Life on Earth: By Chance or By Law?

Good evening, and thank you for coming to today's installment of Life in the Universe, brought to you by UA Science. So first, I'd like to point out that we're very interested in your participation. And in other years, what we've had is answers and questions from the audience, which has become very complicated. This year, we're going to do something slightly different. And what we're going to do is, we're going to have a panel discussion which is going to be broadcast by KUAT on the 20th of March. And what I ask all of you is to send us your questions between now and the 20th, so some of you will be able to ask those questions to the panel. The panel will be on the 20th, as I mentioned, at one o'clock in the afternoon. And I can tell you, because I've been in a room with these folks, that the most interesting conversations I have ever heard have been the conversations between all the speakers talking to each other.

And now, they'll be able to answer your questions. The second that has to do with the participation of the public is, we're going to inaugurate this year an institute at the Biosphere. And if you want to spend three days and two evenings embedded in science with scientists, keep your eyes open. Between now and when the lecture series is over, you'll have information of how you can actually be part of the Biosphere 2 institute this year. And that's to give Aspen Institute a run for their money. I'd like to thank the folks that make it possible for us to do this in a way that it doesn't cost you anything but the parking. And I have to deal with that, but the parking is sort of the final frontier. And there is a list of those that make it all possible.

And in particular, we should give an applause to all of them. And in particular, Ventana, TEP, and the Arizona Daily Star have been with us since day one. They have made it possible. They've made it possible for you to know that this is going on. And I thank them sincerely. Now, another thing that the folks that pay for this allow us to do is to have middle school and high school teachers take part of this experience in a deeper way, so that they can actually create curriculum and take back to their classes. And I'd like to ask the faculty to stand up for a second, so that we can all give you a big applause. And in particular, I'd like two people to stand up. John Pollard, who is a person from the U of A that's teaching them. And Margaret Wilch, teach the most extraordinary high school teacher in the world. It's Dr Wilch, because she has an honorary degree from the University of Arizona in science. All right.

Well, the first day that you came here, the first Monday you came here, you heard all about how little we can actually even define life. And the reason for that lecture is that if we're going to find life somewhere else, we should know what we're looking for. And you heard that lecture, and basically, it's hopeless. But we are trying. The second lecture went the step of how do you actually create life from chemistry. So Donte Loretta pointed out that the elements that are required to create life were not created at the Big Bang, they were created when the stars were formed. And as the stars die, the whole interstellar medium becomes full of these elements, like carbon and phosphorus and nitrogen, which are required to make DNA and so on. So he tried to address the question of, how do you go from chemistry to life? And he was able to at least address, in some ways, how you get the chemistry to maybe RNA or DNA.

And there's still the challenge of how you make cells. He didn't even go there. Today, we're going to talk about another mystery. And that mystery is, how do we get all of the biodiversity that we have in a place like the Earth from the simple building blocks that we're all made out of. So Brian Enquist will tell us today about biodiversity, how it starts, what keeps it going, and the scaling laws in biology. How do we scale up from a very small animal to a very large animal or plant? So the idea of biodiversity, how we make all that we see from the stuff that's Donte Loretta was telling us about last time is an amazing mystery. Now, Brian is an amazing individual from the National Science Foundation. He's a Young Principal Investigator Award.

These are very important things. From the Ecological Society of America, he got one of the most prestigious awards as a young scientist. And more importantly, Popular Mechanics, of all places, named him one of the 10 most interesting young scientists to follow. So there you go, he may have something to do with cars as well. So I leave you with Brian Enquist. Thank you. It's an absolute rush to be here. What an amazing night for science in Tucson. But I think before I get started, this series would not be possible if it weren't for the brain, drive, and passion of Joaquin. So let's thank Joaquin. For not only being our leader in the College of Science but also setting a good example for what science should be, not only in a real democracy, but its role to play in terms of creating dialogue. Unlike most of the other speakers, I'm actually tweeting this, so feel free to send me a tweet if you want. I'll try not to tweet while I'm talking. So what I want to start with today is actually one of my favorite photos of the earth.

This was taken in 1968 by the crew of one of the Apollo missions. This was the first image a human actually saw of the earth. So this is the first photograph of the Earth taken by a human. And what's amazing is that I don't study the universe. I don't build a satellite. I study life on Earth. And so what I would like to do today is tell you a little bit about life on Earth but also what I find absolutely amazing about life on Earth. So let me show you one of the places that we work. This is the Kosnipata Valley, this is in Peru. It's right near Manu National Park. It's perhaps the most biodiverse place on the planet. We study the diversity of plants and trees along an elevational gradient. So we go from tree line at the top of the Andes, we drop all the way down to the Amazon basin.

