William Ratcliff – (Origjina e Jetës) Evolucioni i Jetës Shumëqelizore (diç si pasqyrë diskursive, fjala e fundit e shkencës sa i përket origjinës së jetës)
senad guraziu, ars poetica, mars 2025 – pak fjale, sqarim…
…nuk mohohet fakti se jeta, ashtu në natyrën e vet, gjithmonë ‘enigmatike’, ishte dhe ende është, të jetosh është unikalitet, diç si “dimension” në vete, diç skajshmërisht unike dhe universale, dhe vetëm të ‘gjallosh” është unike, pa le pastaj t’jesh i vetëdijshëm se gjallon (meqë jo gjithçka e gjallë e ka idenë e gjallimit – dhe kjo grupnajë gjallesash pa “idenë e jetës” është jashtëzakonisht e madhe, ishte dhe diç si pararendje e jetës sonë), dmth. jeta ishte dhe ngeli misterioze po aq sa dhe interesante, jo vetëm filozofikisht por dhe shkencërisht, interes dhe enigmë me plotkuptimin e fjalës…
…shikuar me prizmin evolutiv (dmth. të darvinizmit), bën të thuhet se mënyra si do ketë ndodhur ‘evolucioni’ i jetës shumëqelizore akoma është njëra nga enigmat më të mëdha të shkencës moderne, për të ndodhur hapi evolucionar nga njëqelizorja në shumëqelizoren e gjallimit (dhe vetëm ky hap i vetem) mbase do kenë kaluar me qindra miliona vite, ndoshta dhe tejkaluar miliard-vjeçari i zanafilles tokesore si planet… patjetër se ishte hap i madh i çudisë së ashtuquajtur “jetë”…
…siç e dimë, as pas 3-4 shekuj të shkencave, shkencëtarët dhe mendimtarët e botës, më të diturit e më të ndriturit e mendjes njerëzore nuk pajtohen rreth “zanafillës së jetës”, sipas linjes shkencore darvinistike (dmth. evolucionare), organizmat më të hershëm patën lindur spontanisht nga sinteza e materialeve abiotike (dmth. ishte spontanitet, faza bazike si “procese kimike” pakashumë) – mirëpo ata që teorinë e evolucionit e konsiderojnë “përrallë ateistike”, dmth. e qartë se nuk pajtohen me hipotezat abiotike, me lindjen e jetës nga “asgjëja”…
…sidoqoftë, shembulli me nismën e jetës nga “asgjëja” s’është “asgjë” si mospajtim, nëse të krahasohej me faktin që fizikanët e kozmologët modernë madje insistojnë se dhe universi komplet qe nisur nga hiçasgjëja (nga ai “Bangu” i Maaaadh – siç e quajnë ata), pa ku na qenka nisja e një planeti fillikat, pastaj nisja e jetës në atë planet, ehuuu ku është universi e ku 1 planet i vetëm, absurd dhe të “krahasohet”, nëse mund t’niset i gjithë universi, me pluhnajash e me miliarda galaktikash nga “asgjëja” (nga ndonjë ngjeshje e ‘singularizuar’, ngjeshje deri në paskajësi e materies, e padukshme por ekzistente, tekefundit ekzistencë teorike) atëherë pse t’mos niset dhe jeta e Tokës me veset abiotike, pikërisht nga hiçasgjëja…
…ndër “dyshimtarët” më të mëdhenj kuptohet nuk janë vetëm teolog-filozofët, janë dhe vetë shkencëtarët teistë (besimtarë) që ngulin këmbë se “dizajni inteligjent” është shuplaka dhe grushti teistik për evolucionin darvinian, ky evolucion sipas tyre është pra një lloj përralle ateistësh rreth “krijimit”, le që s’përputhet as me Biblen… dmth. me Zotin, por s’përputhet me asgjë – shkurt, këta të fundit pohojnë se hipoteza materialiste që organizmat më të hershëm kanë lindur spontanisht (pa ndihmën e Perëndisë) nga sinteza e materialeve abiotike, është përrallë dhe nonsens…
…nga ana tjetër e marramendjes, në të tashmen, në modernizmin tonë të ngulfatur nga tekno-avancimet dhe me shkencat e ‘panumërta’, sado përparime t’jenë bërë, pa përjashtime dhe pa harruar asgje, në të gjitha fushat e shkencës, pa i shmangur as teknologjikat… qëkur avancimet teknologjike janë njëkohësisht dhe instrumentet e shkencës, misterioziteti i “jetës” ende nuk është kapërthyer me mjetet e biologjisë moderne, mendjet më të mëdha të njerëzimit, pavarësisht në cilin “drejtim” operojnë apo kanë operuar, me qasjet e tyre të interesit intelektual, herë tërthorazi e herë drejtpërdrejt, sikur prore merren dhe me “enigmën interesante”, që ne shkurt e quajmë “jetë”…
