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How Fungi Made All Life on Land Possible


“Mycology” is a branch of biology concerned
with the study of fungi. It might not sound like the most interesting thing to make a
video about, but fungi are some of the most incredible and important organisms on the
planet… and have been throughout the history of earth, possibly having been around for
up to one billion years. Distinct from both plants and animals, fungi
are their own separate ‘kingdom’, with literally millions of species, coming in all
shapes and sizes. One fungus to the next can be as different as… a snake and a giraffe,
or a chicken and a dung beetle. Although we might tend to think of mushrooms
when we think of fungi, this is actually only the fruiting body of the fungus, with much
more going on below the surface. Some fungi are microscopic, invisible to the naked eye…
while one fungus (the “Humongous Fungus”) is believed to be the single largest organism
that exists by area, covering 2,200 acres in the US state of Oregon. For some people, the word “fungus” might
stir up negative connotations, and of course some fungi can cause infections or diseases,
while others are poisonous… and that furry stuff growing on your strawberries, that’s
a fungus too. However, while the bad is often visible and obvious, fungi are very much a
force for good as well. We just don’t notice it. Fungi are not something that we think
about often, but they are (and, have been) immensely important to the ecosystem of this
planet. In fact, it’s no understatement to say, that life as we know it, would not
be possible without fungi. To understand how, we need to go back a few
years… well, a few billion years. Life in the form of simple single-celled organisms
had already been around for a long time in the oceans, but, as far as we know, the land
was largely rocky and barren of life. Things started to change when some early bacteria
developed the ability of ‘photosynthesis’ – the process of converting the sun’s light
into nutrients. Over time, the byproduct of this, more oxygen in the atmosphere, led to
the acceleration of more complex organisms in what is known as the ‘Cambrian explosion’,
about 540 million years ago, but again, this was effectively confined to the water. The transition of more complex life forms
to land was possible because of fungi and their unique ability – fungi can eat rocks,
breaking them down and turning them into soil. This is achieved by secreting digestive enzymes,
as well as through mechanical pressure. Fungi were able to access nutrients that were otherwise
unavailable to any other organism at the time. It was commonly thought that a short 60 million
years or so after the Cambrian explosion, fungi began to move onto land; they would
have most likely benefited from access to more than two billion years of bacteria on
the shore to feed on. Of course, the fossil record from this period has many gaps, so
there’s plenty of uncertainty, with the possibility that fungi had already been on
land a full 500 million years earlier. However long fungi had been on land beforehand,
we are fairly certain that they were eventually followed by small proto-plants, simple organisms
that could photosynthesise. Fungi had minerals and plants had photosynthesis, but they both
needed what the other had to survive. Fungi and plants began to cooperate, in a
process known as “symbiosis”, forming a mutually beneficial relationship. As the
fungi and plants began to spread, colonising the land, they began to turn the Earth green.
The soil became more suitable for many other types of plants that had yet to evolve, eventually
allowing some to become independent of the fungi. With ecosystems becoming more complex, new,
dynamic balances were established. The increase in oxygen produced by plants was balanced out by a growing population of organisms that needed the oxygen to live. Likewise, the organic
matter that started to build up after things would die needed to be recycled so that it
could continue to be used. This is where fungi come in. To put it simply,
fungi “eat death”; by breaking down dead things, they allow the nutrients to be used
again by living things. This set up another important cycle that is fundamental to sustaining
all life on Earth, with fungi serving as one of the final building blocks for the world
we know and love today. Of course, while new cycles were being established
and developed into what would guide our modern ecosystems, older relationships continued
to thrive. In particular, the symbiosis between plants and fungi, called “Mycorrhiza”,
has continued to change and evolve, even to this day, allowing more complex partnerships
to form. There are two types of Mycorrhizae, ecto-mycorrhiza,
in which the fungus wraps itself around the roots of the plant, and endo-mycorrhiza, in
which the fungus will actually penetrate the cell wall of the plant, entwining itself around
the cell membrane. But as invasive as this sounds, this can actually make it even easier
for the plant to benefit. The plant will happily let another organism live literally inside
it, because the fungus helps the plant derive more nutrients. Today, the vast majority of plants benefit
from a symbiotic relationship with various different species of fungi. Some numbers have
suggested as high as 90% of plants in the world. Some plants, after millions of years
of evolution alongside fungi, still rely entirely on them for survival. The orchid plant family,
for example, has virtually no independent energy reserve during its germination stage
– that is, while it’s growing from a seed into… well, a plant. Now, many orchids engage in a symbiotic relationship
with fungi that is not mutually beneficial. Many of these species are actually a parasite
to certain fungi, in which the plant will effectively suck the energy out of the fungus,
in this case referred to as the “host”. At the same time, the fungus may also be involved
in a symbiotic relationship with another plant, so the orchid indirectly gets its energy through
photosynthesis, even while many of these species have actually lost the ability to photosynthesise
themselves. Parasitism can also work the other way around
– with a fungus that is a parasite to a plant. As well as the exchange of nutrients between
fungi and plants, fungi can actually help the plants exchange nutrients amongst themselves.