And we want to understand what not only controls the changes in diversity that we see along these gradients but also how tropical forests, how ecosystems work. And we also want to understand how these organisms work, in particular how these trees work. So when we go out on our various different collecting missions, we sample leaves, we climb trees, we bring them down. And we put them in these pretty fancy bits of equipment here, where we're actually measuring their metabolism, their photosynthetic rate. We then take the leaves, we process them here in a field lab in the middle of the rainforest. Take those leaves, we bundle them up, and we bring them back to the University of Arizona, where we measure other aspects of their metabolism, such as nitrogen content, phosphorus content, isotopic various signatures and so on. And our ultimate goal here is that we want to understand how these trees, these organisms, work. But ultimately, we want to understand how these ecosystems will respond to climate change. But we don't want to understand how they change over time or across elevation, but we want to be able to predict what's going to happen in the future.

Now, to try to bring in the predictive sense, we also work quite a bit on developing mathematical theory. And a lot of this math is aimed to ultimately boil down and kind of get at these measurements that we're taking within the field and take then, those measurements in order to scale up and understand and make predictions about how these tropical forests work but also how ecosystems work. So this is really a search. We're interested in searching for generalities in how life works. We want to understand something as vast as the Amazon ring forest, which also you can think of as the lung of our planet. What happens in the Amazon influences are global climate, it influences the rest of the biosphere. So what can we do in order then, to search for generalities or rules about how organisms work, how diversity responds to changing climates, in order to ultimately provide a more predictive framework for biology? So is Earth the only planet with life? I guess the quick answer is, I don't know. But as a biologist, what I want to know is what is really unique about life on Earth? Now, based on what we know about life on Earth, if we found life elsewhere, what is our best guess or best prediction in terms of what would it be like? And if we could then have some sort of prediction, what would those predictions then be for life? So what I would like to do in this talk is counter two different views about biology and arguably about life in the universe.

But is life on Earth really dictated by chance or is it deterministic? Now, in studying biology for quite some time, what I'm actually impressed with is that I think there's a very big division within biology. Well, maybe division is too harsh of a word. But it seems that biology really does struggle with these two different worldviews on life. In particular, is life due to chance, or is life deterministic? So on the one hand, life on Earth is amazing and intricate and diverse, but the thinking goes that it's actually arisen due to multiple events that are essentially unpredictable. On the other hand, we could also make arguments that life on Earth is highly deterministic, and that we can ultimately boil things down to general rules and principles which have guided the diversification of life. So what about chance? So when we step back and we look at the world, and actually this is one of my favorite paintings I should say, by Jan Brueghel. This is an 1613, Noah's Ark.

When we step back and look at the diversity of life on the planet, it really is amazing. There are small organisms. There's big organisms. There's organisms that fly, organisms that burrow in the earth. All the way from microbes all the way up to the macroscopic, huge organisms that we share the planet with. And it's actually amazing. If you study biological processes, biological processes operate at the subcellular scale, all the way up to the scale of the biosphere. So basically, life operates over 21 orders of magnitude in scale. So that's a number, take a number and move the decimal point 21 times, that's a huge range that biology is operating over. But is it all really by chance? So when I graduated from high school and went to college, I decided that I want to be a biologist, because I loved various organisms, I loved the diversity of the Earth. I love going out and understanding how living things worked.

When I decided to be a biology major, my parents gave me a book, and it was this book. This is a book by Stephen Jay Gould entitled Wonderful Life, The Burgess Shale and the Nature of History. And what's interesting about this book is that it really struck a chord in me early on. And I kind of assumed that well, this is how in general all biologists really think. But it also kind of haunted me, because it spelled out a view of the world that, if true, from my perspective, can't be correct. How can it be that all of life and the diversity that we see today is due just to chance? So according to Gould, chance was and is one of the decisive factors in the evolution of life on Earth. And the paths then, that life has taken since essentially, life arose at the beginning and evolved then, continuously to today, these various paths then were actually constrained by various historical events well outside of the control of anything biological.

Effectively, these random historical events and accidents. So that was in 1989. Now, what's interesting is that Gould takes a very interesting picture about how the world actually works. And within that book, he actually asked the reader to do a thought experiment. In particular, if we were to take the long history of life, all the way from present day all the way back to 3.5 billion years ago, the estimated origin of life, if we redid evolution, that is if we replayed the tape of life– in putting this together, I was thinking about my kids and trying to explain to them this thought experiment about rewinding the tape. And I was like, oh, yes, they don't have tapes anymore. So this is a dated analogy. But anyways, if we rewind the tape, go back to the beginning and play the play of evolution again, would we get the same thing in the end? So that was the thought experiment. So what if we could go back in time, and what if we could watch evolution happen again? What would it be like? So Gould in particular focused on a period of time in the Cambrian here, 541 million years ago.