…gjatë historisë intelektuale të njerëzimit, rreth-e-rrotull “jetës” kanë ardhur në jetë panumër teori, janë kryer panumër studime, kemi sa të duash teori e teoremash, mendime, paragjykime, sugjerime, hamendësime, propozime (herë shkencore, herë pseudo-shkencore, her kuazi-shkencore, herë fot të qëlluara, herë mistike-religjioze, e herë dhe sharlatanizma, as kjo s’duhet mohuar : ) kemi sa të duash fantazi, ëndrra, iluzione, fanta-shkencë, ide fantastike… e çfarë tjetër jo, intelekti s’pushon kurrë, nuk mohohet fakti se 3-4 shekujt e fundit shkencat e mirëfillta kanë përparuar shumë, si linearitet zhvillimor… por dhe si aspekt shumëdimensional (ose dmth. si aspekt i gjithanshëm i përparimit), ndër avancimet kuptohet hyn dhe biologjia, si degë më e madhe nga ku degëzohen plot degëzash e nën-shkencash tjera, mirëpo prapëseprapë mënyra si do ketë evoluar jeta shumëqelizore akoma është njëra nga enigmat më të mëdha të biologjisë…
…më poshtë një material i kohëve më të fundit, intervistë me Profesorin William Ratcliff (GIT – Georgia Institute of Technology) – diç si pasqyrë diskursive, fjala e fundit e shkencës sa i përket origjinës së jetës – ngjitur këtu sa më shkurt që munda, duke i filteruar vetëm përgjigjet e Prof. Ratcliff (William Croft Ratcliff – Associate Professor and Co-Director of the Interdisciplinary Ph.D. in Quantitative Biosciences – https://biosciences.gatech.edu/people/will-ratcliff ) mirëpo ia shtova (diç si parashtesë) shkrimin nga autorja e revistës Quanta, Yasemin Saplakoglu (diç si intro e postuar nga ajo me ndihmën e ‘newsletter’, 24 Mars 2025, qëkur shtojca do shërbente si thuktim abstrakt i gjithë materies së podcast) – dhe dmth. tutje pjesët më të rëndësishme nga biseda me Prof. Ratcliff, gjëra sqaruar nga ai gjatë intervistës për revistën “Quanta”, podcast i hedhur në eter më 20 Mars 2025
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Si ia filloi jeta?
(Yasemin Saplakoglu – Newsletter, Quanta M, 24 Mars 2025)
Një nga ngjarjet më të rëndësishme në historinë e jetës tokësore ishte shfaqja e qelizave shumëqelizore. Për pjesën më të madhe të historisë së jetës, e cila filloi afërsisht 3.9 miliardë vjet më parë, kishte vetëm një mënyrë e të gjalluarit: si një qelizë e vetme. Format e para të jetës ishin, në tërësinë e tyre, njësi mikroskopike të vetme, të përcaktuara qartë, që riprodhoheshin duke u ndarë në dy qeliza të reja, secila prej të cilave e vazhdonte rrugën e saj. Jeta qëndroi e tillë (njëqelizore) për miliarda vjet.
Por më pas disa nga këto qeliza filluan të bashkëpunojnë mes vete. Ato kaluan nga ekzistenca ‘vetmitare’ në jetën grupore. Kur një qelizë u bë ‘dy’ (2 qeliza bashkë) dhe më pas më shumë, ato mbetën së bashku dhe përfundimisht filluan të funksionojnë si një lloj i veçantë grumbulli i gjallë: si një organizëm shumëqelizor.
Pavarësisht suksesit të vazhdueshëm të jetës njëqelizore, shfaqja e jetës shumëqelizore ka rezultuar t’jetë një përshtatje ose adaptim jashtëzakonisht i suksesshëm. Jeta për vete e shpiku ‘përshtatjen’ jo një herë, por të paktën nja 20 herë. Ngjarjet e pavarura evolucionare rezultuan në bimët shumëqelizore të sotme, në kërpudhat dhe në kafshët. Karakteristika kyç e adaptimit shumëqelizor është se mundëson ndarjen e punës (funksioneve): brenda një organizmi të vetëm, qelizat i plotësojnë rolet specifike, duke i përjashtuar të tjerat.
