Where Did Eukaryotic Cells Come From?

Summary

The video script explores the pivotal moment 1.8 billion years ago when a cell consumed another cell and didn't digest it, leading to the emergence of eukaryotes through endosymbiosis. It delves into the symbiotic relationships between organisms like Paramecium bursaria and Chlorella algae, and the benefits of such alliances, including survival and energy production. The script challenges viewers to reconsider the boundaries of what constitutes an organism and ponders the nature of our own mitochondria.

Takeaways

• 🌿 Life on Earth began with prokaryotes, simple unicellular organisms, over 3.5 billion years ago.
• 🔬 A significant evolutionary leap occurred 1.8 billion years ago with the emergence of eukaryotes, which contain cells within cells.
• 🤔 The Endosymbiotic Theory suggests that eukaryotes evolved when one cell engulfed another but didn't digest it, leading to a symbiotic relationship.
• 🌱 Chloroplasts and mitochondria are examples of organelles that may have originated from endosymbiotic events with photosynthetic and oxygen-using bacteria, respectively.
• 🐠 Paramecium bursaria is a single-celled organism that hosts algal cells, illustrating a modern example of endosymbiosis where both parties benefit.
• 🌳 Algae within Paramecium bursaria provide energy through photosynthesis, while the organism offers protection, showcasing a mutualistic relationship.
• 🦠 Anaerobic organisms like Metopus, an anaerobic ciliate, have endosymbiotic relationships with methanogenic archaea, which can produce energy without oxygen.
• 🌈 Multicellular organisms like Hydra also exhibit endosymbiosis, with algal cells providing significant nutritional support.
• 🧬 The presence of algal cells within P. bursaria raises philosophical questions about the definition of an organism and the nature of life.
• 🔬 The study of endosymbiosis challenges traditional views of organism boundaries and suggests a more interconnected understanding of life.

Q & A

When did life on Earth first emerge according to the transcript?

Life on Earth first emerged at least three and a half billion years ago as prokaryotes.

What significant event occurred 1.8 billion years ago in the history of life on Earth?

A significant event that occurred 1.8 billion years ago was the beginning of endosymbiosis, where cells started to contain other cells.

What is the term for cells within cells in eukaryotes?

The term for cells within cells in eukaryotes is 'membrane-bound organelles'.

What is the 'Endosymbiotic Theory' as mentioned in the script?

The 'Endosymbiotic Theory' is the idea that certain organelles of eukaryotic cells, such as mitochondria and chloroplasts, originated from endosymbiotic events where one cell was taken up by another and both lived together.

How did mitochondria and chloroplasts come into existence according to the endosymbiotic theory?

According to the endosymbiotic theory, mitochondria appeared when a cell adapted to live in an oxygen-rich environment was consumed but not digested, and chloroplasts appeared when a photosynthetic cell was swallowed and similarly not digested.

What is an example of a single-celled organism that has endosymbiotic algae?

Paramecium bursaria is an example of a single-celled organism that has endosymbiotic algae, specifically Chlorella, living within its cytoplasm.

What benefits do the Chlorella algae provide to Paramecium bursaria?

The Chlorella algae provide Paramecium bursaria with fuel in the form of sugar and other substances produced via photosynthesis.

How long can Paramecium bursaria with Chlorella survive in a nutrition-free saline solution?

Paramecium bursaria with Chlorella can survive for more than 3 months in a nutrition-free saline solution.

What is the relationship between Metopus and methanogenic archaea?

Metopus, an anaerobic ciliate, has an endosymbiotic relationship with methanogenic archaea, which can use free hydrogen to generate energy and methane.

What benefits do the algal endosymbionts provide to Hydra?

The algal endosymbionts provide up to 69% of the caloric requirements of Hydra through photosynthesis.

What philosophical question does the script raise about the nature of organisms and their components?

The script raises the philosophical question of whether the cells within an organism, such as the mitochondria in humans, are truly part of the organism or if they are separate entities that are simply reliant on the host cell.

