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[Music]

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[Applause]

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I've been looking forward to sharing

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with you some recent work that I hope

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provides a different perspective on the

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origin of cellularity and those of you

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who know me know that my lab usually

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doesn't work on the subject but rather

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on evolutionary history at or around the

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time of the last Universal common

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ancestor and we know a number of things

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about that ancestor one thing we know

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that's relevant to this talk is that

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that ancestor was most likely cellular

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we think that's the case because it had

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a signal recognition particle system

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which is used for delivering proteins to

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the cell membrane and other secreting

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proteins out of the membrane or

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embedding them within the membrane I

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know it says ER lumen here and that's

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true for eukaryotes but in prokaryotes

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this system occurs on the cell membrane

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one of those proteins that was very

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likely embedded in the cell membrane of

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the last Universal common ancestor was

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this ATP synthase motor which is one of

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my favorite proteins and it's

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responsible for forming ATP from ADP and

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phosphate by way of a proton motive

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force that is that is created by a

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proton or sometimes sodium gradient

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that's stabilized by in the case of

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prokaryotes a cell membrane but when we

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think about the origin of life we don't

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know how it happened I'm agnostic about

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how it happened myself because I tend to

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study things that happen much later but

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I hope we can agree that it wasn't the

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type of cellular organism that we see in

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modern life today and in several of the

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most you know popular hypotheses for

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what the simple genetic replicators were

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that led to the origin of life they

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didn't really require cellularity at all

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now that's not to say that the

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environment itself didn't provide some

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of the benefits of cellularity we've

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heard a lot about concentration both of

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this session and at the morning session

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and so these simple replicators likely

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required a rich geochemical environment

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whatever that environment may be that

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provided them

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with the kinds of compound some source

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of chemical potential energy and and

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also mechanisms for concentrating those

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compounds and making reactions

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thermodynamically are kinetically

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favorable but if the last Universal

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common ancestor was cellular and the

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origin of life was non cellular than

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that suggests that cellularity arose in

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between these two stages and maybe

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evolved because it's an adaptive trait

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and if it's an adaptive trait then there

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may have been some selective pressure

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that guided that evolutionary process

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and we were interested in understanding

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what those selective pressures selection

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pressures might be and so we designed

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what's called a digital life simulation

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where we kind of abstract some of the

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activities of organisms and which allows

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us to test hypotheses that we couldn't

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otherwise test in a laboratory setting

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or using phylogenetic analysis and so in

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our digital life system there are

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populations of organisms and they live

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in an environment with a shared food

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stock and some predetermined fixed

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population capacity each organism has a

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genome that has both metabolic genes and

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cellularity genes I'm not going to talk

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about how the metabolism works because I

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tried it over and over again I couldn't

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get it under five minutes so you can ask

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me questions about it later but in

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general the metabolism allows organisms

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to process the food particles and then

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received some energy reward for how well

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they process the food particles that

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energy is required to continue to tick

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through the genome and every time step

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in the simulation costs one each

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organism one energy unit once the

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organisms have accumulated an amount of

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energy that's equivalent to two times

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their initial allotment at the beginning

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of the simulation they automatically

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divide and that energy is split between

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the two resulting organisms I do want to

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talk briefly about the cellular

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Jeanne's so the cellularity of an

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organism is some number between zero and

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one that's defined by the number of

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cellularity genes that follows this

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equation here and a low level of

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cellularity has the consequence that the

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organism is essentially leakier so a low

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level a low cellularity organism will be

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more likely to lose genetic material to

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the environment but also receive genetic

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material that's floating around in the

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environment it will also be more likely

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to lose energy rewards that it got from

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metabolism but also receive energy that

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is floating around in the environment in

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addition to the mutations that a cell or

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an organism might have due to this

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random horizontal gene transfer that's a

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consequence of low cellularity the

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organisms are also regularly are their

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genomes are regularly mutated through

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insertions of any of these genomic

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instruction types deletions of those

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genomic instructions or small

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reconfigurations of the metabolic

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network that they use to process food

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particles and after we had created this

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digital life simulation we were kind of

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looking through the different parameter

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definable parameter spaces and trying to

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determine what would be a good area to

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investigate for selection pressures and

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one thing that we noticed was when we

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made the energy reward from processing

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food very generous we would always see

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an evolution of non cellularity and what

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we thought might have been happening is

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that organisms were starting off with

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zero cellularity it didn't take much

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metabolism basically the metabolism of a

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randomized genome to get a lot of energy

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and then that energy was leaking out

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into the environment and it behooved

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organisms to maintain a low cellularity

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so that well that's not the right way to

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talk about evolutionary processes it's

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selected for organisms that had a low

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level of cellularity because those

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organisms were able to take advantage of

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the energy pool in the environment

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so that led us to a more formal

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hypothesis that under in environments

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where there was a high level of not just

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food but the energy that is a reward

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from processing food so if we give them

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freely available energy in the

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environment that will actually select

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for non cellularity and it's only when

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we limit that energy in the environment

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that it will select for cellularity so

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we set up a series of experiments to

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test this hypothesis half of those

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experiments had simulations where

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organisms began with 0% cellularity and

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we either gave them unlimited food and

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energy in the environment or we gave

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them unlimited food but limited the

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amount of energy that was in the

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environment the other half so that we

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could assign a direction of selection

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here the other half began with a

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cellularity value of eighty seven and a

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half percent which is equivalent to

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three cellularity jeans and then again

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they either had unlimited food and

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energy in the environment or unlimited

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food but limited energy in the

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environment and we ran each of these

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different experiments under three

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different maximum population capacities

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to see whether population pressure

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played any role and then we ran each of