Trupat tanë, psh., përbëhen nga triliona qeliza me identitete dhe detyra të ndryshme. Qelizat e imunitetit i luftojnë trupthat e jashtëm, ‘pushtuesit’ e organizmit. Qelizat nervore na ndihmojnë të lëvizim, të ndjejmë, të mendojmë. Qelizat e zemrës e pompojnë gjakun gjithandej trupit tonë.
Duke i ndarë punët mes qelizave, organizmat shumëqelizorë mund të rriten në dy drejtime, si më të mëdhenj por dhe më kompleksë, e po ashtu dhe të zhvillojnë mënyra të reja jetese. Mirëpo jeta shumëqelizore e ka koston e vet: mbijetesa varet nga funksionimi i një sistemi të ndërlidhur me kërkesa të larta energjitike, në të cilin sistem vdekja e disa qelizave mund ta shkaktojë dhe vdekjen e pjesës tjetër.
Mënyra se si evoluoi jeta shumëqelizore është njëri nga misteret më të mëdha të biologjisë. Afatet, linjat kohore janë të turbullta, të paqarta. Njësoj të paqarta janë dhe arsyet. Provat si ‘dokumente’ fosilike për pjesën më të madhe të historisë së jetës thjesht nuk janë ekzistente. Të dhënat fosilike që i kemi në dispozicion sugjerojnë se jeta shumëqelizore filloi t’jetë më e ‘zakonshme’ rreth 600 milionë vjet më parë. Por prova të tjera evidente sugjerojnë se organizmat shumëqelizorë të thjeshtë mund t’kenë ekzistuar dhe rreth një miliard vjet më parë.
Kjo pasiguri nuk i ka penguar shkencëtarët të cilët duan ta gjurmojnë origjinën dhe historinë e jetës komplekse. Disa prej tyre kërkojnë të gjejnë fosile organizmash të vegjël apo dhe molekula, ndërsa të tjerë përpiqen të kultivojnë strategji laboratorike shumëqelizore nga organizmat njëqelizor. Teoritë janë të shumta dhe shumica e teorive nuk përshtaten së bashku. Mirëpo duke patur parasysh që jeta shumëqelizore ka evoluar aq shumë herë, atëherë s’është nevoja që shkencëtarët të kufizohen në një shpjegim të vetëm. Çdo studim i ri dhe eksperiment ideatik na afron më shumë drejtë të kuptuarit të momentit kritik evolucionar që e mundësoi ekzistencën tonë.
Në “Eksperimentin e Evolucionit Afatgjatë Shumëqelizor”, Will Ratcliff (Multicellularity Long-Term Evolution Experiment – Georgia Institute of Technology) merret me kultivimin e ‘kolonive’ njëqelizore në shkallët kohore të 10.000 brezave, me qëllim për të evoluar forma të reja të shumëqelizore. Në v. 2021 ai publikoi një zbulim të madh, duke treguar se në vetëm dy vjet, “tharmi” (njëqelizor) u rrit në njësi shumëqelizore aq të madhe sa të shihej me sy të lirë.
Ratcliff merret gjithashtu me qasje teorike. Kohët e fundit ai përdori një model kompjuterik për t’kuptuar pse qelizat prokariote, siç janë bakteret – të cilat, ndryshe nga qelizat tona eukariote, s’kanë bërthamë dhe organele – nuk e evoluan kurrë versionin e tyre shumëqelizor.
Fizika e ujit të ftohtë të detit, qindra miliona vjet më parë, mund t’ketë qenë një tjetër faktor për evolucionin e jetës shumëqelizore të kafshëve, raportoi Veronique Greenwood për Quanta, Korrikun e kaluar. Uji i ftohtë, siç psh. uji në Tokë kur planeti ishte i mbuluar me akull rreth 700 milionë vjet më parë, është më i dendur dhe për këtë arsye më i vështirë për t’notuar në të nga organizmat njëqelizor.
Në një eksperiment, paleobiologu Carl Simpson (Uni. of Colorado Boulder), pa se algat njëqelizore filluan të sillen kolektivisht teksa notonin përgjatë shumë brezave, duke e përshkuar masën xhelatinore gjithnjë e më të dendur në pjatat laboratorike, duke i simuluar kushtet e ngjashme me ato që, eventualisht mund ta kenë shkaktuar të paktën një formë shumëqelizore.