Outlines

00:00

🌿 The Birth of Eukaryotes and Endosymbiosis

This paragraph delves into the pivotal moment in Earth's history when prokaryotes, simple unicellular organisms, underwent a transformative event 1.8 billion years ago. The narrative explains how these early cells began to contain other cells, leading to the emergence of eukaryotes. The 'Endosymbiotic Theory' is introduced, which posits that a cell consumed another cell but allowed it to reproduce within, eventually leading to a symbiotic relationship. This theory is exemplified by the development of mitochondria in oxygen-rich environments and chloroplasts in photosynthetic cells. The paragraph also discusses the controversial nature of this theory and how it has been supported by evidence such as the unique DNA of chloroplasts. It concludes with examples of endosymbiosis in single-celled organisms like Paramecium bursaria, which houses algal cells that provide it with nutrients and in return receive protection, highlighting the mutual benefits of such relationships.

05:01

🐠 Endosymbiosis in Diverse Organisms

The second paragraph explores the prevalence of endosymbiosis across various organisms, including anaerobic unicellular organisms like Metopus, which form symbiotic relationships with methanogenic archaea. It touches on the lesser-known domain of life, the archaea, which are prokaryotic like bacteria but distinct in their own right. The paragraph also discusses the benefits of endosymbiosis for organisms like Hydra, a freshwater relative of jellyfish and sea anemones, which rely heavily on algal endosymbionts for up to 69% of their caloric needs. The narrative ponders the blurry lines between different organisms and the concept of identity within a cell, challenging the viewer to consider whether components like mitochondria are truly part of an organism or independent entities. The paragraph ends with a call to subscribe for more content and a plug for SciShow's documentary on the moon landing's 50th anniversary.

Keywords

💡Prokaryotes

Prokaryotes are simple unicellular organisms that lack a nucleus and membrane-bound organelles. They are one of the earliest forms of life on Earth, existing for nearly half of the planet's history. In the context of the video, prokaryotes represent the initial stage of life's complexity before the advent of eukaryotic cells. The video script mentions that life began with prokaryotes, setting the stage for the evolution discussed throughout the narrative.

💡Eukaryotes

Eukaryotes are organisms whose cells contain a nucleus and other membrane-bound organelles. They represent a significant evolutionary leap from prokaryotes, allowing for more complex life forms. The video emphasizes the importance of eukaryotes by discussing how they contain cells within cells, a concept central to the endosymbiotic theory. The script describes eukaryotes as having 'membrane-bound organelles,' highlighting their role in the diversification and complexity of life.

💡Endosymbiotic Theory

The endosymbiotic theory posits that certain organelles within eukaryotic cells, such as mitochondria and chloroplasts, were once free-living prokaryotic organisms that were engulfed by other cells but not digested, eventually becoming essential parts of the host cell. This theory is central to the video's exploration of how life evolved from simple to complex forms. The script uses the example of mitochondria and chloroplasts to illustrate how endosymbiosis led to the development of these organelles.

💡Mitochondria

Mitochondria are organelles found in eukaryotic cells that are responsible for producing energy through cellular respiration. They are considered the 'powerhouses' of the cell. The video script discusses mitochondria as an example of endosymbiosis, suggesting that they originated from a cell that was consumed and adapted to live in an oxygen-rich environment, thus becoming an integral part of the eukaryotic cell.

💡Chloroplasts

Chloroplasts are organelles in plant cells and some algae that carry out photosynthesis, converting light energy into chemical energy. The video script mentions chloroplasts as another example of endosymbiosis, where a photosynthetic cell was engulfed and eventually became a permanent part of the host cell, allowing it to perform photosynthesis.

💡Paramecium bursaria

Paramecium bursaria is a single-celled protozoan that has a symbiotic relationship with algal cells from the genus Chlorella. The video uses this organism to illustrate a modern example of endosymbiosis, where the Chlorella cells live within the cytoplasm of Paramecium, providing it with nutrients and protection in exchange for a safe environment. This relationship exemplifies the互利共生 concept discussed in the script.

