Chris Kempes

Good. Now, it is tricky. Bacterial biofilms do have... Lots of really interesting responses within them. And there's been debates about all sorts of higher order functions that they might be performing that look like cell differentiation. So I think there is some trickiness there, but maybe staying in that aggregate or getting... irreversibly committed to that aggregate maybe is the hard part.

Good. Now, it is tricky. Bacterial biofilms do have... Lots of really interesting responses within them. And there's been debates about all sorts of higher order functions that they might be performing that look like cell differentiation. So I think there is some trickiness there, but maybe staying in that aggregate or getting... irreversibly committed to that aggregate maybe is the hard part.

Maybe once you get big and complex and differentiated, there's no way to regulate it in prokaryotes. I think that would be a leading idea. But yeah, I think that's still a frontier in my opinion.

Maybe once you get big and complex and differentiated, there's no way to regulate it in prokaryotes. I think that would be a leading idea. But yeah, I think that's still a frontier in my opinion.

Yeah, exactly. And so, yeah, so we... You know, sort of running with this idea of how physical constraints might set up a evolutionary transition. We worked with some paleontologists who were very focused on the fact that one of the huge expansions of multicellularity seems to happen just after Snowball Earth.

Yeah, exactly. And so, yeah, so we... You know, sort of running with this idea of how physical constraints might set up a evolutionary transition. We worked with some paleontologists who were very focused on the fact that one of the huge expansions of multicellularity seems to happen just after Snowball Earth.

And so Snowball Earth was this phase of Earth history where the Earth was mostly covered in snow and ice, including the oceans. So the glaciers come, even the ocean glaciers, sea ice, come all the way down to the equator. And then people debate about whether this is a hard snowball or a soft snowball with some amount of open equator and some amount of open patches of water around the planet.

And so Snowball Earth was this phase of Earth history where the Earth was mostly covered in snow and ice, including the oceans. So the glaciers come, even the ocean glaciers, sea ice, come all the way down to the equator. And then people debate about whether this is a hard snowball or a soft snowball with some amount of open equator and some amount of open patches of water around the planet.

But the point is it's mostly covered in ice and snow. And as an aside, the way that you get out of that is volcanic activity, which keeps pumping CO2 into the atmosphere with no way to take that CO2 up into the ocean and into life. And so then you rewarm the planet through a greenhouse effect, and that pulls you out of snowball Earth.

But the point is it's mostly covered in ice and snow. And as an aside, the way that you get out of that is volcanic activity, which keeps pumping CO2 into the atmosphere with no way to take that CO2 up into the ocean and into life. And so then you rewarm the planet through a greenhouse effect, and that pulls you out of snowball Earth.

Otherwise, with no volcanoes, you just get stuck there, which is a sad end to an interesting planet up to that point. So... We said, well, you get this huge explosion of multicellular organisms just after snowball. That doesn't seem coincidental.

Otherwise, with no volcanoes, you just get stuck there, which is a sad end to an interesting planet up to that point. So... We said, well, you get this huge explosion of multicellular organisms just after snowball. That doesn't seem coincidental.

And so can we take our models of the biophysics for unicellular organisms, say eukaryotes, and simple multicellular organisms and ask what would happen to those as the world cools down, as you go into one of these snowballs? And so we sort of invented a sort of toy model organism to represent the multicellulars, which is this swimming spherical shell of cells.

And so can we take our models of the biophysics for unicellular organisms, say eukaryotes, and simple multicellular organisms and ask what would happen to those as the world cools down, as you go into one of these snowballs? And so we sort of invented a sort of toy model organism to represent the multicellulars, which is this swimming spherical shell of cells.

Now we see multicellulars in the modern world that look quite a lot like this. And so we said, that seems like a good model. It's hollow on the inside. It's these single cell aggregates that could decide to go back to being single cells if they wanted to, but they also can swim around and collectively feed and that sort of thing. And so then we said, and then you also have large single cells.

Now we see multicellulars in the modern world that look quite a lot like this. And so we said, that seems like a good model. It's hollow on the inside. It's these single cell aggregates that could decide to go back to being single cells if they wanted to, but they also can swim around and collectively feed and that sort of thing. And so then we said, and then you also have large single cells.

And so now what happens as the world freezes? Well, temperature starts to go down first. So you first get this cooling ocean temperatures. And then eventually you start to form more and more ice. And when that happens, sunlight and what we call primary productivity, which is just how much sunlight is being captured and turned into biomass and producing sugars and minerals

And so now what happens as the world freezes? Well, temperature starts to go down first. So you first get this cooling ocean temperatures. And then eventually you start to form more and more ice. And when that happens, sunlight and what we call primary productivity, which is just how much sunlight is being captured and turned into biomass and producing sugars and minerals

fixed carbon in the ocean, that then starts to go down as the ice covers the ocean. So in this cooling phase where only temperature is changing, Two things are sort of happening. The metabolic rate, that metabolic power that we were talking about, is getting slower and slower because it depends on temperature. And as temperature cools, that rate goes down.

fixed carbon in the ocean, that then starts to go down as the ice covers the ocean. So in this cooling phase where only temperature is changing, Two things are sort of happening. The metabolic rate, that metabolic power that we were talking about, is getting slower and slower because it depends on temperature. And as temperature cools, that rate goes down.