Shumë shkencëtarë supozojnë se organizmat e parë shumëqelizor filluan si kolektivitet i qelizave identike, përpara se qelizat t’ia fillonin me ‘specializimet’. Mirëpo provat e fundit sugjerojnë se krijesat e lashta njëqelizore ishin tashmë çuditërisht komplekse. Ato qeliza e bartnin potencialin e ‘specializimit’ gjatë gjithë kohës, duke u veçuar si forma të reja për t’kryer detyra të caktuara, përpara se t’riktheheshin në formën standarde – të ngjashme me qelizat tona staminale – siç raportoi Jordana Cepelewicz për Quanta, në v. 2019. Këto gjetje sugjerojnë se specializimi qelizor mund t’kishte ekzistuar shumë më përpara se jeta shqumqelizore ta bënte atë si veçori më të përhershme.
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Origjina e Jetës – Diskutim rreth Evolucionit të Jetës Shqumëqelizore, me Will Ratcliff (Georgia Institute of Technology) – pak dhe për arsye të gjatësisë së tekstit… ngeli në Anglisht sepse duhej kohë, më vjen keq, më së miri do t’ishte në Shqip, por s’pata kohë më shumë, artikullin si ‘intro’ më lart po jua nis të përkthyer, ndërsa për materien nga Podcast-interview po e ngjes edhe linkun edhe tekstin e shkurtuar…
[ https://www.quantamagazine.org/how-did-multicellular-life-evolve-20250320/ – Podcast hosts: Janna Levin, Steven Strogatz, March 20, 2025 ]
We contain approximately 40 trillion cells. We want to understand how initially dumb clumps of cells… can evolve into increasingly complex multicellular organisms, with new morphologies, cell-level integration, division of labor, and differentiation.
Cells have a nucleus, which contains the DNA that encodes the genetic information that the cells use to perform their basic functions that, you know, then makes proteins that are the action parts of a cell. And so, cells are these fantastic biological machines, right, in which you have this concentrated soup of highly functional macromolecules.
Life wasn’t always cellular. Cells are like one of these great innovations of life. And once sort-of cells evolved, they really took off, and it has been the sort-of basic building block of life for the last three-and-a-half billion years. Multicellular organisms are a kind of organism that is built upon the basis of cells, but where many cells live within one group and function essentially collectively. So, we are a multicellular organism, we contain approximately 40 trillion cells, which divide labor and perform all these various functions to allow us to do things in the multicellular, you know, environment – run around, have eyes, see things, talk on podcasts – that wouldn’t be possible for single-celled organisms, right? So, the evolution of multicellularity is a way of increasing biological complexity by taking what were formerly free-living individuals and turning them into parts of a new kind of individual: a multicellular organism. And it’s evolved, not once or twice, but many times. We don’t really have a great number, because we keep discovering more, actually. But there’s at least 50 independent transitions to multicellularity that we know of.
As people, we tend to be very animal-centric, but then there’s a whole slew of things that are a little bit more esoteric. There’s cellular slime molds that live on land that, you know, move around like a slug, and then will grow as single cells and come together, like a transformer, to then do something as a group, you know. So, there’s different flavors of multicellularity that have evolved in different lineages. And I think partly we’ve known about this for a while, but especially as we develop the tools to understand bacteria and archaea – the big domains of single-cell life that have been around for a very long time – we’re finding more and more types of multicellular bacteria and archaea that we just didn’t know existed, because, unless you’re looking at them with a high-powered microscope or using other advanced techniques, you can’t just see it.
We have reasons to think that cellular life exists around three-and-a-half billion years ago, and Earth is only four-and-a-half billion years old total. So, it’s fairly early in Earth’s, you know, history as a planet. But it probably happened earlier, and by that time they’ve already done the things that are required to evolve cells, and have all these basic building blocks of life, like DNA, which contains the, sort-of, code of the organism.
The evolution of multicellularity is broader than just animals. It’s a process, through which lineages that are single-celled can form groups, which then become units of adaptation. Evolutionary units that can get more complex through, you know, natural selection. And the Cambrian explosion is an incredible period where animals, which had already been around for probably 100 million years or more, just start to figure out all of these innovations which are hallmarks of extant animals. Before the Cambrian explosion, things were soft and gelatinous and didn’t have eyes or skeletons. It’s questionable if they had brains. They don’t have any of these things. And then in a relatively short period of time, just a few tens of millions of years, all of these things show up. And we think it’s probably due to these, like, ecological arms races, where you have predators attacking prey. The prey start evolving defensive mechanisms. You have just this explosion of animal complexity in what appears to be a very short period of time in geological terms.
