Ergodicity, Evolution, and Aliens
This article considers whether or not evolution is predictable.
In some ways it is, in others not. That is, you can predict certain things, but life adds levels of complexity that unalive systems just don't have. So even though they're using some of the same elements and forces, you see some different behaviors.
The things that are the most predictable are those driven by the laws of physics. A flier has to be aerodynamic. A floater or glider has somewhat more options, but still has to contend with lift and gravity, thrust and drag. While there are various ways to achieve true flight, all of them involve aerodynamics. They strongly tend to outperform floating or gliding in terms of power, maneuverability, speed, etc. Similarly, a swimmer's ability depends on how hydrodynamic it is. There are tolerably okay swimmers, like dogs, who can do it but aren't really adapted for it; good swimmers like otters, who are somewhat adapted; and excellent swimmers like dolphins who are fully adapted. That process of adaptation is basically just species squeezing themselves toward an optimum swimming shape, which tends to be torpedo-ish.
So looking at these things, and the general tendency of life to evolve toward greater complexity, we can confidently predict than in a world conducive to flight (e.g. a reasonably thick but not soupy atmosphere, moderate to light gravity, and moderate to calm weather) the most efficient aerodynamics are likely to occur and outcompete other features. That means a streamlined surface, airfoils, etc. Similarly in an environment suited to swimming (e.g. a habitable liquid medium) we can predict hydrodynamic results: a streamlined surface and a handful of shapes optimized for different styles, such as the torpedo, the disc, and the ribbon.
The biggest difference between a nonliving system and a living system is that the latter has to succeed. This means that, while life tries many many iterations, only a small subset of those are viable and an even smaller subset are competitive. The nonviable ones leave no trace, and even the noncompetitive ones usually don't. What we see are the successes, which show extreme diversity, which indicates a whole lot of frobbing around.
This brings us to the fossil record, which is largely how we study the history of evolution. It's incomplete. We know this, but people tend to forget its implications. What we have are a few fragments representing a tiny fraction of individuals from a very small fraction of species. We also know its deeply biased, because of how fossils work. Things with a lot of hard parts such as shells or bones fossilize much more easily, and thus frequently, than things with mostly or all soft parts. So we know a lot more about hard-part lifeforms than soft-part lifeforms, because the latter only fossilize under very rare circumstances. That means the vast majority of life's iterations don't show up for this reason, in addition to most of them not showing up for the previous reason.
When you've got only a handful of puzzle pieces, it's extremely difficult to figure out much of how the puzzle works. We've got enough to piece together things like evolutionary adaptation, but we have nowhere near enough of its iterative outcomes to prove or disprove ergodicity. We can, however, observe that life is extremely iterative and extremely ingenious. We've identified a number of supercreative instances (e.g. the Cambrian Explosion) where everything exploded into variety, and then later collapsed into much fewer species as the more effective ones outcompeted the less effective ones. "Life finds a way" and "anything that can be eaten, will be eaten" are robust behavioral patterns.
The aliens you see in science fiction movies and TV often look a lot like us: two arms, two legs, and a head (but with pointy ears). While the reason for this has everything to do with limited budgets and not science, these representations do raise a deeper question about what’s called convergent evolution. If Darwinian evolution works on other planets, will they lead to forms of life - literally how it appears - like we find on Earth?
So let's look at what this means for extraterrestrial life ...
* Different base media can have different patterns, but are still bounded by the laws of physics. It's just that those laws sometimes behave quite differently in odd circumstances than in common circumstances. You can float a boat on most liquids, but not a superliquid because it will crawl into the boat. Most frozen things are heavier than liquid things, but water ice floats. That means similar environments tend to produce similar results, but there are also lots of truly bizarre environments with exotic results. So don't assume you know anything about life in general much beyond its extreme creativity; assume the universe has a bigger imagination than you do.
* The most common life is the simplest because that's where it starts out. Single-celled creatures are the first to evolve, and you'll find them in places too hostile for anything else. Most life in the universe is basically algae or the like. Slightly more complex multicellular microorganisms are also quite common.
* Designs that work well will consistently appear. That means things like shells, skeletons, fur, scales, leaves, seeds, etc. will be common in ecosystems that advance far enough for them.
