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 algae, 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.

Thank you for coming on this journey with us as we explore the unseen world that surrounds...and inhabits us. Journey to the Microcosmos is produced by Complexly, which produces over a dozen shows on YouTube, including Scishow. And we wanted to let you know that the SciShow team has just put out a really interesting new episode.

This year, of course, marks the 50th anniversary of the first time humans walked on the moon. And to celebrate, SciShow made their first documentary. The team traveled throughout the US.

I even went to the UK to talk to experts. trying to figure out whether the moon landing was actually a good idea. and they got some really interesting answers, but I won't spoil them. You can watch the episode at YouTube.com/SciShow or by clicking that link in the description. If you want to see more from our master of microscopes, James check out Jam and Germs on Instagram.

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