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Endosymbiotic theory, is an evolutionary theory which explains the origin of eukaryotic cells from prokaryotes.

Key organelles of eukaryotes originated as symbiosis between separate single-celled organisms.

According to this theory, mitochondria and chloroplasts are descendants of formerly free-living bacteria.

These bacteria were taken inside another cell as an endosymbiont, and over time lost their ability to live on their own, but gained a stable protective environment.

Would have occured around 1.5 billion years ago.



Uprooting the Tree of Life

Scientific American, Feb 2000 W. Ford Doolittle

Scientific American,
Feb 2000 W. Ford Doolittle


Teaching guide

How do we, as science teachers, get this across to students? Using just a single image or animation is a good start, but that also is an over-simplification, which can lead to serious misconceptions.

However, if we have step-by-step descriptions, or animations, it is easier for students to see the big picture correctly.

Here is one useful animation of “the first symbiotic event”

This animation gets part of the idea across very easily. However, some students may get the idea that “MIRACULOUSLY this happened by random chance, and that’s ANOTHER example of how evolution is no different than religion. Wishful thinking and super lucky events!” Which isn’t correct. So we need to teach this in context, so they hear what we are really saying. To do this we need to understand what misconceptions people are likely to have, probe for them, and then address them.

* (Student thinks/says) “So this happened just once? And then all of our eukaryotic cells descended from this?”

Teacher: Nope, consider that cells lived all across the Earth, in millions of local environments, in billions of gallons of water and watery-regions.  All of them had lipid bilayer membranes, with proteins that let things get in and out. These proteins weaken the bilayer, so that it can be broken. 

Also, over hundreds of millions of years, trillions and trillions of cells bumped into each other all the time. Some of these cells could easily fuse together, in certain circumstances. Some of these cells could easily engulf other cells in certain circumstances.

* (Student thinks/says) “Oh! So it’s not just one lucky event. It’s non-stop, normal events that happen all the time, most of which lead to _nothing special_”

Teacher: Now you’re getting it.

* (Student thinks/says) “Oh, so once this happened, then that one special cell led to all of the eukaryotic cells that we have today.”

Teacher: Maybe…. but it’s also likely that even after this happened…. nothing lucky occurred.  That cell-within-a-cell probably didn’t reproduce to make perfect copies of itself, that first time.  It may not even have reproduced at all.

* (Student thinks/says) “What the hell, man?!  So this is how it happened once, but then didn’t necessarily lead to us?”

Teacher: We don’t assume, or even require, lucky events.  It’s not a one-in-a-trillion event. Do some math: Calculate the volume of the oceans. Then calculate how many cells could live in the ocean at once.  Then calculate how many cells could live and die in the Earth’s oceans per year. It will be a GINORMOUS number of cells. Then multiply that by a BILLION years.  That’s the number of opportunities for cool stuff to happen.

So are the odds for this event happening once one-in-a-trillion? Or are the odds, way, way better? 🙂

Hint: They are not only much better, but given the chemistry, size and age of the Earth, perhaps these events are almost certain to happen?

* (Student thinks/says) “So…. you’re saying that endosymbiotic events didn’t just happen once, they probably happened many time.”

{And if no students say this, prompt them with Socratic questioning to them to realize this}

* (Student thinks/says) “So, we don’t need “lucky events”.  Maybe most of these endosymbiotic events didn’t lead to many descendants.  Only some did. And out of billions of times this happened, all we need is just one success.”

Teacher: Yes – and that’s an important idea about evolution itself: We don’t require lucky events. Evolution “works” because there are billions of events happening all the time, over huge volumes of the Earth, and it is Ok that most of them don’t lead to anything different. As long as one event, one time, leads to something being successful (having healthy offspring, that themselves have healthy offspring) then that’s how new things develop.

Learning Standards

HS-LS4-2. Construct an explanation based on evidence that Darwin’s theory of evolution by natural selection occurs in a population when the following conditions are met: (a) more offspring are produced than can be supported by the environment, (b) there is heritable variation among individuals, and (c) some of these variations lead to differential fitness among individuals as some individuals are better able to compete
for limited resources than others.

HS-LS4-4. Research and communicate information about key features of viruses and bacteria to explain their ability to adapt and reproduce in a wide variety of environments.

HS-LS4-5. Evaluate models that demonstrate how changes in an environment may result in the evolution of a population of a given species, the emergence of new species over generations, or the extinction of other species due to the processes of genetic drift,
gene flow, mutation, and natural selection.

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