I'm sorry, going to the Precambrian. So during this time, this was essentially then, the biological equivalent of the Big Bang. During this time, the diversity of life really exploded, and we see all of these various different forms of life. So Gould focused on one example, in particular the organisms and the animals then found then within the Burgess Shale. The Burgess Shale then stems from the middle Cambrian, it's about 505 million years old. It's perhaps one of the most famous fossil beds in the world. The Burgess Shale is located here in Alberta, Canada, it's in the Canadian Rockies. And so you can go here and sample, and you get these amazing well preserved fossils. These fossils preserved soft parts then of the body. And you can go through and you can really get a really good snapshot of essentially a whole living ecosystem, all the various different organisms then that lived during that time.

So during this time, life really was a bizarre and alien world. If we were able to time travel and go back and look at these various different organisms, many of us would think that we were in an alien world. They look very different to anything that we have today. The name of this organism, this is Hallucinogenia, mainly because it perplexed people quite a bit. We're not sure exactly what it looked like. But over time, as more and more fossils were found, they're able to get a really good kind of construction, kind of an idea, a mental idea of what this organism actually looked like. And so the first then drawings then of the organism looked something like this, bizarre. Well, then that caused quite a bit of debate and stir. More fossils were found, and they decided, well, actually, they drew this upside down. So this interesting little critter, instead of having these little spines as legs actually had these spines on the back.

And now the thinking is that these are actual predator avoidance spines, effectively. So when you see something like that today, it basically means, don't eat me, don't touch me right. Which is interesting, because now we can start to read into the ecology of what that world was like, going all the way back about 500 million years ago. So these organisms were quite bizarre and alien. And the more we know, the more bizarre and alien they look. But what's interesting is that they were really only just a lucky few of these organisms during this time that actually made it through, left descendants that today partially helped fill our world. So over the course of history, chance alone wiped out many of these groups, they're now extinct, they're no longer with us. So chance. So you don't have to go very far back in recent biology to find some pretty prominent thinkers in biology arguing that chance and unpredictability is a very central component of biology. So this is Watson and Crick, co-discoverers of the structure and shape of DNA.

In 1981, Crick then wrote a book, Life Itself, Its Origin in Nature. And so he has this great quote in terms of the origin of life. And so he says here, the origin of life appears at the moment to be almost a miracle. Effectively, punting the ball. I don't know. We've got DNA, that's enough, right? Here's Jacques Monod or Monod. Life is only the result of pure chance. So the biosphere doesn't contain any kind of set or class of predictable objects, chance. So does life really come down to a roll of the dice? Well, like anything in biology, to make sense of not only the organisms today but also of organisms in the past and the origin of life in general, we ultimately have to understand the unique selective pressures of different environments but also how life evolves. So this is Dobzhansky, he was one of the main architects of modern evolutionary theory in terms of our modern understanding and our modern underpinning of the role of evolution within biology. And he has this fantastic quote that says, nothing in biology, underscore nothing, makes sense except in the light of evolution. So what about evolution and chance? So 1859, Charles Darwin publishes The Origin of Species.

And so chance is actually an important component of evolution by natural selection. Now, what's interesting is I have here Darwin's algorithm. And I do that because, if you read the last paragraph in The Origin of Species, Darwin actually outlines then the theory of evolution by natural selection as an algorithm. You can almost think of this as a computer program. So if a few truths are correct, then the end result must be true, and that is natural selection. Now, there is going to be a quiz on this later on. So please take some notes. And I actually ask my students, incoming students, if they can basically outline Darwin's algorithm. I do that in the start of every class. And none of them have gotten it right at the start, but they get it right at the end of course.

So what's the algorithm? Well, Darwin observed that organisms or any organism has the propensity to ultimately grow in its population size exponentially. So take this very cute example of a bunch of ducks on a pond. Given enough time, we're going to be not only filling the pond with ducks, but very quickly, that population is going to be growing exponentially. Any population, bacteria, ducks, elephants, given enough time, that population rate of growth is going to be growing exponentially to the point where it could be growing at the speed of light. Imagine populations of elephants growing at the speed of light, clearly that's impossible. So this is the insight of Darwin. Then he said, OK, if that's true and all these individuals at some point are going to be competing, competing either for mates, space, resources, that at some point, because of this competition for limited resources, that exponential growth has to change. There's a leveling off, there's a carrying capacity or a limit to the number of individuals and that you can support.

