Do you get freaked out by an unexpected spider roommate or maybe a wasp that is just too close for comfort? If you are one of these people, perhaps you can take some comfort in knowing that eons ago, there were even crazier insects that ruled supreme: welcome to the Carboniferous!

The Carboniferous Period was part of Earth’s paleozoic epoch, and ran from roughly 360 to 299 million years ago, after the Devonian period and before the Permian period. The continents were arranged in a manner very different from what we have today, and much of the world was covered in heavily forested swamps with towering trees.

Perhaps the most ubiquitous of these “trees” (which really weren’t trees at all,) were the lepidodendrons, or scale trees. Accounting for almost half of the biomass in some areas, these trees were able to cross into almost all landmasses, aided by the positioning of the continents. Scale trees would also be one of the earliest vascular plants, having xylem for transporting water and nutrients throughout the plant, and with the rigid material lignin, also known as cellulose, holding it in place. Reaching up to 30 meters tall, these plants were massive, with juvenile plants growing leaves straight from their bark in a staircase-like manner. This left divot-like scars on the adult trunks, which would inspire the name “scale trees”. Unlike true trees, these plants reproduced by spores, with their closest relatives actually being the lycopsids, or club mosses. Massive horsetail plants and tree ferns grew as well.

Their success came with a cost. Microbes that could break down the tough organic bonds seen in cellulose and other similar structures did not yet exist, allowing dead organic materials to just pile up rather than decompose, locking away carbon from cycling through. Over the eons, this would lead to an increase in oxygen that was a 160% increase of what we have today (35% atmospheric oxygen versus our contemporary 21%). In time, this would bring on the aforementioned giant bugs and friends.

Terrestrial arthropods do not have lungs, and instead rely on laterally lying holes on their bodies called spiracles to draw in oxygen and directly diffuse it in small tubes called tracheoles, which their internal structures. This method is all good and dandy for small arthropods, but if they grew too large, oxygen can not be effectively distributed, which would lead to the death of the animal. With higher oxygen levels, arthropods were less constrained in their growth and reached sizes never before seen in their kind: Griffinflies, relatives of dragonflies but the size of falcons. The alligator-sized millipede Arthropleura scuttled along the forest floor munching on ferns, while Pulmonoscorpius, a giant scorpion over two feet long, stalked the primal jungles, as well.

Early amphibians flourished in these ancient swamplands, and were also able to grow to massive sizes thanks to their enormous insect food sources. The earliest amniotes (vertebrates that lay hard shelled eggs) were making their appearance in this time frame, too. This unique environment would not last forever. Remember that whole thing of the plants not decomposing and carbon just getting stored away? Well, history caught up, and with the depletion in carbon dioxide, the climate cooled and dried. This increased volcanic activity and led to a global ice age. This whole debacle led to what is one of the only two known substantial extinction events in plants, in an event now called the Carboniferous Rainforest Collapse.

This prehistoric event would change the course of life on Earth; with forests being more fragmented, and the land more dry, conditions were playing more favorably for the amniotes, who were not bound to the water for reproduction like the amphibians. Oxygen levels plummeted, terminating the reign of giant insects. The stage was now set for the ascension of amniotes, which in due time, would lead to humans. The conditions that defined the Carboniferous Period were likely a one-time-thing for our planet’s biodiversity, as now multiple kinds of microbes flourish in breaking down woody plants, inhibiting the lack of decomposition that would produce the remarkable atmospheric conditions previously seen. The Carboniferous still impacts us to this day; all the coal burned and utilized by humans for our industrial activities was deposited in this time, long before we existed.

A reminder of how all chapters of life on Earth are impactful and unforgotten. Does anyone else find it mildly ironic that the release of the carbon that cooled the Carboniferous in times past is what’s warming the planet in present times?