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those simulations three times so each of

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these experiments has nine different

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simulations and all of the results of

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those were pretty similar so I want to

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direct your attention to panel B here in

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which the organisms begin with a

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cellularity of zero percent but we limit

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the amount of energy in the environment

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and as our hypothesis would predict we

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see an evolution toward cellularity

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these are population averages of the

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cellularity across all of the organisms

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in the simulation and the cellularity

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reaches near 100% I also want to direct

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your attention to this panel where we

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where the organisms begin with a high

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level of cellularity but we give them

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both unlimited food and unlimited energy

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and in every one of these simulations

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the cellularity value of the average

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cellularity of the entire population

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settles at pretty close to 0%

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cellularity so

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our predictions were borne out by these

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experiments at the same time that we

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were measuring cellularity we were also

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measuring how good their metabolism was

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and we did that by pausing the

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simulation and giving the each organism

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1000 food particles that they would

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process and then we took the average

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quality of their solution to the food

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processing and then we averaged that

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across the whole population and we came

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up with a value of metabolic proficiency

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now a metabolic proficiency of zero

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means no metabolism is happening a

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metabolic proficiency of eight for

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reasons of how the metabolism works that

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I didn't discuss in this talk a

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metabolic proficiency of eight means a

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perfect metabolism and a randomized

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genome like the ones that they start

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with is equivalent to a little under

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four so in every scenario where we

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limited the amount of energy in the

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environment organisms had to process

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food to get energy that's panel B and E

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and in all of those cases the the

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metabolic proficiency increased over

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time but in every case where they gave

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them unlimited energy metabolism

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metabolic proficiency did not increase

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in the population so they settled at

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basically the random value R the value

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of a randomized genome so this shows

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that and these results aren't too

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surprising but taken together this shows

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that when there's energy in the

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environment that selection is that

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natural that the selection within our

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digital life simulation is selecting for

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populations of organisms that have both

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a low level of cellularity and a

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metabolic proficiency that's equivalent

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to a randomized genome when energy is

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limited in the environment organisms

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have to process food metabolically and

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so we see metabolites metabolic

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proficiency increased but we also see

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cellularity increase across the

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population

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and between these two extremes we see

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that these two traits are linked and so

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this is just the experiment in which

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organism started with zero percent

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cellularity and we gave them unlimited

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food but we limited the amount of energy

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in the environment and as cellularity

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increased so did metabolic proficiency

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so these are the the data points for all

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of all nine of the simulations in this

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experiment and you see that they're very

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strongly correlated these two traits are

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very strongly correlated with one

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another as they are evolving and so we

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take this as evidence that within the

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context of our digital life simulation

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the cellular the cellular T and

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metabolism are actually Co evolving with

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one another and we think this is

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important for the context of the origin

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of life because if you consider the kind

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of geochemical environment that we'd all

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like to see as an origin of life setting

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one that was very rich that was

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providing the early simple replicators

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with all of the materials and energy

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that they would need to keep going it's

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not just the case that the simple

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replicators were non cellular when they

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first arose but that this environment

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would have strongly selected for non

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cellularity and it wasn't until a change

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occurred in that environment that

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cellularity and metabolism would have

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co-evolved I think for the interest of

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time because I'd like to take questions

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I'm going to skip some other experiments

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and just point out that there were

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several co-authors on this work unit

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ACOG II who's shown here in my lab circa

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2015 also sitting right there as yab

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nuyen who is sitting right there or here

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from my lab this year and then Tom

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Wexler is a colleague who was at Oberlin

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College but has since left for Google

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also some funding sources who made this

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work possible and thank you for your

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[Applause]

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multiple the ability to do a metabolism

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is not the only benefit of a membrane

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for example a membrane protects yourself

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against molecular parasites and a

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membrane helps you to define yourself

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for evolution which means I think your

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your conclusion is right that once you

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need to have a metabolism you need a

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membrane but I would not make the other

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conclusion that without the necessity

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for metabolism you don't need a membrane

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actually you're absolutely right and I

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skipped over the experiment that

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demonstrates that so apologies for that

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so we actually turned off because it's a

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digital life simulation we can do that

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we turned off the consequences of

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randomized horizontal gene transfer and

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when when there is unlimited energy in

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the environment and organisms don't have

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to do metabolism we still see a strong

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selection for non cellularity but when

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there is limited energy in the

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environment and they have to do

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metabolism with horizontal gene transfer

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turned off it doesn't really settle the

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cellularity doesn't increase to a

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hundred percent or near a hundred

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percent it actually kind of fluctuates

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around and you can see we've extended

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the simulation how to ten to the fifth

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time steps and it's still fluctuating

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around and so we think that in the

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situation where there's unlimited energy

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there's selection for not having a very

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good metabolism or there's not selection

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at all for metabolism it seems to be the

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equivalent of a random randomized genome

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but there's selection for non

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cellularity to take advantage of the

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energy in the environment but when we

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limit the energy in the environment the

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selection is for having a proficient

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metabolism and cellularity just as you

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suggested is actually a mechanism for

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providing genetic fidelity to the

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a there's an interesting set of

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simulations now one of the things that

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you point out is that there's a

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selection pressure away from the

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cellularity and so my question is are

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there you know systems and modern

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biology high-energy lots of food that

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you can look to and see and it doesn't

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make sense to look at modern well you

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know I think viruses don't have to do

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their own metabolism and they need to

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have a little in a spacesuit to get from

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cell to cell but once they're in the

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cell they kind of become these non

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cellular entities within the cell and so

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that might be I can't think of any

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others off the top of my head but that

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might be a nice analogy to the kind of

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[Applause]