And this is his other insight. If you look then within any population, when organisms are than competing then for limiting resources, offspring are going to differ in their ability then to compete and obtain resources. So some individuals are better in terms of finding mates, competing for resources. Now we know that that variation is ultimately due to genetic variation, ultimately random mutation, chance events introducing this variation then into the population. So according to Darwin, if and all these things are true and that genetic variation is heritable, then, given all of the above, you're going to see differential survivorship and/or reproduction. Some individuals will survive better as well as reproduce better, some combination of the two, because of which the population phenotype of the genotype will then change. But then the last step, go back to one, and repeat infinitely. So that was Darwin's insight.

So I want to emphasize here that chance events enters in terms of how variation is entered into the population. But I also want you to note that individuals are competing for limited resources that ultimately, then they're using to fuel then their bodies. This is the end result. This is the tree of life. So what I'm showing you here at the base then of this plot, this is time going out in either direction. So this is the birth of the earth then going out all the way into present day. So present day then is the tips of the trees all the way at the top, ranging from bacteria, eukaryotes, plants, fungi, all these invertebrates, sharks and fish, amphibians, reptiles, birds, and mammals, very impressive sweep. So what's beautiful about Darwin's insight is that all of the organisms today, all that we know, all can be traced back to one common ancestor.

That ancestor then left behind descendants who left behind descendants that ultimately survived to today, populating the world that we live in. Now, I want to point out several very interesting things about this plot. If you'll notice, it's kind of like an onion. There's these different kind of circles in there, right? These onions are actually marking some very important Earth history events. In particular, here is the Cambrian Explosion, where all of these different body plans and body types come about. If you follow any one branch, most of those branches actually end. The ending then the branches then is an extinction event. But if you'll notice, there's some pretty large extinction events. There have been five mass extinction events on the earth, because of which, on the order of 90% to approximately 99% of all species that have ever lived on the planet are now extinct.

So literally, we are the 1%. So chance events. Many of these mass extinctions are now pointing back to, just by chance, a large asteroid hit. So 65 million years ago, there was a very large one that wiped out quite a bit of life on Earth. But also 65 million years ago, also ushered in a new group of organisms who before were relatively rare. So this extinction event ushered in the age of the mammals and the extinction of the dinosaurs. And it also ushered in a very interesting niche art of extinction events on the internet. And I have to say that these are all very bad pieces of art. I wouldn't want to put these on my wall, but somebody spent a lot of time in designing these. Maybe you saw this recent study. So this came out just a few weeks ago. This was in the New York Times, a very large study on risk of cancer.

What they find is that random mutations, chance events, account for about 2/3 of the risk of getting cancer, which leaves the other suspects, heredity, environmental factors, accounting for a very small fraction of your risk of getting cancer, chance events. So two fundamentally different views of biology. Chance, but now let's talk about determinism. Is life on Earth organized by a set of general rules and principles? The thinking here is that, well, maybe, the winners and the losers may change, but the rules of the game have stayed the same. And if true, what are the rules? Another book that I was influenced by in graduate school was actually a book by Harold Morowitz published in 1970. And so what we can ask are, what are the general aspects of life? So if we went back and replayed the tape of life again, would we likely see biological constants, things that repeatedly evolve? So Morowtiz has a great quote. The energy that flows through any sort of system ultimately organizes that system. So he was think of physical systems, but also biology is no different. Biology ultimately does work. Biology consumes resources, converts then those resources into offspring, and does work.

We can think of biology as also having a very physical work component. So here's the hypothesis. The hypothesis is that life on Earth has emerged in part deterministically from the laws of chemistry and physics but also evolution by natural selection to maximize organismal power, given the constraints of the environment. So what I would like to do is work through several different examples, what appear to be very deterministic, very rule-like aspects of biology. The first is something known as the citric acid cycle. It's a part of your core metabolism. So when you take introductory biology, you often start with a review like this. So this is a typical cell. You can see all the different organelles, the nucleus, the mitochondria, all the various different components. Within the nucleus of course, that's where the DNA is. But we can take a completely different view of all the amazing biology then that's within the cell.

Now, when I was taking introductory biology as well as molecular biology and biochemistry, when I went in and I saw it in his chart, I was like holy bejesus, do I have to learn all of that? Very daunting, but this is also one of the pinnacles of biology. This is what happens inside each of your cell effectively constantly, every one of you is doing this, and yet you're totally unaware. So biology has been so successful and very Byzantine in drilling down and working out all then of the connections of the biochemistry then of a typical cell, very complicated. Now, let's zoom in on one part of this. If you notice this circular part right here, this is something known as the citric acid cycle. What's interesting about the citric acid cycle is that it's ubiquitous, from what we can tell, across all of life. Doesn't matter if you're bacteria, unicells, multi-cells, animals, plants, we have the citric acid cycle.