The interesting thing about multicellularity, it’s evolved in very different time periods and different lineages. So, cyanobacteria were evolving multicellularity with honest-to-goodness development and cell differentiation around 3 billion years ago. It doesn’t take that long after you get cells that you start to get multicellular organisms evolving.
The red algae, which are a seaweed, they begin evolving multicellularity around a billion years ago. The green algae start doing it around then too. Fungi, probably anywhere between a billion and half a billion years ago. Plants, we know that pretty well, that’s about 450 million years ago. Animals, they really start to take off around 600 million years ago. Again, it’s really hard to put an accurate date on that, so we have to be, sort of you know, hedgy. And then the brown algae – the most complex kelp – they actually only began evolving in multicellularity around 400 million years ago.
I think we should not think of it as one process, but something where there are ecological niches available for multicellular forms, and there has to be a benefit to forming groups and evolving large size. That benefit has to be fairly prolonged. And most of the time, there isn’t, but occasionally there will be an opportunity for a lineage to begin exploring that ecology and not be inhibited by something else that’s already in that space. That might be why something like animals has only evolved once, because once you already have an animal, then it suppresses any other innovation to that space, like a first-mover advantage.
John Tyler Bonner is an evolutionary biologist, who worked on multicellularity decades ago, and he has this quote, that there’s always room one step up on the size scale. The ecology of single-celled organisms, that’s a niche that’s been battled over for billions of years. And there’s lots of ways to make a living in that space and that’s why we are in a world of microbes. But, once you start forming multicellular groups, you can participate in a whole new ecology of larger size. You might be immune to the predators that were eating you previously, or maybe you’re able to overgrow competitors for a resource like light. If you imagine that you’re an algae growing on a rock in a stream, single-celled algae will get the light but, hey, if something can form groups, now they’re intercepting that resource before it gets to you. They win. Groups also have advantages when it comes to motility and even division of labor and trading resources between cells.
There’s many different reasons to become multicellular. And there isn’t just one reason why a lineage would evolve multicellularity. But what you need for this transition to occur is those reasons have to be there, and that benefit has to persist long enough that the lineage sort of stabilizes in a multicellular state and doesn’t just go back to being single-celled or die out. You can imagine there’s lots of ephemeral reasons to become multicellular, and then they go away, and then the single-celled competitors just win again.
Big picture, we want to understand how initially dumb clumps of cells, cells that are one or two mutations away from being single-celled, don’t really know that they’re organisms – they don’t have any adaptations to being multicellular, they’re just a dumb clump – how those dumb clumps of cells can evolve into increasingly complex multicellular organisms, with new morphologies, with cell-level integration, division of labor, and differentiation amongst the cells. Just like, we want to watch that process of how do these simple groups become complex.
And this is, like, one of the biggest knowledge gaps in evolutionary biology. I mean, in my opinion. But it’s something where we can use the comparative record. We know multicellularities evolved dozens of times, and the only truly long-term evolution experiments we’ll have access to are these ones that happened on Earth over the last hundreds of millions or billions of years. But because they’re so old, and because those early progenitors, those early transitional steps, aren’t really preserved, we don’t really know the process through which simple groups evolve into increasingly complex organisms.
What we’re doing in the lab is: we are evolving new multicellular life using in-laboratory directed evolution over multi-10,000 generation timescales, to watch how our initially simple groups of cells – dumb clumps of cells – figure out some of these fundamental challenges. How do you build a tough body? How do you overcome diffusion limitation when you, after you’ve built a tough body and made a big group? How do you start to divide labor amongst yourselves when you only have one genome? How can you make that one genome be used for different purposes in different cells to underpin new behaviors at the multicellular level? Does this thing become entrenched in a multicellular state which prevents it from ever going back, or at least going back easily, to being single-celled?
We’re watching this stuff occur with a long-term evolution experiment, which, we’re now on generation 9,000 of what we call the Multicellularity Long-Term Evolution Experiment… M.U.L.T.E.E… MuL-TEE… absolutely a pun. It’s also named in homage of the long-term evolution experiment, which is a 70,000 and counting generation experiment with single-celled E. coli, started by Rich Lenski and now run by Jeff Barrick. So, we’re basically trying to do something similar, but in the context of understanding how multicellular organisms evolve from scratch. How they can, sort of, co-opt basic physics and bootstrap their way to becoming organisms.