* Consider the arrangement of limbs and head. Earth actually played around with many iterations, some of them quite bizarre, before settling into a favored range. We have everything from creatures with one leg like snails to hundreds of legs like millipedes. But the really numerous creatures tend to have 4, 6, or 8 legs. Reason being, some redundancy is good in case of injury; but every limb uses resources and provides opportunity for injury. Multiples also allow some specialization, like the wings of birds or hands of humans. So there's a sweet spot where you have enough limbs for fault tolerance without overspending the energy budget. Some planets lean toward one or the other end of the spectrum, based on what survives evolutionary challenges there; but there are concrete advantages on part of that spectrum which make those areas more likely. Much the same is true of a head. It conveys advantages and mitigates risks, so it's a very common feature; but it's an energy hog and easily injured, so few species have more than one.
Bipedalism is a bit different. The height advantage is clear, it frees up grasping organs if any, but it's hard to do. It requires active rather than passive stability, and that kind of balance is much harder to achieve than something with more feet on the ground and a lower center of balance. So while it's fairly popular, what you often see is actually ambipedal species like bears or squirrels who can stand up or run on all fours as desired. Advanced alien species are likely to include one or both of these options. Another very common version is the hexapedal form, like insects. Look at a preying mantis and you can see the sort of centaur-effect with a very stable four-legged lower body leaving upper limbs free and head higher. It does have a heavier energy cost for those extra limbs, but they can usually pay for themselves through increased benefits. And those aliens are going to wonder how the hell humans stay upright.
Remember too that aliens might be terrestrial, aquatic, or aerial; and the last two almost certainly won't be human-shaped since it's poorly adapted to liquid or gaseous locomotion. Bipedal with wings is possible, though: multiple iterations of that exists in birds, bats, etc.
As the system changes in time, its representation in phase space will visit every point available to it in those 6 x 1023 dimensions.
See now, here we have to define what "visit" means, and that's where life and nonlife are very different. Nonlife can bounce around purely in response to its environment; life has to work, it has to be viable. So has an instance been "visited" if it has, ever in evolution, had a cell exist or attempt to exist? Or does it have to achieve birth? Live to reproduce? Be recorded? Those numbers get smaller all the way. And we have genetic examples of restricted manifestation in the form of nonviable combinations, such as Lethal White Syndrome, where one copy of the gene is viable but two copies are nonviable. The result is a full-term foal that dies shortly after birth. So the instance is created, but cannot continue -- and you wouldn't see it in a fossil record.
We can predict a significant amount of what's likely to happen when instances take a particular path. For example, the domestication syndrome predicts things like smaller brain/skull size and floppy ears, which appear across multiple species.
For Kauffman, the most important aspect of evolution is its path dependence, its history. Run the history of the Earth over again and you would get something different.
That's because life has to survive. Certain traits work for or against survival, and some of those are rockers that depend heavily on environment. Being a generalist is a robust choice: there are always generalists in an ecosystem, even if specialists may be able to exploit some resources they can't. Specialists are most vulnerable to disruptions, and generalists least vulnerable. Being warm-blooded makes you faster and more independent, a clear advantage in an environment with plenty of energy; but being cold-blooded means you can survive a much longer period of low input. Extinction events have shown cases where the only warm-blooded survivors were quite small, whereas larger cold-blooded species survived due to needing less energy. Enter luck. If an asteroid hits Continent A, the survivors will be different than if it hits Continent B. Some of the possibilities are removed.
If we pour out some of the coffee from the cup before the molecules finish visiting all their instances of hyperspace, does that make the energy system of the cooling coffee non-ergodic? No, because they would've finished doing that if not interrupted. The same may (or may not) be true of life.
“If the evolutionary ergodic search time of a genome subspace for any corresponding phenotype can be calculated, then… we expect that a fitness optimum can be found, if one exists. This would provide a conceptual basis for understanding convergence in evolution…”
Now there's a good, life-relevant approach. For a given type of creature in a given environment, can we identify an optimum format that evolution will lean toward? Well, one good way of doing that is to look for things that have survived a very long time. That doesn't happen unless they're excellent survivors. Take sharks. They are basically eating machines, in a torpedo shape we have already identified as hydrodynamic. They come in all sizes, so they can use as much food as is available. And they've been around for ages. This qualifies "shark" as a type of "optimum fish" although there may be other types too. We can expect that aquatic habitats will tend to develop something like "shark" if it is possible to do so. And that means, when you are exploring an alien planet, keep your limbs inside the boat so you don't look like food to whatever is in the liquid habitat. (If the "shark" is big enough, the boat will look like food, but that's a different issue.)
We can't predict evolution precisely. We can't predict weather precisely. But we know enough to make very useful predictions that are much better than nothing.
