Our brains are composed of three central elements:
Neurons, the cells that transmit information from one place to another;
Synapses, the connections between neurons;
and the Circuit, the specific pattern of connections between the neurons.
Without any one of these our brains would fail.
Without synapses, neurons couldn't transmit information between them,
without neurons there would be no electrical activity,
and without a specific circuit no meaningful computation can be performed.
Another wonderful example of irreducible complexity.
So how did it all evolve?
First came the neurons.
A neuron is simply a cell that transmits an electrical impulse from one location to another.
The electrical impulse is simply a passive wave of ions.
Once started, it will propagate through the cell on its own,
just like ripples on the surface of a pond.
All the cell needs is a means of starting such a wave,
and a means of using the electrical wave to do something useful.
In order to start an electrical wave an ion channel is needed.
Ion channels are proteins in the cell's membrane
that allow one or more types of ions to move in or out of the cell
in response to a particular stimulus.
DNA sequences reveal that the ion channels used in the neurons of animals
had their origin in bacteria
where they were used to regulate ion concentrations inside the cell.
At the opposite end of our early neuron existed a special type of ion channel,
called a voltage-gated ion channel.
These open in response to a passing electrical wave
and can allow special ions like calcium to enter the cell
which goes on to activate a multitude of cellular processes.
Again, DNA sequencing reveals that voltage-gated ion channels
also had their origins in bacteria.
So the first neurons were simply co-opted bacterial ions channels
put to the new task of transmitting information.
Such basic electrical signaling evolved even before the appearance of multicellular animals.
The single celled Paramecium
generates a voltage change when it bumps into an obstacle.
The wave of ions travels across the cell reversing the beating of its cilia,
allowing it to change direction.
Moving to multicellular organisms,
such proto-neurons would have been highly useful even without synapses.
An electrical wave can traverse macroscopic distances
orders of magnitude faster than simple diffusion.
Therefore, as body size enlarged,
the distance between locations where a stimulus may be sensed
and where an action is required
such as sensing a touch and contracting a muscle
would also enlarge,
thus requiring a means for rapidly transmitting information.
Early multicellular animals through gene duplication, mutations, and natural selection,
co-opted preexisting ion channels originally evolved in bacteria
to produce proto-neurons to accomplish just that.
Over time the family of ion channel genes would have enlarged
allowing for more specialized functions
such as the active propagation of electrical waves known as action potentials.
Action potentials involve specialized sodium and potassium ion channels,
that for an electrical analogy act as repeaters boosting the signal as it travels,
and can therefore transmit information faster
and over longer distances than simple passive waves.
Without which organisms could never have achieved the size they did.
Next came synapses.
Early synapses would have been simple pores between neighboring cells.
Such pores can be formed by gap junction proteins
which evolved around the time multicellular organisms first appeared,
allowing molecules to diffuse between neighboring cells.
These pores, called electrical synapses,
allow the electrical wave to travel from one cell to another
and can still be found in the brains of almost all animals today.
So the first synapses were co-opted molecular pores
put to the new task of permitting electrical waves to travel between cells.
The other type of synapse, called a chemical synapse, is a bit more complex.
Here the electrical wave in one neuron
causes that cell to release a chemical (called a neurotransmitter).
That chemical then binds to receptors on a nearby neuron.
When bound those receptors open their ion channels starting a new electrical wave.
Again, such a system appears irreducibly complex.
Why release the neurotransmitter if there is no receptor for it to bind.
Why have such a receptor if no cells are releasing the chemical.
However, if we actually look at the proteins involved in making a chemical synapse
their origin becomes apparent.
The receptor came first.
Analysis of DNA sequences reveals that glutamate receptors
existed before the divergence of plants and animals
and therefore before the emergence of multicellular life.
And the glutamate binding domain goes as far back as bacteria.
Early unicellular organisms would have used this receptor to sense glutamate,
an important cellular metabolite, in their environment.
Later multicellular organisms co-opted this protein
by evolving cells to release glutamate
near another cell already expressing glutamate receptors.
And thus the chemical synapse was born.
Over time through gene duplications, mutation, and natural selection,
different types of neurotransmitters and receptors evolved.
Some allow positively charged ions to flow into the cell
leading to an electrical wave resulting in the release of neurotransmitter from that cell,
and are thus term excitatory synapses,
but others allow negatively charged ions to flow into the cell.
These can essentially cancel out any positive waves,
thereby preventing the cell from releasing neurotransmitter,
and are therefore called inhibitory synapses.
But what was the use of synapses if complex circuits weren't already genetically encoded.
It was the origin of synapses
that allowed circuit patters to begin evolving under natural selection.
And as we will see, even extremely simple circuits can be useful to an organism.
The earliest circuits transduced sensory input to motor output in a reflexive manner,
basically input-output with little to no computation in the middle.
An example are the nerve nets in sea anemones.
More complex circuits can be completely genetically encoded
such as the "brain" of the nematode C. elegans which only contains 302 neurons.
But even simple 4 neuron circuits can make decisions,
1 neuron is enough to store short-term memory,
and 1 synapse is enough to store long-term memory.
Don't believe me, here's how.
Here we see two simple input-output circuits.
An input on the left results in the left muscle contracting.
An input on the right results in the right muscle contracting.
Now let's add an intermediate neuron that also happens to synapse onto itself.
Now an input on either side not only causes that muscle to contract,
but because the intermediate neuron continues to excite itself,
the muscle remains contracted,
hence the circuit remembers the stimulus long after it has gone away.
But how can a simple circuit make decisions.
Let's image a case where stimuli come from both sides
and the animal needs to contract only the muscles
on the side where the stimulus is greater,
it would be a waste of energy for the organism to fight itself and contract both muscles.
Here we can add two more neurons,
but these are inhibitory, not excitatory,
and they connect the left and right circuits together.
Now any input to the left inhibits the right circuit,
and any input to the right inhibits the left circuit.
When the stimuli are perfectly balanced they cancel out.
However, if one side receives slightly more stimulus,
it can beat out the opposite side, resulting in only one muscle contracting.
Learning can occur through modification of the network
or plasticity of the strength of synaptic connections.
Each of these processes has co-opted preexisting cellular machinery.
If we add two more excitatory connections, here connecting the right and left circuits,
we now have a complete circuit that can modify the weights of its synaptic connections
to either allow the animal to move towards the stimulus as shown here,
or if aversive, to move away from the stimulus as shown here.
As you can see, a circuit composed of only 4 neurons
is capable of learning, making decisions, and storing memory.
So we have seen how complex neural circuits,
capable of learning, remembering, and deciding,
composed of neurons expressing a suite of ion channels,
connected by chemical and electrical synapses
can evolve through a series of gradual, logical, and biologically plausible steps.
Each step involves the co-option of genes that already existed in the organism,
some dating back as far as bacteria.
So, how did our brains evolve from these simple nervous systems?
And what about our thoughts, our ability to be self-aware?
That will be the topic of my next video, the Origin of Cognition.
Until then, remember to exercise those neural circuits
Think about it.