So here's the argument maybe the TCA cycle is an energetic inevitability, maybe the TCA cycle actually originated before life that we know originated. Here within the TCA cycle, what you actually see is several different molecules linked to the other in a series of chemical reactions. There is some input here, mainly pyruvate, acetyl CoA. But don't worry about all those details, but if you know a little bit of biology, you should recognize this, because this is one of the key components of aerobic respiration. So if you break down these sort of sugars and these sort of fats or anything, ultimately, you're going to be feeding then molecules into the TCA cycle. Two Nobel prize is were given for, effectively, the discovery of this. Adolf Krebs then was the one that worked out the full cycle was also awarded then the Nobel Prize. It is also known as the Krebs cycle. So the core reactions of the citric acid cycle likely existed before cells, before RNA, before DNA.

The thinking is that this is an autocatalytic network, an autocatalytic set then of reactions where newly created molecules, then catalyze then the reaction of the next molecule. In general, this is an energetically favored set of reactions, effectively, it gives off then free energy by this set then of autocatalytic reactions. So you can think of this as a way to start creating order before a lot of these big biological macromolecules, before cells, before molecular replication, effectively at the start then, the beginning of life. Now, what's interesting is that the derivatives then of the TCA cycle then feed into that very complicated map that I showed you earlier. Effectively, sugars, fatty acids, the production of all of these different molecules, nucleic acids, amino acids, all these various different cofactors, critical things that your body needs to complete all the biochemistry. This effectively, you can think of this as kind of like the fountain of all origination of biological biochemistry.

So the question is, this is really the first engine then of biological synthesis. So I'd like you to think of this as a spinning wheel. Every time then this wheel is cranking around, producing not only a bunch of biological molecules, but we're ultimately creating stored energy. Within then the stored energy ultimately is used to produce ATP, which is effectively how we're powering then all metabolic reactions. So we're creating then a bunch of biological molecules, we're also then creating then and preserving stored energy. What's interesting is that the rate then of the spin of the TCA cycles influences everything, such as how fast you age, the rate of senescence, the risk of cancer, and the overall organism's energy status. So how fast the cycle spins is ultimately related to a bunch of other biological phenomena that we study. Now, one of the big surprises is that there are bacteria then on Earth who actually don't rely on an oxygen-rich environment but actually use the TCA cycle, but they run it in reverse, really cool.

So it runs in reverse, and it does the opposite. So unlike aerobic respiration, where you breathe in oxygen and you breathe out CO2, if you run it in reverse, you can actually suck in carbon dioxide, and you can then create a bunch of additional organic molecules, things like vinegar and additional molecules as well, again, some basic building blocks of life. So very interesting, you can run it forward, you can run it backwards. And depending on the local environment, that would then select for whether or not you're running this forward or backwards. So here, I'm showing you a deep sea hydrothermal vent. So these hydrothermal vents then are at effectively, the bottom of the ocean. This is where your plates then are splitting apart. So we have rising magma pushing our various continental plates apart but here, because of all of the magma underneath in the Earth, we're getting this really superheated water in to a bunch of dissolved minerals and other molecules. And they're then reacting to create a very interesting mix of chemical reactions.

The thinking is that in this environment, these hydrothermal vents, when we actually look today, we see life teeming around these hydrothermal vents. And so what's interesting is that we can support an entire ecosystem based on energy that's actually not ultimately from the sun which is most of what's life in the oceans and the terrestrial environment relies on, but ultimately, the source of energy here is geothermal. So the thinking is before, there was life on earth, these early reactions of the TCA cycle were likely present in these hydrothermal vents. All of the molecules that you need in order to get the TCA cycle going look to be present within these hydrothermal vents. Now, what's interesting– we can go back.

I forgot to one mention one thing. If you spin this thing in reverse, in order to spin it in reverse, you're actually not creating energy, but actually you have to input energy. And so that's actually very interesting. So if we run it forward, we're creating ATP. If we run it backwards, we actually have to have energy put in. So where did this initial set of molecules to power the TCA cycle come from? Well, the thinking is that this ultimately came from acetyl phosphate. And acetyl phosphate is actually very interesting, because if you heard the talk previous to me, Dante talking about the early Earth chemistry, based on the chemistry of meteorites, if you then look at the chemistry of meteorites, you'll find that there's a very large supply, apparently, of phosphorous coming into early Earth. Those early phosphorus compounds in combination with water looks to easily give you acetyl phosphate. So we have all the ingredients. We have the basic molecules to start the TCA cycle, but we also have acetyl phosphate to power the cycle.