In some ways it is, in others not. That is, you can predict certain things, but life adds levels of complexity that unalive systems just don't have. So even though they're using some of the same elements and forces, you see some different behaviors.
The things that are the most predictable are those driven by the laws of physics. A flier has to be aerodynamic. A floater or glider has somewhat more options, but still has to contend with lift and gravity, thrust and drag. While there are various ways to achieve true flight, all of them involve aerodynamics. They strongly tend to outperform floating or gliding in terms of power, maneuverability, speed, etc. Similarly, a swimmer's ability depends on how hydrodynamic it is. There are tolerably okay swimmers, like dogs, who can do it but aren't really adapted for it; good swimmers like otters, who are somewhat adapted; and excellent swimmers like dolphins who are fully adapted. That process of adaptation is basically just species squeezing themselves toward an optimum swimming shape, which tends to be torpedo-ish.
So looking at these things, and the general tendency of life to evolve toward greater complexity, we can confidently predict than in a world conducive to flight (e.g. a reasonably thick but not soupy atmosphere, moderate to light gravity, and moderate to calm weather) the most efficient aerodynamics are likely to occur and outcompete other features. That means a streamlined surface, airfoils, etc. Similarly in an environment suited to swimming (e.g. a habitable liquid medium) we can predict hydrodynamic results: a streamlined surface and a handful of shapes optimized for different styles, such as the torpedo, the disc, and the ribbon.
The biggest difference between a nonliving system and a living system is that the latter has to succeed. This means that, while life tries many many iterations, only a small subset of those are viable and an even smaller subset are competitive. The nonviable ones leave no trace, and even the noncompetitive ones usually don't. What we see are the successes, which show extreme diversity, which indicates a whole lot of frobbing around.
This brings us to the fossil record, which is largely how we study the history of evolution. It's incomplete. We know this, but people tend to forget its implications. What we have are a few fragments representing a tiny fraction of individuals from a very small fraction of species. We also know its deeply biased, because of how fossils work. Things with a lot of hard parts such as shells or bones fossilize much more easily, and thus frequently, than things with mostly or all soft parts. So we know a lot more about hard-part lifeforms than soft-part lifeforms, because the latter only fossilize under very rare circumstances. That means the vast majority of life's iterations don't show up for this reason, in addition to most of them not showing up for the previous reason.
When you've got only a handful of puzzle pieces, it's extremely difficult to figure out much of how the puzzle works. We've got enough to piece together things like evolutionary adaptation, but we have nowhere near enough of its iterative outcomes to prove or disprove ergodicity. We can, however, observe that life is extremely iterative and extremely ingenious. We've identified a number of supercreative instances (e.g. the Cambrian Explosion) where everything exploded into variety, and then later collapsed into much fewer species as the more effective ones outcompeted the less effective ones. "Life finds a way" and "anything that can be eaten, will be eaten" are robust behavioral patterns.
The aliens you see in science fiction movies and TV often look a lot like us: two arms, two legs, and a head (but with pointy ears). While the reason for this has everything to do with limited budgets and not science, these representations do raise a deeper question about what’s called convergent evolution. If Darwinian evolution works on other planets, will they lead to forms of life - literally how it appears - like we find on Earth?
So let's look at what this means for extraterrestrial life ...
* Different base media can have different patterns, but are still bounded by the laws of physics. It's just that those laws sometimes behave quite differently in odd circumstances than in common circumstances. You can float a boat on most liquids, but not a superliquid because it will crawl into the boat. Most frozen things are heavier than liquid things, but water ice floats. That means similar environments tend to produce similar results, but there are also lots of truly bizarre environments with exotic results. So don't assume you know anything about life in general much beyond its extreme creativity; assume the universe has a bigger imagination than you do.
* The most common life is the simplest because that's where it starts out. Single-celled creatures are the first to evolve, and you'll find them in places too hostile for anything else. Most life in the universe is basically algae or the like. Slightly more complex multicellular microorganisms are also quite common.
* Designs that work well will consistently appear. That means things like shells, skeletons, fur, scales, leaves, seeds, etc. will be common in ecosystems that advance far enough for them.
