Humans could breathe 300 million years ago if humans were around then.
Although the experience of breathing 300 million years ago would have been potentially hazardous and uncomfortable.
During the Carboniferous period of 300 million years ago, the oxygen levels in the air were much higher than they were today and peaked around 30 percent to 35 percent when compared to the current 21 percent of oxygen levels we have now.
This high concentration of oxygen levels 300 million years ago, would not make someone instantly asphyxiate, it would however present several major issues for human survival which includes oxygen toxicity, hyper flammable world and giant insects.
The extra oxygen in the air 300 million years ago is also what had allowed insects in that era to grow to gigantic sizes and if humans were around 300 million years ago humans would also be sharing the same planet earth with dragonflies the size of hawks and millipedes as long as cars.
And because oxygen fuels fires, Earth would also have been highly hazardous to humans as lightning strikes could even cause massive and uncontainable forest fires and lead to an atmosphere that would be thick with smoke.
And breathing air with the amount of oxygen that was around 300 million years ago, would cause dizziness, headaches and even lung irritation within minutes.
Humans did not exist 300 million years ago.
Anatomically modern humans (Homo sapiens) only emerged about 300,000 years ago.
300 million years ago was the Paleozoic Era (specifically the Carboniferous period).
Earth during this time was dominated by vast swamp forests, giant insects, and early amphibians and reptiles; the first dinosaurs wouldn't even appear for another 60 million years.
The Carboniferous Period was an ancient geologic era famous for vast, swampy forests that created the world’s major coal deposits.
The Carboniferous Period was defined by soaring oxygen levels, the dominance of giant insects, the rise of the first reptiles, and the onset of a massive ice age
1. The Climate & The "Coal Age"
The Carboniferous period gets its name from the large amount of carbon-heavy coal deposits formed during this time.
The Coal Forests: The Earth’s equatorial regions were covered in expansive, tropical swamp forests.
Carbon Drawdown: These dense, rapid-growing forests—made up of giant club mosses, tree ferns, and horsetails—absorbed massive quantities of atmospheric CO₂.
Ice Age: This drastic drop in greenhouse gases, coupled with rising oxygen levels (which peaked around 35%), ultimately triggered the Late Paleozoic Ice Age.
2. Explosion of Life
High oxygen levels in the atmosphere allowed for the evolution of terrestrial giants.
Giant Insects: Due to the efficient oxygen intake, bugs grew to massive proportions.
The giant dragonfly Meganeura had a wingspan of roughly 2.5 feet, and the millipede Arthropleura grew up to 8.5 feet long.
Rise of Reptiles: Amphibians dominated the swamps and shallow waters, but the most important evolutionary leap was the emergence of the first amniotes—reptiles and the ancestors of mammals—which could reproduce away from water.
3. Tectonics and Geography
The continents were actively shifting toward one another to form the supercontinent Pangaea.
Collisions: North America and Europe collided with the southern landmass of Gondwana, leading to the formation of the Appalachian mountain range (the Variscan-Alleghanian orogeny).
4. Geologic Subdivisions
Geologists typically divide this 60-million-year period into two primary sub-periods based on the distinct rock and coal layers:
Mississippian (Lower Carboniferous): Characterized largely by shallow, marine environments and extensive limestone deposits.
Pennsylvanian (Upper Carboniferous): The primary "Coal Age," where the vast swamp forests and coal seams formed.