For most of Earth's 4.5 billion year history, the only living things were microscopic single-celled
organisms. But once the Paleozoic era began about half a billion years ago there was an explosion
of complex life in the oceans, and over the next several hundred million years, life started to
slowly colonize the land. But these assemblages of life were still mostly relegated to the edges
of the water. But by the time the Permian had begun, the last pieces of the puzzle had fallen
into place: fully terrestrial water-independent land ecosystems. These ecosystems would continue
to grow until they faced an extinction larger than any in the history of complex life.
The Permian period is the sixth and final period of the Paleozoic era, lasting from 299 to 252
million years ago. At this time, the Earth's land masses are mostly clumped together into
the supercontinent of Pangea, although Siberia and several large islands remain independent. At
the beginning of the Permian, the Ice Age that began the later Carboniferous is still ongoing,
and the South Pole is still covered by a massive ice sheet. Unsurprisingly, conditions
at this point in prehistory are much colder than usual, similar to today's climate. But that's
where the similarities end. Because of the recent proliferation of land plants across the continents
during the Carboniferous, the atmospheric oxygen levels are at their highest ever
point in the Earth's history, at about 35% of the atmosphere, compared to today's 21%. And although
the Earth's spin had slowed down significantly since the beginning of the Paleozoic, the days
are still under 23 hours long, while sea levels are 200 feet higher than today's. The interiors
of Pangaea are filled with dry areas, but the wetter areas still harbor swamps that are a lot
like those of the Carboniferous. Because of the high levels of oxygen, giant insects and other
arthropods are still flourishing, although most insects from this time are ancient relatives of
the Polyneopterans. Among the vertebrates, there are still many primitive amphibian-like tetrapods,
but now they have been joined by more advanced species that can lay eggs on land.
There were forests of early seed-bearing plants, which could also reproduce without as much need
for water. These adaptations would come in handy for later, because these colder, wetter conditions
would not last long. The Permian is divided into three epochs: the Cisuraelian, Guadalupean, and
Lopingian. Each of these is divided into stages, and as the Permian goes through these stages,
the climate becomes much warmer and drier. The forests of Lycopods and other primitive plants
would continue to disappear, and the oxygen levels would drop, meaning no more giant arthropods.
Among the insects, Endopterygotes, which are the ones that undergo four-step metamorphosis,
would become more common. Seed plants would also do much better in these situations than their
competitors, and it's during the Permian that we see early conifers and cycads proliferate.
Land ecosystems will become dominated by large carnivorous and herbivorous amniotes. Despite
their outward appearance, these creatures weren't reptiles per se, but were primitive
relatives of a very different branch of the vertebrate family tree. Meanwhile the
ancestors of dinosaurs and other reptiles would diversify, with some of them becoming aquatic,
joining marine communities full of strange Paleozoic sea life. The Permian seas contained
reefs populated by a menagerie of ancient mollusks and arthropods, as well as sharks,
giant seagoing proto-amphibians, and more modern fish than previous periods. But the sea level
itself would fall, and end up dropping from 200 feet to only 60 feet above modern levels, due
to the ocean floor deepening. Increased volcanic activity would keep releasing greenhouse gases,
raising the temperature higher and higher and turning most of Pangaea into a desert. Eventually
these factors would lead to the largest mass extinction of multicellular organisms of all time.
But during these 47 million years, the groundwork was being laid for the future of life on Earth,
and the period represents a culmination of 290 million years of Paleozoic evolution.
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