Chances are, if you’ve turned on a lightbulb or a computer, you’ve harnessed the power of some very ancient, and very strange … trees. The vast majority of the electricity produced
in human history has come from coal. And like all fossil fuels, coal is made from
the remains of extinct organisms that have been exposed to millions of years of heat
and pressure. But in the case of coal, these organisms consisted
largely of some downright bizarre plants that once covered the Earth, from Colorado to China. Meet the Lepidodendrales, also known as scale
trees. Named for their scaly looking bark, scale
trees were tall plants topped with crowns of branches that sported long, narrow leaves. They thrived during the Carboniferous Period,
from 299 million to 359 million years ago. And they’re partly why we call the Carboniferous
… the Carboniferous! This period is named after the carbon-rich
layers of sediment that formed from countless millions of scale trees, ferns, and other
early plants that died, piled up, and fossilized. You know these layers as coal. During the Carboniferous, much of the Earth
was covered in lush forests and swamps, and this is where the scale trees flourished,
sometimes growing as tall as a 15-story building. Their remains have been found throughout the
Northern Hemisphere, but they’re especially common in North America, where fossilized
roots, trunks, and leaves have been found from almost every stage of the plant’s life
cycle. And because we know so much about the anatomy
of these trees, we know that they were very different from the trees that you and I know
today. For one thing, they had no need for seeds! This is because scale trees were a type of
plant called lycopods, which originated way back in the Silurian Period — before seeds
were even a thing! Some lycopods are still around today, like
the low-lying, water-loving club mosses and quillworts. But instead of having seeds, these plants
reproduce by releasing tiny, single-celled spores. Scale trees produced cone-like pods that dropped
their spores into the water below, to be whisked away to the nearest patch of soil. But lycopods were also the earliest vascular
plants, which means they were the first plants to contain xylem, a special tissue that
moves water and nutrients from the roots to the leaves. This tissue is what allows modern trees to
grow tall and straight, because it contains lignin, a rigid organic polymer that gives
the plant structural support. But scale trees only produced a small amount
of lignin. And they also had much shallower anchoring
roots than today’s trees. So how did they manage to grow so large without
simply falling over? Their secret was in their bark. The outer layer of the scale tree, the periderm,
was extremely thick and actually made up most of the trunk. This allowed scale trees to reach great heights
— some growing as large as 50 meters high and more than 2 metres across! But they were still a lot more delicate than
the hardwoods you find in today’s forests. A single storm could probably knock down a
whole swath of them. And they didn’t always look quite so majestic. As juveniles, scale trees looked more like
giant spikes sprouting out of the ground. At this stage, they had no branches to speak
of, so their leaves just grew right out of the bark, in a spiral pattern. As the trees matured and shed their leaves,
diamond-shaped marks were left in the bark, called leaf scars, giving the scale trees
their name. So, scale trees were weird by the standards
of your modern oak or pine. But when you’re talking about evolutionary
history, you can’t really argue with results. Scale trees were built to grow fast and die
young, allowing them to spread across North America, Europe and Asia, leaving huge swaths
of organic matter in their wake. At their peak, they made up nearly half the
biomass in Europe and North America, and provided food and shelter for the giant insects and
early reptiles that called these forests home. But the scale trees’ success may also have
led to their downfall. Their vast forests pumped out huge amounts
of oxygen, which was great for us animals. But at the same time, the scale trees were
also sucking up tons of CO2 from the atmosphere. And back in the Carboniferous, many of the
decomposing microbes that could break down the hardy lignin — and release CO2 as a result
— had yet to evolve. So, over time, scads of scale trees collapsed
and piled up, without decomposing, keeping much of the CO2 that they had taken up, locked
away in their tissues. In addition to really putting the carbon in
the Carboniferous, this storage of CO2 also created a sort of reverse-greenhouse effect. Falling CO2 levels, combined with a sudden
burst in volcanic activity, plunged the Earth into an early ice age. The resulting extinction became known as the
Carboniferous Rainforest Collapse, and the scale trees were among its casualties. A few species did manage to hang on, in places
like Asia and South America, into the Permian Period, but by 272 million years ago, the
last of the scale trees were gone. Nonetheless, these bizarre plants left behind
a big legacy. After living and dying in prolific numbers
for millions of years, scale trees and other early plants formed a thick layer of carbon-rich
peat across much of our swampy world. The peat was eventually buried, compacted,
and exposed to extreme heat, turning into coal. And eventually, some hairless ape dug it up
and burned it, releasing the energy that the scale trees absorbed from the Carboniferous
sun over 300 million years ago. Think about that the next time you turn on
a light, or charge your phone, or … watch a video about natural history. As plants go, scale trees were powerful. What do you want to know about the story of
life on Earth? Let us know in the comments. And don’t forget to go to youtube.com/eons
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