For the last seven years, researchers at the Prakash lab at Stanford University have sampled
rocks, algae mats, and ocean reefs in search of a tiny and mysterious multicellular organism
called Trichoplax.
Trichoplax is a species of Placozoa, the simplest animal at the base of the tree of life.
They don’t have a nervous system, making them living fossils from a critical time in
life’s evolution.
When the researchers looked at Trichoplax under a microscope, they were shocked to see
its millions of cells engaged in complex activity.
Trichoplax was able to coordinate its movement, strategize how to find food, and even reproduce
by ripping itself in two.
But how can Trichoplax exhibit these complex behaviors without a brain to call the shots?
The answer could shed light on the origins of the nervous system.
Traditionally, when you think about intelligence, you think of the brain.
Trichoplax stretches that intuition, stretches your imagination to ask how intelligent can
a collection of cells with no neurons, no muscles actually be?
How much intelligence can a mechanical collective embody?
To answer these questions, the researchers tried to understand the simple mechanics of
the Trichoplax, starting with the tiny waving filaments called cilia.
Cilia I've often described as the atom of motility.
For the last 50 years of the work in ciliary biology and ciliary biophysics, we have thought
of cilia as an actuator that enables swimming.
Typically in organisms with nerves and muscles, cilia help move fluids within the body.
But when the Prakash lab studied the cilia on the underside of the Trichoplax, they discovered
something unexpected.
I explicitly remember a moment when we started imaging the organism and literally watch the
cilia.
You could see that what we observed was a walking gait and the realization was that
the organism does not swim.
It actually walks.
The researchers designed a mathematical model to measure the orientation, height, and beat
frequency of each individual cilium as it balanced torque from its neighboring cells
with the force of its attachment to the surface below.
The result was a regular pattern of locomotion that the researchers called “walking.”
As they walked, these cilia cells exhibited dynamics similar to those seen in individual
neurons.
When you stimulate the neuron with a voltage, it executes a spike and that spike passes
information through the network.
Similarly, what we find in this mechanical system of a walking cilia where // just the
right pull in the right direction leads to a transition of a cilia from a stall state
to a walking state // And it actually starts “firing” in the sense that it starts “walking”.
But just how these individual cilia cells coordinated their movement remained a mystery.
We started characterize our work as asking the question, "How do you walk with a million
legs?"
"How can many of these cilia work together?
When the researchers looked at all of the ciliated cells at once, they saw something
they never thought was possible: the cilia synchronized their movement like a flock of
birds.
To literally be able to watch this kind of what we end up describing as a flocking dynamics
was surreal because you could see it.
You know, when I see bird flocks I can imagine, "Aha, there is a controller.
There is a brain in there that's making these sets of decisions.”
Here, we are talking about individual flocks of these in cilia that are seemingly just
connected by springs and twirling in remarkably organic ways.
You can think about this coordinated activity like a group of castor wheels attached by
springs.
Even if they start off facing in different directions, a force applied to the system
will eventually reorient each individual wheel so that they all point in the same direction,
and the system travels as a whole.
Similarly, each ciliated cell in the Trichoplax bounces up against its neighboring cells in
a dance between responsiveness and stability.
Individual cells, the researchers found, could self-assemble into collective systems capable
of a wide range of behaviors, including some that allowed them to be responsive to their
environment.
In other words, basic mechanics could give rise to the kind of incredibly complex behavior
we normally expect from brains.
The observation of Trichoplax ciliary flocking really brought to the fore for us this idea
that mechanical systems could somehow be intelligent.
They could somehow seek out food.
They could somehow come to consensus across many cells.
And they could somehow find ways to keep an organism alive as a whole, instead of just
its individual parts.
Trichoplax may have set the stage for the rise of animals with nervous systems by taking
an important first step: embodying simple mechanics that work together to produce complexity.
One of the implications of our work // is that it really has pulled us in new directions
as how we think about computation and intelligence in physical matter.
We want to make soft robotic, self-oscillators.
We want to make caster robots that come to consensus and work together to make fungible
cargo transport.
And we want to think about how we can use these internal dynamics to make predictive
machines.
That is a very exciting notion because we have built an entire world out of passive
materials.
What if these materials had just the tiniest sliver of so-called intelligence?