It is a wild idea to try to make multicellularity happen in the lab – it’s kinda directed evolution. How to encourage this transition? We start out with a single-celled yeast. We did some preliminary experiments where we evolved them in an environment – it’s just a test tube that’s being shaken in incubator – where it’s good to grow fast, because they have access to sugar water, and the faster you eat the sugar water, the more babies you can make. And it’s, you know, scramble competition, everyone has access to the same food. And then at the end of the day, we put them through a race to the bottom of the test tube, where we just put them on the bench for initially five minutes, but as they get better and better at sinking quickly, we make that time shorter and shorter to keep the pressure on them. And here, there’s an advantage to being big, because big groups sink faster through liquid media than small groups. This is just due to, you know, surface area-to-volume scaling relationships. Bigger groups will have more, you know, gravity pulling them down relative to the friction from their surface. You take the winners of that race to the bottom, the best ones. They go to fresh media and you just, kind of, keep repeating this very simple process.
Yeast have a budding mechanism, where a mother cell will pop off a baby, from one of their poles, and then they can keep dividing and adding new cells to the same cell. In our early experiments that were just open-ended, we got these simple groups forming that have this beautiful fractal geometry. We had this easy mutation – it turns out it’s just one mutation in a regulatory element of the cell – that prevents daughter cells from separating. Super simple. Every time the cells divide, they pop off a baby but remain attached. And so, you get this sort of growing fractal branching pattern. Imagine something like a coral, or maybe like a branching plant. They kind of look like that, and they end up becoming more spherical with these you know nice branches. We call our yeast snowflake yeast. And you have this life cycle where they grow until they start to have packing-induced strain, they run out of space. And now if they add more cells, they just break a branch. And so, you have this emergent life cycle where they’re growing, they’re jamming, they’re breaking branches. Those little baby snowflakes pop off. And they even have a genetic bottleneck in this life cycle, in that the base of the branch that came off is one cell. So, as mutations arise, they get segregated between groups, and every group is basically clonal. Every cell in the group has the same genome.
The big mutation is the one that doesn’t let the daughter detach from the mother. That’s the key thing for forming simple groups. We figured out what this mutation was, and when we started our long-term evolution experiment, we started them with basically one genotype, so one clone, that already had this mutation engineered into it, but with replicate populations. Because what we want to understand is, how do these simple groups of cells evolve to become more complex? And I don’t want that to be confounded by the mechanism through which they form groups in the first place. We have actually 15 parallel evolving populations, that started out the same in the beginning, but we actually have different metabolic treatments for them. One of them, is taking all their sugar, and they are burning it up with aerobic respiration, using air from the environment to respire their sugar. One of them, we broke their mitochondria in the very beginning, so they don’t get to use respiration, they can only ferment, and they get a much lower energetic payoff from that. But they don’t have to worry about oxygen diffusion anymore. So, sort of a trade-off there. And then one of them can do both; it first ferments and then it respires.
We thought initially, that the ones that could use oxygen would be the ones that evolved the most interesting multicellular traits. But it turns out that they’ve actually stayed very simple for almost 10,000 generations. They haven’t done that much in the last 8,950 generations. They peaked early, and they’re only about six times bigger than the ancestor, and we don’t see any beginnings of cell differentiation. They’re just simple kind of bigger snowflakes. The anaerobic ones, they have evolved to be more than 20,000 times bigger than their ancestor.
It turns out that this is because there’s a trade-off that’s introduced by oxygen. If you form a body, and oxygen is this valuable resource that if you get it you can grow a lot more, but it can’t diffuse very far into the organism, then all of a sudden, the bigger you are, the smaller a proportion of your cells are able to access this really valuable resource, and your growth rate just falls off a cliff.
Your interior is so small compared to your surface. The bigger you are, the larger your radius is, the smaller a proportion of your biomass has access to oxygen. And so, in our case, the anaerobic line, they’ve done the interesting things because they’re not being constrained by oxygen. They’ve evolved large size. They’ve evolved all these interesting behaviors. And they’re solving all these fundamental multicellular problems.