For example, a small tree in a forest with limited access to sunlight, could be ‘fed’
more nutrients to help it, thus growing tall enough to be able to photosynthesise on its
own. These more complex types of interactions between
a fungus and potentially many different plants, from the smallest flowers to the tallest trees,
are known as “common mycorrhizal networks”. Fungi are capable of connecting entire forests,
which can sort of be thought of as “Nature’s Internet”, or somewhat more comically, the
“Wood Wide Web”. Fungi are able to facilitate communication between plants, which can be
especially strong if the plants are of the same species. Now, obviously plants aren’t literally having
a conversation with each other, they’re not sentient. However, the communication they
are capable of is still pretty impressive. While plants are able to communicate via the
air as well, via the fungi is much more effective. Signals and cues are transferred between plants
which can influence behaviour. For example, fungi can mediate the transfer
of chemicals that plants produce to stunt the growth of their neighbouring, rival plants,
such as by depriving them of nutrients or inhibiting their photosynthesis. These so-called
“allelochemicals” may also be used against herbivores, such as insects, that might want
to eat the plant. Alternatively, fungi can also facilitate the
warning signals that affected plants send to unaffected plants, triggering the plants’
defensive response. Such a response could come in the form of a chemical that acts as
a repellent to the attacker, which could be a pathogen—something that can give plants
diseases—or, again, a herbivore. But as well as merely helping the plants communicate,
fungi can actually directly protect the plants. For example, a fungus can secrete a compound
that both kills pathogens and strengthens the plant’s immune system. Of course, this
isn’t just the fungus being altruistic. It’s just in the best interest of the fungus
that the plant survives, allowing the continuation of their mutually beneficial symbiotic relationship. Fungi have played a key role in the development
of the world as we know it. They may not always be visible, and they’re not usually something
we think of as being all that important… but fungi are an unseen cornerstone of their
ecosystems. Silently pulling strings ‘behind the scenes’, forging relationships with
many other organisms, both alive and dead. Of course, it’s not just plants that fungi
interact with, us humans also have a long history with fungi as well. Whether it be
as a food source, or the yeast we need for our beer and bread… their medicinal purposes,
pest control, and so many other uses, fungi have always been there… always there as
an influential part of not just the history of our planet, but of human history as well
– both for good and bad. The topic of fungi has proven to be one of
the most surprisingly interesting topics I’ve made a video about. This video was actually
inspired by a very compelling documentary I watched called The Kingdom: How Fungi Made
Our World. The documentary is available on CuriosityStream, which is the sponsor of this
video. The documentary takes a much deeper look into
the history of fungi, and their symbiosis with plants, as well as how fungi have revolutionised
medicine, their many other uses, and also the dangers posed by some fungi. This video
was barely able to scratch the surface. CuriosityStream is an online streaming service
with over 2,400 documentaries and non­fiction titles, with content spanning science, nature,
history, technology, society, lifestyle, and much more. Membership starts at just 2.99 per month,
and you can get started completely free with a 31-day trial, by signing up at CuriosityStream.com/wonderwhy,
and enter the promo-code “wonderwhy” during the sign-up process. A big thank you to CuriosityStream for sponsoring
this video, and for letting me use parts of the documentary in this video. And always,
thanks for watching, and I’ll see you next time.

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