and 36°C, and the Early Devonian sat well on the warmer end of that spectrum. Atmospheric carbon dioxide concentrations hovered around 2,000 parts per million, roughly five times the level we have now. That CO2 would gradually drop below 1,000 ppm by the Late Devonian, partly because of the very plants that were starting to spread across the continents.
The continents themselves were arranged in unfamiliar positions. A massive southern supercontinent called Gondwana dominated the Southern Hemisphere, while the landmass of Laurussia (a combination of what would become North America, Greenland, and northern Europe) sat closer to the equator. Between them lay the Rheic Ocean, its warm, shallow margins perfect for reef growth. There were no polar ice caps, and sea levels were high.
The Age of Fishes
The oceans 400 million years ago were dominated by fish in a way they never had been before. Jawed vertebrates were diversifying rapidly. Among the most prominent were placoderms, heavily armored fish with bony plates covering their heads and upper bodies. These ranged from small bottom-dwellers to apex predators that would eventually grow several meters long. Placoderms were the most diverse group of jawed fish during this period, filling ecological roles from filter feeders to crushing-jawed shellfish eaters.
Alongside them swam early sharks and two groups of bony fish that would prove more consequential in the long run: ray-finned fish and lobe-finned fish. Ray-finned fish, the group that today includes everything from goldfish to tuna, were just appearing in definitive fossil form around this time. Lobe-finned fish had already been around for roughly 23 million years, with fossils dating back to about 423 million years ago. Their muscular, fleshy fins, supported by internal bones, would eventually prove critical. These were the ancestors of all land-dwelling vertebrates.
Massive Reef Systems
The Devonian produced the most significant expansion of animal-built reefs in the entire Phanerozoic, the last half-billion years of Earth’s history. These reefs stretched across what is now Europe, North America, North Africa, Australia, Siberia, and China. The primary builders were not the stony corals we know today but two now-extinct groups: tabulate corals, which grew in colony formations, and rugose corals, which came in both solitary horn-shaped forms and branching colonial varieties.
Recent research published in Nature has revealed something striking about these ancient reef builders. Chemical analysis of their fossils shows that tabulate corals and some branching rugose corals hosted photosynthetic algae inside their tissues, the same kind of symbiotic relationship that powers modern coral reefs. Solitary rugose corals, by contrast, did not appear to have these partners. This photosymbiotic relationship, once thought to have originated much later, may have been fueling reef growth as early as the Middle Devonian, around 383 million years ago. These thriving reef ecosystems would eventually be wiped out during a mass extinction event around 372 million years ago.
The First Plants on Land
If you could stand on dry land 400 million years ago, the landscape would look strikingly barren by modern standards. There were no trees, no grass, no flowers. The most advanced plants were small, leafless stems that branched repeatedly and topped each branch with a spore-producing capsule. These earliest vascular plants, known as polysporangiophytes, represent a critical evolutionary leap: they had internal plumbing (vascular tissue) that could transport water and nutrients, allowing them to grow taller than their moss-like predecessors.
The fossil record captures this transition beautifully. Cooksonia, one of the earliest known vascular plants, ranged from less than 2 millimeters to about 6 centimeters tall. Even smaller precursors like Partitatheca stood only about 3 millimeters high and straddled the line between mosses and true vascular plants, possessing stomata (tiny pores for gas exchange) and branching stems but no internal vascular tissue. Aglaophyton, preserved in exquisite detail in Scotland’s Rhynie chert fossil deposit, showed a similar mix of primitive and advanced features. These plants were building complexity piece by piece: first branching, then stomata, then vascular tissue, then greater height. Leaves and roots would come later.
Small as they were, these plants were beginning to transform the planet. Their roots broke down rock into soil. Their tissues, when they died, added organic carbon to the ground. Over the next tens of millions of years, their descendants would grow into the first forests, pulling enough CO2 from the atmosphere to cool the planet dramatically.
Animals Venture Onto Land
The earliest land animals at this time were arthropods. Fossil evidence from the Pragian stage of the Early Devonian, right around 400 million years ago, includes springtails (Collembola) and even an incomplete insect fossil called Rhyniognatha, found in Scottish rock of this age. Rhyniognatha’s jaws are particularly interesting because they show features associated with winged insects, hinting that the diversification of insects may have begun far earlier than once thought. A more recent discovery of a complete Late Devonian insect with omnivorous jaws confirms that insects were diversifying through this entire period, not just at its end.
Vertebrates had not yet made the transition to land, but their future was swimming in Devonian waters. Lobe-finned fish like Kenichthys and, later, Eusthenopteron and Gogonasus were developing skeletal features in their fins that foreshadowed the limbs of four-legged animals. These fish could likely prop themselves up in shallow water or mudflats. The full transition to land, marked by fossils like Tiktaalik and Panderichthys, would come later in the Devonian, but the 400-million-year mark is when the lineage that would produce all amphibians, reptiles, birds, and mammals was taking shape in warm, oxygen-poor coastal waters.
Why This Period Matters
The world 400 million years ago was a planet in transition. The oceans were rich and complex, filled with armored fish, early sharks, sprawling coral reefs, and the lobe-finned fish that would eventually crawl ashore. The land was just beginning to turn green, colonized by tiny, leafless plants and the first wave of arthropods. The atmosphere held far more CO2 than today, the climate was warmer, and the continents were in completely different positions.
Nearly every major group of land-dwelling organisms traces a critical chapter of its origin story to this window of time. The vascular plants that now blanket the Earth were centimeters tall. The fish that would become the ancestors of all land vertebrates were testing the limits of their muscular fins. Insects were beginning a diversification that would eventually make them the most species-rich group of animals on the planet. Four hundred million years ago, the modern world was being assembled from spare parts in shallow seas and on barren, rocky shores.