* Consider the arrangement of limbs and head. Earth actually played around with many iterations, some of them quite bizarre, before settling into a favored range. We have everything from creatures with one leg like snails to hundreds of legs like millipedes. But the really numerous creatures tend to have 4, 6, or 8 legs. Reason being, some redundancy is good in case of injury; but every limb uses resources and provides opportunity for injury. Multiples also allow some specialization, like the wings of birds or hands of humans. So there's a sweet spot where you have enough limbs for fault tolerance without overspending the energy budget. Some planets lean toward one or the other end of the spectrum, based on what survives evolutionary challenges there; but there are concrete advantages on part of that spectrum which make those areas more likely. Much the same is true of a head. It conveys advantages and mitigates risks, so it's a very common feature; but it's an energy hog and easily injured, so few species have more than one.
Bipedalism is a bit different. The height advantage is clear, it frees up grasping organs if any, but it's hard to do. It requires active rather than passive stability, and that kind of balance is much harder to achieve than something with more feet on the ground and a lower center of balance. So while it's fairly popular, what you often see is actually ambipedal species like bears or squirrels who can stand up or run on all fours as desired. Advanced alien species are likely to include one or both of these options. Another very common version is the hexapedal form, like insects. Look at a preying mantis and you can see the sort of centaur-effect with a very stable four-legged lower body leaving upper limbs free and head higher. It does have a heavier energy cost for those extra limbs, but they can usually pay for themselves through increased benefits. And those aliens are going to wonder how the hell humans stay upright.
Remember too that aliens might be terrestrial, aquatic, or aerial; and the last two almost certainly won't be human-shaped since it's poorly adapted to liquid or gaseous locomotion. Bipedal with wings is possible, though: multiple iterations of that exists in birds, bats, etc.
As the system changes in time, its representation in phase space will visit every point available to it in those 6 x 1023 dimensions.
See now, here we have to define what "visit" means, and that's where life and nonlife are very different. Nonlife can bounce around purely in response to its environment; life has to work, it has to be viable. So has an instance been "visited" if it has, ever in evolution, had a cell exist or attempt to exist? Or does it have to achieve birth? Live to reproduce? Be recorded? Those numbers get smaller all the way. And we have genetic examples of restricted manifestation in the form of nonviable combinations, such as Lethal White Syndrome, where one copy of the gene is viable but two copies are nonviable. The result is a full-term foal that dies shortly after birth. So the instance is created, but cannot continue -- and you wouldn't see it in a fossil record.
We can predict a significant amount of what's likely to happen when instances take a particular path. For example, the domestication syndrome predicts things like smaller brain/skull size and floppy ears, which appear across multiple species.
For Kauffman, the most important aspect of evolution is its path dependence, its history. Run the history of the Earth over again and you would get something different.
That's because life has to survive. Certain traits work for or against survival, and some of those are rockers that depend heavily on environment. Being a generalist is a robust choice: there are always generalists in an ecosystem, even if specialists may be able to exploit some resources they can't. Specialists are most vulnerable to disruptions, and generalists least vulnerable. Being warm-blooded makes you faster and more independent, a clear advantage in an environment with plenty of energy; but being cold-blooded means you can survive a much longer period of low input. Extinction events have shown cases where the only warm-blooded survivors were quite small, whereas larger cold-blooded species survived due to needing less energy. Enter luck. If an asteroid hits Continent A, the survivors will be different than if it hits Continent B. Some of the possibilities are removed.
If we pour out some of the coffee from the cup before the molecules finish visiting all their instances of hyperspace, does that make the energy system of the cooling coffee non-ergodic? No, because they would've finished doing that if not interrupted. The same may (or may not) be true of life.
“If the evolutionary ergodic search time of a genome subspace for any corresponding phenotype can be calculated, then… we expect that a fitness optimum can be found, if one exists. This would provide a conceptual basis for understanding convergence in evolution…”
Now there's a good, life-relevant approach. For a given type of creature in a given environment, can we identify an optimum format that evolution will lean toward? Well, one good way of doing that is to look for things that have survived a very long time. That doesn't happen unless they're excellent survivors. Take sharks. They are basically eating machines, in a torpedo shape we have already identified as hydrodynamic. They come in all sizes, so they can use as much food as is available. And they've been around for ages. This qualifies "shark" as a type of "optimum fish" although there may be other types too. We can expect that aquatic habitats will tend to develop something like "shark" if it is possible to do so. And that means, when you are exploring an alien planet, keep your limbs inside the boat so you don't look like food to whatever is in the liquid habitat. (If the "shark" is big enough, the boat will look like food, but that's a different issue.)
We can't predict evolution precisely. We can't predict weather precisely. But we know enough to make very useful predictions that are much better than nothing.