Sciency Words: Endosymbiosis

Sciency Words MATH

Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us all expand our scientific vocabularies together. Today’s term is:


The story goes like this: long ago (roughly 1.5 to 2 billion years ago), two single-celled organisms reached a mutually beneficial arrangement.

Jn17 Endosymbiosis 1

Or perhaps one single-celled organism imposed a mutually beneficial arrangement on another.

Jn17 Endosymbiosis 2

Whichever way it happened, the result is what we now call endosymbiosis. It’s a term which comes from a bunch of Greek words meaning “living together” and “inside.”

The word endosymbiosis can be used to describe any mutually beneficial (non-parasitic) relationship where one organism lives inside another. Think of the bacteria living in your stomach helping you digest your food.

But typically, the term seems to be reserved for such relationships between single-celled organisms. One cell is called the host. The other—the one living inside the host—is called the endosymbiont.

Each individual cell in your body contains tiny internal structures called organelles. This is true not only for humans but all animals and plants, and many microorganisms, such as the amoeba or the paramecium. The presence of organelles is one defining characteristic for all eukaryotic life forms on Earth (as opposed to prokaryotic life forms like bacteria or archaea).

It is believed that, at some point in evolutionary history, all these organelles started out as endosymbionts. Some organelles, like mitochondria and chloroplasts, still have their own DNA separate from the DNA of the cell nucleus.

But at what point did the transition occur? At what point should we stop calling something an endosymbiont and start calling it an organelle? That question gets into some murky territory for biology. The distinction between organelles and endosymbionts is rather too poorly defined at the moment for a Sciency Words post.

6 thoughts on “Sciency Words: Endosymbiosis

  1. Interesting. I had heard of the theory with regard to Mitochondrias, but not in reference to other organelles as well. Do you know how far back in the evolutionary chain those other organelles would have had to have been? It would seem to me pretty early, maybe even before cells had evolved into something that could actually be considered life, but I don’t know much about single-cell history.

    Liked by 1 person

    1. Most of the numbers I’ve seen range between 1.5 and 2 billion years ago. I’m not sure if that time range covers all organelles or just mitochondria and chromatophores.

      The fact that mitochondria and chromatophores haven’t given up their separate generic identities yet might suggest that they joined team eukaryote more recently than other organelles (although that’s hardly certain at this point, according to the article I linked to at the end of this post).


  2. I always thought that concept sounded so cool when I first heard about it. It helps explain how complicated life formed as quickly as it did.

    Liked by 1 person

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