Thread by @LyingWrongAgain

I was criticized for not inserting detailed info about how a complex molecular machine *could have* evolved into someone’s brain

against their will

in tiny understandable tweets

without them having to read an actual science paper on it

So, ok, here’s the story in crayons 🖍️

🚨Disclaimer, please read

🚨More disclaimers

‼️Warning, oversimplified concepts and just-so stories ahead

⚠️Proceed with caution 😂

First: what constitutes an explanation of how a flagellum evolved?

We have to start somewhere

We start with bacteria that had no flagellum

We are not attempting to explain how the first cells arose

So what counts as an explanation?

We have to show how ancient bacteria which have no flagellum can have parts that can change over generations in a stepwise fashion

And each step has to have some function that is adaptive (helps survival)

Essential cell biology background:

We start w bacteria. They make proteins

Regions of DNA called genes determine the proteins

There are many variations of genes and proteins that can result in many different chemical proclivities & assemblies

Example: DNA > copies of a protein > multiple copies tend to get together into a ring

The outside of the ring is hydrophobic (hates water)

So, this ring tends to embed in the cell membrane (away from water)

Such rings can serve as ion transport pores in membranes

Ancient bacteria had some proteins that form such pores

Other proteins form fibers. Copies > tend to bind end to end in a helical fashion to form rods

Imagine a conga line

Why would a pore and a fiber get together?

Initially it could happen infrequently by chance, as they float around, and the binding might be unstable

Suppose a mutation in DNA forms fibers that happen to be able to stick out of some pores. Maybe they did this poorly at first

Now you have a spiky bacteria. Maybe only a little spiky

This is possible from a genetics and biochemistry perspective

But if it occurred, why would it persist?

It would persist if it was adaptive. That is, if it helped survival - even only a little

How could spiky-ness help survival?

One answer: adhesion

Maybe the bacterial colony found a great food source. Those who stick around do better. Those who get washed away by rain and currents do worse

Other possible answers:

if the spike has a hollow tube it’s now a needle that can transport stuff out of the cell

spiky-ness could also offer some defense, like a porcupine or puffer fish

(Remember I am *in good faith* oversimplifying for simplicity)

Also: bacteria have ion channels in membranes. Think little rings that let ions flow through and convert that flow into a little mechanical “push”

Much like a windmill or wind turbine converts the flow of air or water into a mechanical rotation

Sometimes our spike and those ion channels might encounter each other in the membrane

Imagine a mutation that causes them to tend to encounter more often

Sometimes, the “push” from our ion channel jostles the spike

How could this be adaptive?

Maybe the result is this spiky bacteria sticks around … but not forever

It sometimes wiggles its spikes and can break free again

That could be adaptive if a good survival strategy is a balance between “stick around forever” and “float on the mercy of the currents”

Or if wiggling spikes are a better defense than motionless ones (“get away from me, this food source is mine”)

Now imagine mutations improve the coupling of the ion channels and the spike. A mutation of some other protein helps bridge the two

Now it’s very wiggly, not just a little

Mutations could couple the wiggles better to rotation of the spike

Further mutations make the spike itself more flexible

Now instead of a wiggly spike for “Goldilocks adhesion” & maybe dynamic defense

you have a rotating string that acts more like a propellor

How could this change be adaptive?

Maybe the ion channel originally allowed lots of different kinds of ions to pass but now it’s inner channel shape has been tweaked to be very selective for only 1 ion

That 1 ion tends to accumulate and flow more when, let’s say, times are bad

This causes all the spinning spikes to propel, causing a random tumbling motion - you can get the heck outta Dodge

Now you have something like a flagellum that turns on and off with selective signal

But what about irreducible complexity?

Irreducible complexity means if you remove one part the whole thing no longer functions

How could that evolve?

Many ways actually. Here’s one. Remember that “helper protein” I mentioned that helps the ion channel get together with the spike?

We assumed originally, the ion channel *sometimes* poorly bound to the spike when both floated in the membrane, providing just a wiggly spike

But the helper protein came later and made this a sure thing: now all those ion channels bind to all the spikes, improving wiggly-ness

So originally we had A+B=functional, now we have A+B+C=more functional

Now suppose A and B get tweaked a bit such that they do their jobs better. A is a better ion channel, B is a better spike, C is a better helper to bring and keep A and B together

“Division of labor”, everybody wins

But now, suppose A and B have become critically dependent on C. Without C, they bind so poorly they no longer function at all

In fact A hardly works as an ion channel unless stabilized by C and B can’t properly form the spike without C

It’s called evolved codependency

Other examples of evolved irreducible complexity are elimination of redundancies

Maybe originally, a few variants of C were helpers, C and C+

If you removed one of the variants, the whole assembly could still function - but less quickly, less reliably, less stably

Later, C got really good at its job and C+ was no longer needed. Mutations stop C+ entirely or give it mutations to help some different pathway

So, the original assembly A+B+C+C’=functional was not irreducibly complex

bc you could remove at least one part (C’) and it works (albeit poorly)

But the later evolved assembly A+B+C=functional is irreducibly complex

Remove C, and it fails completely

This can happen, it’s called elimination of functional redundancy

So what do we end up with?

We end up with a flagellum that assembles step-by-step in a manner analogous to how it evolved

- membrane pores
- pores + spikes
- Ion channels + spike rotation
- Flexible longer spikes
- Stabilizing proteins
- Codependency/redundancies

But still we’ve grossly oversimplified because there is not “a” flagellum

There are many variants of bacterial flagella

Here’s actual data (greyscale images) of their structures

More pretty pictures (actual data in greyscale) of the variety of ways these motors can be formed in bacteria

Some are simpler; some are more complex

Simpler might be better if they are “cheaper” for cells to make

Complex ones provide faster swimming speed

Note that if our “just so” story of bacterial flagellum evolution is true, it makes testable predictions

It should be possible to fit the many different flagella into a nested tree, based on (1) overall structure and (2) the genes that code for the proteins

So to summarize: what “good” was an ancestral version of this motor?

It helped bacteria be wiggly

And before that, spiky and sticky

And before that, forming channels in its membrane and using ion channels to do a little work (provide a “push” like a water wheel)