So the TCA cycle appears to have originated before the origin of cells, before RNA, and before DNA. And it's really this geothermal energy that's effectively provided the environment and the initial chemistry to get these initial reactions going. So remember, if you get these reactions going, all one has to do is simply put those reactions inside a cell in order to contain it and then control the production of these early biological molecules. So if this is correct, our initial view of the start of life has actually changed. What we can actually then see is that this core metabolism of the TCA cycle appears to have been present at the start. Once we have the ability to create some basic biological molecules associated with these autocatalytic set of reactions, somehow, if we then contain and harness this core metabolism within cells, have a set of genetic material in the form of either DNA or RNA, to somehow not only control those reactions but also have natural selection taking over at that point.

So if we go from back to the origin of life 3.8 billion years ago, all the way to the Cambrian Explosion, what we actually see is this core metabolism staying put and life just building more and more complexity on top of that fundamental core. Now, within you, what's interesting is that you run your a cycle within these organelles known as mitochondria. What's interesting is that the mitochondria is essentially this long ghost of an ancient bacterial symbiosis. Ultimately your mitochondria were at one time free-living bacteria. Tight living associations between bacteria probably provide beneficial associations, so much so that the fact that now, you're organelles within your cells appear ultimately to be traced back to these early symbiotic relationships between bacteria. What's interesting, if we look at the genes that are ultimately underlying not only the production of energy within the mitochondria but ultimately the TCA cycle, this one associated with ultimately harnessing this energy gradient, this gene cytochrome oxidase is another ubiquitous gene across biology.

Very little genetic variability in general. And we find this gene that ultimately is responsible for generating energy ultimately associated with TCA cycle being present throughout most if not all of life. So ubiquitous across all of life. It's fundamental to organismal metabolism. There's even indication that the TCA cycle itself may have evolved multiple times. So ultimately, we're talking about core metabolism producing, storing energy to fuel everything that an organism does. OK. So what's the second component of life? What we see repeatedly across the evolution of life is very strong selection to get big. Multicellularity has evolved multiple times independently. So going from unicellular organisms to multicellular organisms again and again appears to offer a selective advantage. So today, the organisms that we share our planet with are actually the largest that have ever lived on Earth. And you may say, well, gosh, the dinosaurs are pretty big, right? Well, they're easily dwarfed by this guy, the blue whale. But the blue whale is actually dwarfed by the General Sherman tree, an order of magnitude larger.

So why is larger better? Throughout time, we see this increase in the size of an organism. So if you get big, you can overcome various physical forces like viscosity. If you're very small and you're in the water, that can provide a really hard situation for you in terms of how you go about your day to day business. So if you get bigger, you can overcome these forces like viscosity. But also if you're bigger, you can dominate and kind of maintain larger territories and control more and more resources. If you're also bigger, you can avoid the competition of smaller individuals. If ' use resources that are differently available to you, being larger often offers quite a few opportunities. If you're bigger, you can usually do more tasks. Larger things tend to have more different cell types, larger things tend to respond to the environment differently. You can also process more and different types of information. But also, if you're bigger, you live longer.

And because of which, you often then have access to resources that smaller things don't have. It's probably not a big surprise that the biggest city that's ever been on earth is actually alive and well today. This is Tokyo, 37.6 million. And just for chuckles and grins here, what's the biggest corporation? Actually, I had no idea what the Sinopec group was, but it makes $486 billion per year. It's the national petroleum company of China, which I guess makes sense. They're making a bunch of stuff which then leads to the second largest company on Earth, which sells the stuff. $476 billion per year, bigger, better, can dominate, control more resources. So another aspect of general items that we see across life, fractals. Fractals are interesting because they seem to be underlying a lot of themes within biology. But unlike the genetic code, which to our knowledge has only evolved once, we see a lot of fractal structures that have mathematically very similar properties for many different functions that have evolved multiple times that are effectively identical to each other. So you're a great example.

If I boiled away your skin and looked in your body, you'd look like this. You're a sitting tree. Leaf veins. Leaves are nice and green, but if you take away that outer skin, they're beautiful, and this is what you're left with. Your lungs. This could be a root system, it could be a branching architecture of a tropical tree. Within your brain, information networks, these transport electrical signals for not only your motor control but also your learning and your sensory perception. What's interesting is that we can describe all of these different networks and find that they have very similar mathematical signals associated with them. Here's an aquatic example. This is a modern filter feeder. This a gorgonian in a coral reef, And this is a fun one. So let's go back to the Burgess Shale. So they think that this organism, it's a fossil, was some sort of an alga, some algae of some sort.