The anaerobic ones, because they don’t get this a sugar high from the availability of oxygen early on, they have to be resourceful. They have to come up with all kinds of other innovations, and they do. The ones that have access to oxygen, as they get bigger and bigger, their slower and slower growth rates really push back against them, and kind of act in the opposite direction of any benefits that come from size. But if you remove oxygen, now bigger is better. The smaller ones go extinct and the bigger ones win. And then they figure out a way to get bigger. And they can really push the envelope on size and explore large size in a way that the ones with oxygen can’t, because they’re getting pushed back on by growth rate. But then as they get bigger and tougher, they actually start to have real trade-offs that are created by forming big bodies. They’re so big that now they’re struggling to bring sugar into these groups, because they’re actually becoming macroscopic. You know, they’re bigger than fruit flies now. They’re large.
They also face another constraint. I mentioned that they grow and would normally break due to physical strain arising from packing problems. But they solve that, by figuring out how to make tough bodies, by making their cells long enough that they actually wrap around one another and entangle. This is now a vining procedure where, if you break one branch of a vine, you know, the ivy is still not coming off your shed. Because entanglement percolates those forces throughout the entire, entangled structure. And so now, you don’t just break one bond to break apart the snowflake yeast, you have to break apart hundreds of thousands. And it becomes much, much tougher as a material. And we even understand the genetic basis of this, all the way up to the physics, it’s really cool to be able to watch mutations arising that change the properties of cells that underpin emergent multicellular changes, which natural selection can see and can act upon, and can, sort-of, drive innovation in that multicellular space.
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[ Janna Levin – Comment: ]
He’s got this hypothesis going on on the basis of what we believe about the importance of oxygen, and we even talk about it when we’re looking for other planets and life on other planets. Will there be oxygen, and is there water? And all this stuff that we’re really so certain is what’s needed to really accelerate life and life radiating. But now, he’s amazingly saying, well maybe, maybe that’s just not the case here. You have these oxygen hogs that got stuck. Evolution is not just mutation. It’s mutation and environmental pressure. So, it’s the hostility of the environment in some sense that drives the mutation.
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Clusters versus organisms. What would make an organism different than a colony? And how do you know which kind of thing you’re getting through these selection experiments? This question really cuts to the core of what do we mean by multicellularity. And I think a lot of confusion in my field, for the last half a century, has come down to poorly resolved questions of philosophy about what do we mean by these words, and people inadvertently speaking at cross purposes.
Part of this is that the word multicellular really means three different things, and we’re not very clear with our language. It’s treated as a noun in English to say, you know, multicellularity, but it’s really an adjective which modifies different nouns. So, you could have a multicellular group. That’s just, you know, a group that contains more than one cell. You could have a multicellular Darwinian individual, and that is a multicellular group which participates in the process of evolution as an entity at the group level. So, something which reproduces, where mutations can arise which generate novelty in a multicellular trait, and which natural selection can act on and cause evolutionary change in a population of groups. That’s adaptation at the group level so that would be a multicellular Darwinian individual.
And then you have multicellular organisms. And the sort of philosophical distinctions of what’s an individual and what’s an organism, there’s been a lot of work done in the last 20 years, and I’m pretty happy with the results of where that field is right now, which is that organisms are functional units. Organisms have integration of parts and work well at the organismal level with, you know, high-function minimal-conflict.
We are all three. We’re a group. We’re a Darwinian individual. And we’re organisms. And so, the distinction is that are, sort of, progressively higher bars for how you get to these additional steps, and they tend to occur sequentially. The first step would be forming a group. The second step would be making that group capable of Darwinian evolution. And then, as a consequence of group adaptations, you can get organisms, which would be functional integration of cells, which are now parts of the new group organism.
And so, a trait that would be diagnostic of that would be cellular specialization or differentiation, especially if it comes down to reproductive specialization. People love that in evolutionary biology because if cells give up their direct reproduction, they’re no longer making offspring, that’s something which is a behavior that you really can’t ascribe to the direct fitness interests of that cell.
Your skin cells will never make a new You. Never. They are entrenched in the, not on the line of descent. But it’s okay, because they are helping you make you know your reproductive cells reproduce. And so, the vast majority of our cells are not directly on the line of descent, but that is a derived state.
Originally, every cell made copies of itself. They were on the line of descent. Originally, simple groups don’t have this kind of reproductive specialization. But over millions of generations of multicellular adaptation, you get organisms that have, now, cellular parts, where those parts work together to allow the organism to do things that it couldn’t have done before, and an important part of that is specialization.