💡Anaerobes

Anaerobes are organisms that do not require oxygen for growth and can be negatively affected or even killed by the presence of free oxygen. The video script introduces Metopus, an anaerobic ciliate, to discuss how some single-celled eukaryotes form symbiotic relationships with prokaryotes, such as methanogens, which can generate energy and methane using free hydrogen.

💡Hydra

Hydra is a freshwater organism, a relative of jellyfish and sea anemones, that contains algal endosymbionts. The video script describes how Hydra benefits from its symbiotic relationship with algae, with studies showing that up to 69% of its caloric needs are met by these symbionts. This example underscores the video's theme of how symbiotic relationships can be essential for survival and growth.

💡Symbiosis

Symbiosis refers to a close, long-term interaction between two different species, which can be互利共生 (mutualistic), commensal (one benefits, the other is unaffected), or parasitic (one benefits, the other is harmed). The video script explores various forms of symbiosis, such as the relationships between Paramecium bursaria and Chlorella, and Hydra and its algal symbionts, to demonstrate how these interactions can be beneficial for both parties involved.

💡Methanogens

Methanogens are a group of archaea that produce methane as a byproduct of their metabolism, often using hydrogen and carbon dioxide. The video script mentions methanogens in the context of their symbiotic relationship with anaerobic ciliates like Metopus, where the methanogens provide energy benefits to the host organism, illustrating the diverse ways in which symbiosis can manifest in the microbial world.

Highlights

Life on Earth began with prokaryotes, simple unicellular organisms, around 3.5 billion years ago.

A major evolutionary leap occurred 1.8 billion years ago with the emergence of cells containing cells, leading to more complex life forms.

Eukaryotes, cells with membrane-bound organelles, allow for different conditions necessary for various cellular activities.

The Endosymbiotic Theory suggests that mitochondria and chloroplasts originated from endosymbiotic events.

Chloroplasts have their own DNA, which is used to produce proteins essential for photosynthesis.

Paramecium bursaria, a single-celled protozoa, hosts algal cells that provide it with photosynthetic products in exchange for protection.

The symbiotic relationship between Paramecium bursaria and Chlorella algae is hereditary and mutually beneficial.

Testate amoebae, a type of amoeba, also have endosymbiotic algae that produce food and share it while receiving protection.

Anaerobic organisms like Metopus form endosymbiotic relationships with methanogenic archaea for energy production.

Multicellular organisms such as Hydra also engage in endosymbiosis with algae, with significant nutritional benefits.

Endosymbiosis can be temporary or hereditary, and some organisms are entirely dependent on their symbionts for survival.

The line between different organisms blurs as endosymbiosis challenges traditional views of individual organisms.

The mitochondria within our cells might be considered as separate prokaryotic species that are highly reliant on their host.

Our cells may contain more than just 'ourselves,' suggesting a complex ecosystem within our own bodies.

Journey to the Microcosmos is produced by Complexly, which also produces SciShow and other educational YouTube series.

SciShow has released a documentary for the 50th anniversary of the moon landing, exploring whether it was a good idea.

James, the master of microscopes, can be followed on Instagram for more fascinating microscopic insights.

Transcripts

Life on Earth emerged at least three and a half billion years ago as prokaryotes.

These are the simple unicellular organisms.

They have a membrane on the outside and a wash of cellular machinery inside, all mixed

together and touching, and sharing the same environment.

This worked, certainly.

Life chugged along this way for nearly half of the history of life on earth.

But then, 1.8 billion years ago, something remarkable happened.

Something that led to a tremendous shift in the scope and complexity of life.

Something we should all be grateful for because, without that leap, we would not exist.

Cells started to...contain cells.

Now this isn’t generally how it’s talked about in science class.

There you hear that Eukaryotes have quote “membrane-bound organelles.”

These are areas of the cell that are separated from the rest of the cytoplasm by membranes,

just as the cell itself is separated from the rest of the universe by its membrane.

It turns out, different activities require different conditions, and these cells within

cells allow for those different conditions.

That, in short, is the secret of Eukaryotic success.

But how did it happen?

Well, over decades of study we have determined something shockingly peculiar.

Something so odd that it makes us kind of mad that we now discuss it as if it isn’t

the miracle it is.