It's now extinct. But what's very interesting is that if we analyze the branching structure of these organisms, you'd be hard pressed to tell this apart from a lung or a tree, oak tree, that you'd find growing in your yard. So why these treelike networks? So one of the projects that I decided to focus on when I was in graduate school was essentially asking this question. Why is it that we tend to see these branching morphologies throughout most of biology? It doesn't matter if you're a plant or an animal or even within a cell, we tend to see these sorts of structures. So here's the argument. So evolution by natural selection actually favors this sort of network, because it's optimal. And we would expect to see these networks anytime in biology where resources are limiting and where competition or limiting resources is keen. Because these networks, they tend to be space filling, they fill some sort of space. But what's also interesting is that– here's a little bit of lingo– they tend to be area preserving.

And these are just fancy ways of saying that, by space filling, these networks maximize resource exchange surfaces. Organisms are exchanging resources from the environment into their body, and they're transporting within their body or their cells, ultimately to fuel metabolism. These networks are also designed so that they minimize transport costs. So you can boil this down. These networks give you the biggest bang for the buck. You can take in a bunch of resources or information, transport them within the body to fuel all of your central nervous system or your metabolism at a minimum cost. So the result then are networks that maximize energy throughput at a minimum cost. We're in the homestretch here. Last part. Life is characterized by a set of scaling laws. And as a result, and this is really interesting, the rate of aging is actually scaled in a way so that all of life actually fluxes about the same amount of energy per unit mass across all living things. And they say, my goodness.

That sounds really quite remarkable. But I would like you to consider for a moment these wonderful and amazing organisms. They do very different things. They feed on different resources. Some of them live a short time, others live a long time. So what I'm to do is only take all those organisms, and I'm going to put them on a graph. So at the bottom, I have their body size or their mass. You pick them up, you weigh them, you put them on a scale. On the y-axis, I'm going to plot a measure of how fast they're producing biomass, how fast they're growing. So remember that growth is fueled ultimately by the TCA cycle. So let's look at insects, birds, fish, plants, unicellular all the way up to multicellular plants, mammals, protists, zooplankton. And what was really interesting is that recently, we were wondering if we could put long-extinct organisms on this plot. And what's really interesting, in the bones of dinosaurs, if you cut open their bones and their fossils and you look then at the bones, what you'll actually find with things that look like tree rings. So just like a tree when you cut it down it has growth rings, a lot of different organisms than that leave behind bone, you can also measure their growth rate.

If some basics of how to convert that to mass, we have estimates of where the dinosaurs would be, things long extinct. So this is interesting. All organisms are described, apparently, by an approximate general scaling function. As you go across scales of size, we know how they grow. What's interesting is that plot says that the total amount of energy a very small organism or a very big organism uses per unit mass for their entire lifespan is approximately the same. It's actually 8 times 10 to the fifth kilojoules per kilogram. So this is like the Hitchhikers Guide to the Galaxy. Why that number? I have no idea, but it exists. Within a mouse, the TCA cycle is spinning very rapidly. Within an elephant, it spins very slowly. An interesting outcome of that, if you calculate the total number of heartbeats in a mammal, you can do this for birds too or anything with a heart– plants don't have hearts, there are actually very similar numbers for them but for different quantities. If you actually plot out the total number of heartbeats across all the mammals, there's some variability.

But it's about 7.3 plus or minus 5.6 times 10 to the eighth, about a billion. Now, I'm sure some of you, quantitatively astute, have probably calculated your number and found that you well exceed a billion. That's all right, there is some slop in these numbers. I'm sure you're going to be fine. But this is what's interesting. Why is it about a billion? Why isn't it 100? Why isn't it a million? Why isn't it a trillion? Why that number? Well, probably, it ultimately has to do with the fact that– you can just think of any car that you buy. At some point, it's going to wear out, entropy. But what's interesting is that we don't really have a really good biological understanding for why these numbers. Last thing that I'm going to end on, ecosystems. So if you think about an ecosystem, you can have a trophic network, that is who eats who. So you can have plants, herbivores eat the plants, larger carnivores eat the herbivores, you have secondary carnivores, tertiary carnivores, carnivores eating carnivores.