What does it mean to be in the line of descent, in relation to skin cells versus what, like gonadal cells? Sperm and eggs, for example, and this isn’t a strict requirement. You could have something like plants that don’t have this type of line of descent segregation. But nonetheless, you know, if you look at a tree, it makes flowers, it makes seeds. You have this differentiation into cells that will be the reproductive structures, and those that don’t. If you’re a wood cell, you just give up your life to make wood. Wood is basically a series of tubes. You differentiate into a tube, then you die. They’re doing it for the good of the multicellular group, and it’s also for the good of their own genome. Because usually those that are on the line of descent are related to them. And that’s how you, kind of, square it. So, there’s apparent altruism at the level of the cell, but there isn’t really altruism at the level of the genome.
When talking about Darwinian adaptation at the level of the group, Richard Dawkins said that there’s no selection except at the level of the gene. And Stephen Jay Gould would say there’s no selection except at the level of the individual. I think there should be some sociological studies on this, in evolutionary biology, because it has been much more, do you believe the consensus rather than, like, actually rigorously thinking through it. And in the last 15, 20 years, I’d say the anti-group selection sentiment, that was very powerful all the way up through the 2000s, has mostly melted away, as people have embraced more pluralistic philosophies that, like, there is sort of one evolutionary process, you can view it through different perspectives, sometimes it makes more sense to use a group selection model. And, I think if we’re thinking about individuals, in this, in the Gould sense, selection acting on the traits of individuals, for multicellular organisms those individuals are groups.
It’s always a little bit of a confusing distinction, I mean, the individual is made of other things. People are happy to sort-of round them up to just one, but there was a point where it wasn’t just one. It was a simple group, and it wasn’t so clear that that group was an individual. Like a snowflake yeast, you can break off any cell, put it into its own flask of media, and it’ll turn back into another snowflake yeast. That wouldn’t happen with one of my arm cells. If you go for a really long time in my experiment, that stops happening. But in the beginning, cells are just in groups as vehicles. And then over time, they gain enough adaptations, as a consequence of selection acting on the traits of groups, and really caring about the fitness of groups, that cell-level fitness, outside of the context of groups, starts to really take it on the nose. They don’t do so well as being outside of groups anymore. And you know, they’re evolving, the beginnings of division of labor, different cell states from one genome. This is unpublished work, so I want to be appropriately hedged here. But we’ve done like single-cell RNA sequencing, and we can see new cell states evolving over the five thousand-generation timescale. We go from one, sort of, putative cell type to three. And we think we know what they’re doing, like we think it is actually adaptive differentiation, as opposed to just sort of noisy chaos. The cells have differentiated in their gene expression, into different sort of behaviors.
By seeing these interesting transitions in the lab, by inducing them through the selection the researcher is putting on – to what extent these multicellular transitions that are being provoked shed any light on what happened historically in the wild? This question is an important scientific question. In order for our experiments to have meaning, they need to be somewhat generalizable. Now, I think the caveat here is that there is no one answer to how multicellularity evolved. It likely evolved in very different ways, and for very different reasons, in plants and animals and mushroom-forming fungi. It’s not a single thing.
But nonetheless, the thing that does unite it all is this evolutionary process. You have to have group formation, those groups become units of selection, and they turn into organisms as a consequence of group adaptation. And that evolutionary process, while it might play out in different ways in different lineages, some of these things are fundamental. So that transition to individuals that become organisms, that’s universal. And size is universal, and the physical side-effects that come with size, scaling laws, challenges with diffusion, and the opportunities that come to break those trade-offs through innovations, those things are all generalizable, even if they take different paths in different lineages, because they’re all proximate creatures of their environment and their gene pool. And we’ve never seen those processes play out in nature. And I don’t know that we ever will, because they’re historical things that we don’t have the actual samples to see it.
And one of the things that we can do is, while we’re not saying this is how multicellularity evolved in any one lineage, what we’re saying is this is how multicellularity can evolve, and this is how some of these things that, maybe looking in hindsight, you think you need really complex developmental control… oh, actually it turns out you don’t, because physics gives you all these things for free, that are kind of noisy, but they work, and you can bootstrap those into your evolutionary life cycle and build upon them, without necessarily having to evolve those traits for a reason. A lot of things in our experiment have turned out to be easier than we expected, and while the details may differ, I suspect that some version of these things that we’re seeing in our experiment play out in the different transitions in nature.