1.8 billion years ago, a cell consumed another cell...but then it didn’t digest it...it

let it reproduce inside of it, and they lived together, and, over time, became the same

organism.

Or did they?

This is what we call “Endosymbiotic Theory.”

Mitochondrion appeared when the consumed cell was adapted to live in an oxygen rich environment

and chloroplasts appeared when the swallowed cell was photosynthetic.

This idea was deeply controversial when it was first proposed, but as data have continued

to come in, endosymbiotic theory has been able to explain more and more about the realities

we find.

For example, that chloroplasts have their own DNA which they use to create the proteins

required for their function.

And as we dive deeper into the microcosmos, it just becomes obvious that this happens.

This is Paramecium bursaria, a single-celled protozoa, that has several hundred algal cells

from the genus Chlorella living in its own cytoplasm, making it green.

The algae live inside Paramecium bursaria providing it with fuel in the form of sugar

and other substances produced via photosynthesis.

And Paramecium bursaria provides protection for the algae from algae eaters and viruses.

P. bursaria is regarded as a predatory protozoa, it feeds on bacteria, small organisms, and, yes, algae

and because of that it's often thought that the algae in it are temporary symbionts

engulfed by Paramecium bursaria’s feeding behavior.

But in fact, many other protozoa acquire algae in that manner for temporary use, but that

is not the case for P. bursaria; its symbionts are continuously inherited from generation

to generation through cell division.

The symbiotic Chlorella guide the Paramecium to well lit areas, so they can photosynthesize

more efficiently.

The mutual relationship is extremely beneficial for the Paramecium.

Even when the Chlorella-containing Paramecium cells are put in nutrition-free saline solution they

can survive for more than 3 months while cells that didn't have Chlorella died within a week!

This is another single-celled organism with endosymbiotic algae, it's a testate amoeba.

A kind of amoeba that builds itself a shell.

This species, like some kind of sculptural artist, pulls bits and pieces of mineral from

its environment to create these amazing looking homes.

You can see the amoeba extending from the opening of the shell and you can see the green

algae in its cytoplasm.

Just like Paramecium bursaria, the algae use sunlight to produce food sharing it with the

amoeba while the amoeba provides protection.

Some unicellular organisms don't need oxygen for growth, indeed the presence of free oxygen

can affect them negatively or even kill them.

These organisms are known as anaerobes.

Such as this one, Metopus.

It is an anaerobic ciliate we find in pond sediment and it has an endosymbiotic relationship

with methanogenic archaea.

Now we haven’t talked much on this channel about archeans, but they are the third domain of life, along

with bacteria and eukaryotes and, like bacteria, they are prokaryotic.

We can’t wait to do our episode on them someday soon.

Many of the single-celled eukaryotes living in anaerobic environments contain symbiotic

prokaryotes, some of these prokaryotes are methanogens, meaning they can use free Hydrogen

to generate energy and methane.

The advantages of having these symbionts are not fully understood

but while Metopus can live without the symbionts

they grow faster when they have them.

Endosymbiosis occurs in multicellular organisms as well.

This is a freshwater relative of jellyfish and sea anemones, Hydra!

It's simply stuffed full of algal endosymbionts.

We collected this Hydra from a nearby pond and cultured it in our aquarium.

The benefits provided by the symbiotic relationship here have been well documented, with scientists

actually tracking how carbon moves from the environment, into the algea, and then into

the hydra, and studies have shown that up to 69% of the caloric requirements of the

hydra is satisfied by its algal symbionts.

Nice.

So, we see, some organisms temporarily pull in symbionts, others pass them from generation

to generation.

Some can survive without them, and some cannot.

When we look at the algal cells in P Bursaria, we’re forced to ask if those cells

are part of the organism, or if they’re simply cells of one species living in the

cells of another.

If that’s the case, it’s worth asking whether the mitochondria in you are you at

all, or if they are just another extremely successful species of prokaryote that is particularly reliant on

its host cell.

As we look deeper and deeper down, the line between organisms is harder and harder to find.

Which is why, if you think hard enough you might begin to feel like our cells are more

than just ourselves.

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