It gets really messy. Now, if you think about an ecosystem, that dissipation of energy, either from the deep sea vents or from photosynthesis from the sun, is dissipated then through the biosphere through these trophic interactions of who eats who. But remember, that energy is all being paced and scaled ultimately by what's happening not only in your mitochondria but also down to the core metabolism of the TCA cycle. So we can talk about ecosystem-scale metabolism. We can talk about the metabolism of the biosphere. So what I'm showing you here actually, this is some work led in part by Neil Martinez. Neil is one of my colleagues here at the University of Arizona. And Neil and his colleagues went through and they did a fantastic study. So they looked at these networks of interactions. So you can kind of think of this as any other kind of network of interactions, like on Facebook or the internet, kind of like who's connected to who.

But here, what we're doing is, we're looking at a trophic network in terms of having the plants here at the bottom, then the herbivores, who eats who in terms of which herbivores are on which plants but also the top carnivores here in terms of who are they eating. So what's actually very interesting is, I'm showing you two of these different trophic networks or food webs. One of them is from the Burgess Shale. The other is from a lake, probably in Wisconsin. Now, what's interesting is that statistically, you can analyze than the degree of connectedness then of this network. And in general, you can't tell them apart from each other. Their network of interactions are organized in a very similar way. So when we look at these networks of interactions remember, we're seeing the dissipation of metabolic energy throughout the ecosystem. And those interactions are organized in a way so that metabolism is then passed through all the way from herbivores up to the carnivores. So what I'm showing you here is one view of the metabolism of the earth. This is carbon assimilation, how much carbon is coming into the terrestrial biosphere via photosynthesis.

And this movie is looping through at a monthly timestep, every month. And so if you watch it, what you'll see is you'll see seasonalities during the year. So the Northern Hemisphere will green up in the summer, but when winter comes, it'll brown down. You'll also see within the tropics and South America and Australia and Asia. You'll see, even within the tropics, the pulsing of terrestrial metabolism to flux throughout the year. And you can watch this. This goes out actually, for many, many different years. But this is variation in ecosystem metabolism. Now, if you're interested, we can talk about this later, but I don't have the time. But I will tell you that you can take what we know of core metabolism, you can take our knowledge of how fast the TCA cycle spins, you can take the different sizes of organisms that we know exist to actually predict the metabolism at the scale of an ecosystem. And we can trace that metabolism all the way down to the core metabolism ultimately down to the TCA cycle.

So the Earth is literally a metabolizing entity, ultimately fueled by a set of core reactions that are ubiquitous across all of life. And that dissipation of energy, both within organisms and across ecosystems, appears to also be governed by general rules with how natural selection has maximized not only distribution networks, but also control the interactions of who eats who and how energy is dissipated. So to summarize, general aspects of life. Life does work. Metabolism maintains homeostasis. So right now, your body is working very hard to maintain your core temperature around a very small degree of fluctuation. Any deviation from that is deadly. You spend a lot of energy trying to maintain your core metabolism temperature at a very constant level, homeostasis. Life in general evolves, becoming increasingly larger and larger and larger. This doesn't mean that small organisms disappear. There's plenty of different places for small organisms to live. But larger organisms effectively by becoming larger, they avoid the competition from smaller ones but can exploit and dominate more and more resources. Life uses fractal networks.

They appear to optimize transport in terms of time and flow. Again, natural selection appears to be doing the optimization. Life itself is scaled by core metabolism. This has remained constant across all scales. But life's interactions in terms of the struggle for resources to fuel metabolism is ultimately scaled and paced by the core rate of metabolic reactions. So in general, the diversity of life is shaped by this flow and dissipation of energy. So I started with this dichotomy within biology that we have both a deterministic and also a chance view on life. So determinism, there are regular deterministic processes that shape the diversity of life, the laws of chemistry, physics, combined with evolution by natural selection appears to be very general in terms of many different aspects of life. Again, these rules appear to ultimately determine not only the pace of life but also the functioning of Earth's ecosystems. But we also know that there is a big component of chance as well. Random mutations are a key component of evolution.

Extinction at many levels happened constantly. Because of these extinction events, ecosystems undergo rearrangements. So let's try to put this together. Well, life in general is both by chance and deterministic. Even though the players of the game have changed, it looks as if there are some general rules that have remained the same. And I've showed you five general aspects of life on Earth which are tantalizing and suggest that if we did find life elsewhere, and if resources are limiting, so there is competition for limiting space, resources, or mates, we may expect to see these aspects of life repeated there. And so this is interesting to ponder once we do start looking for life in the next several talks, what that life may look like. So I would like to thank many folks who have inspired not only this research but also this talk, in particular Eric Smith and Geoffrey West at the Santa Fe Institute, but also my lab group, Joaquin, and many folks that underlie the production staff here at the science series.

But with that, I want to say thank you for your time. It's been a pleasure. Thank you. .