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What Does It Mean To Be An Organism? The Parable of the Chloroplast
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Maximus Thaler
Maximus Thaler
is a PhD candidate at Binghamton University studying cultural evolution.

There is a story I like to tell called the Parable of the Chloroplast.  Single celled organisms don’t feel things like humans do, and they certainly don’t speak, but, for the sake of the story, for now let’s pretend that they do.

Here’s the story:

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Around 2 billion years ago, was a single celled organism called Chloroplast. Chloroplast made a living for herself by floating in the sea, transforming sunlight into sugar. One day, she was voraciously consumed by a bigger cell, named Plant. But before Plant’s digestive enzymes had completely dissolved Chloroplast’s body she called out, “Wait! You can eat me, but don’t digest me just yet! Instead, let me live inside you. If you protect me, and make sure I have enough water and sunlight and minerals, I will grow and transform those things into sugar, just for you! You will have all the sugar you could ever want!”

Plant considered this proposal and after some deliberation, she decided it was a good deal. The modern eukaryotic cell was born, a symbiotic merger of two previously autonomous bacteria.

Now of course, this is not actually want happened. No formal deal was struck in a day, and it took literally millions of years of coevolution for this interpenetrative relationship to develop. But the story is worth condensing in this way because doing so sheds light on another, related story. Here’s a second story:

Around 10,000 years ago, there was a humble plant called Wheat. Wheat made a living for herself by growing in Mesopotamian soils, transforming sunlight into sugar. One day she was voraciously consumed by a bigger organism, called Human. But before Human’s digestive enzymes had completely dissolved Wheat’s body she called out, “Wait! You can eat me, but not all at once. Instead, plant some of me by your house. If you protect me, and make sure I have enough water and sunlight and minerals, I will grow and transform those things into sugar, just for you! You will have all the sugar you could ever want!”

Human considered the proposal and, after some deliberation, she decided it was a good deal. Domestic agriculture was born, a symbiotic merger between humans and their crops.

Now of course, this is not exactly what happened either. No formal deal was struck in a day, and it took thousands of years of cultural evolution for this interpenetrative relationship to develop. But the story is worth condensing in this way in order to highlight the common features of two very different branches on the tree of life.

There are other places where you could tell this story. A similar deal between alga and fungus has evolved hundreds of times, producing the many flavors of lichen. And something like it has also evolved between fungi and members of the atta genus of ants. What these examples suggest is that there have been times throughout the history of life when autonomous creatures have merged together to form a new, higher kind of organism.

Describing these examples in the form of a parable highlights that the critical dynamic acting in all of these cases is a tension between conflict (being eaten) and cooperation (being cultivated). As biologists have come to recognize the importance of these organismal fusions, there has been a renewed emphasis on studying how conflict and cooperation interact to create the kinds beings which we call organisms.

A notable voice in this conversation has been Dr. Joan Strassmann. She has published several papers which advance a theory which defines organisms as a bundle of adaptations among which there are high levels of cooperation and low levels of actual conflict. This definition recognizes that organisms exist on a spectrum, and that the idea “organism” itself is better thought of as a verb, some action that life does, rather than a noun, some thing that life is.


What makes an Organism by Nicole Chapko

If you’d like to learn more about these ideas, a good place to start would be Dr. Strassmann’s paper Beyond society: the evolution of organismality. Dr. Strassman presented on this paper for the EvoS Seminar Series. Her full lecture is below.

The EvoS Seminar Series is an online class which presents evolutionary theory as a chain of linked ideas that connect science to story. We provide articles (sometimes with silly parables), explainer videos, live lectures, and peer reviewed papers all in one central place, to allow our audience to engage with the material on multiple levels of complexity. We believe that the best way to learn science is to learn how to tell stories about it.

You can subscribe to our Youtube channel for even more links in the Science to Narrative chain. We’re also on facebook.

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