IN 1948, cybernetics pioneer Ross Ashby built a curious machine. The was constructed from four interconnected bomb-control units scavenged from the UK鈥檚 Royal Air Force. It featured four pivoting magnets, the position of each being determined by that of the others and guided by feedback mechanisms generated using a table of random settings. When Ashby turned the machine on, the magnets would start to oscillate wildly. Sometimes they would return to a stable equilibrium position. If not, Ashby had wired the Homeostat to reboot itself with a new selection of random settings. Over time, this basic algorithm 鈥 if unstable, try again 鈥 always eventually led to equilibrium. That was the machine鈥檚 sole purpose: to show that a simple, dynamic system would regain stability in response to changes in its environment.
Ashby believed this 鈥渦ltrastability鈥 to be a governing principle in nature, explaining, among other things, the adaptation of species to their niche 鈥 a process that appears purposeful, but actually arises from random processes. It may seem a stretch to describe the Homeostat鈥檚 change over time, from wild motion to stability, as 鈥渆volution鈥. After all, it lacks all the trappings we associate with Darwinian evolution 鈥 such as life and reproduction. Yet, there is a growing belief that the same forces driving Ashby鈥檚 machine hold the key to a wider concept of evolution, one that can encompass semi-living and even nonliving systems. This new view may prove essential to understanding the functioning of ecosystems and even the origin of life. Most intriguingly, it bolsters the Gaia hypothesis, the controversial idea that the biosphere acts like a giant organism, one that self-regulates to keep conditions just right for life.
Darwin鈥檚 original formula for evolution by natural selection works as follows: organisms vary, those with more favourable traits leave more offspring on average and these offspring are likely to inherit their parents鈥 favourable traits. This explains why organisms are well adapted to their environments 鈥 why seed-eating birds have thick, strong bills, why flowers produce sugar-rich nectar to attract pollinators, and countless other traits. As our evolutionary thinking has developed, natural selection has proved remarkably adaptable. It can explain the evolution of 鈥渟elfish genes鈥 鈥 an idea popularised by biologist Richard Dawkins in the 1970s 鈥 because, like individuals of the same species, a gene may exist in various forms, some of which are more likely to survive and be passed on to future generations. Under the right circumstances, natural selection can even extend to discrete groups of organisms, as when more cooperative populations do better against less collaborative ones. However, this Darwinian dynamic breaks down at the level of Gaia, the whole planet.
Advertisement
As far as we know, Earth is a one-off: there is no population of competing, reproducing planets for natural selection to choose between to form the next generation. And yet, like a superorganism honed by evolution, Earth seems to self-regulate in ways that are essential for life. Oxygen levels have remained relatively constant for hundreds of millions of years, as has the availability of key building blocks of life such as carbon, nitrogen and phosphorus. Crucially, Earth鈥檚 surface temperature has remained within the narrow range that allows liquid water to exist. It is true there have been upheavals: during a 鈥渟nowball Earth鈥 episode about 700 million years ago, for example, almost the entire surface was frozen. 鈥淏ut the key question is, why does it spend so much time in a stable state and not just flying all over the place?鈥 asks at the University of Exeter, UK.
This question has stumped earth scientists since James Lovelock first proposed the Gaia hypothesis in the 1960s. There is, after all, no obvious way for such self-regulation to evolve. This is particularly true because the processes that underpin Earth鈥檚 temperature, oxygen levels and the like 鈥 which include things like plate tectonics and erosion 鈥 operate over millions of years. That is far too long for the adaptation of individual organisms to their environments through natural selection to make a difference. This conundrum has led most evolutionary biologists to entirely reject any notion of Gaian evolution. 鈥淵ou simply cannot get an adaptation at the planetary level,鈥 says at the University of Vermont.
But there might be another way, says Lenton. What if Gaia works like Ashby鈥檚 Homeostat? In other words, he suggests, Earth and the early life on it might have interacted haphazardly at first. Unstable configurations 鈥 those, say, with little or no cycling of key elements such as nitrogen 鈥 would have failed quickly, requiring life to reboot nearly from scratch. Eventually, though, the system must have stumbled on a stable configuration, with better cycling and tighter regulatory mechanisms. It should be no surprise, then, that the planet of today has strong regulatory systems.
鈥淟ike a superorganism honed by evolution, Earth seems to self-regulate鈥
This process, called 鈥渟election by persistence鈥, evades the requirements for competition and reproduction that make natural selection so problematic as a mechanism for explaining the evolution of Earth. 鈥淚 think of it like a search algorithm,鈥 says Lenton. 鈥淸Earth] can undergo repeated trials over time until it falls into a stable configuration. And once it does, that tends to persist.鈥
The process doesn鈥檛 stop there either, according to Lenton. Once the system is in a stable state, it can accumulate further changes 鈥 new regulatory mechanisms, for instance 鈥 that stabilise it still further. He calls this 鈥渟equential selection鈥. As a result, stable systems not only persist, they get better at persisting over time too. , he and others describe how this gradual improvement adds up to an evolutionary process that could lead to Gaian self-regulation. Lovelock, for one, is enthusiastic about the idea. 鈥淭he problem with Gaia and other dynamic feedback systems is that they can be described but cannot be explained,鈥 he says. 鈥淟enton and his colleagues have extended the Gaia theory.鈥
However, although selection by persistence may make perfect logical sense, is it feasible in practice? Self-regulation arises easily in relatively simple systems, as Ashby鈥檚 Homeostat illustrates. But as systems become more complex, the number of possible states they can have rises dramatically 鈥 and the odds of randomly stumbling on a stable one go down just as steeply, says , a complexity scientist and evolutionary theorist at the University of Surrey, UK. 鈥淚s it plausible within the lifespan of the Earth that we would end up there?鈥 she wonders. As far as she knows, no one has answered this.
鈥淪table systems don鈥檛 just persist, they get better at persisting over time鈥
Sceptics also question the claim that systems should become more robust over time by accumulating stabilising changes. 鈥淭he longer you鈥檙e in a system, the more time you can destabilise it, too,鈥 says , a geochemist at Harvard University. That鈥檚 true, Lenton concedes. But his mathematical models, described in a recent paper, suggest that the in the long run.
There are other reasons to expect stability to increase over time. If a system is to persist rather than collapse in the face of change, it needs to be robust. Three features make an ecosystem, or a planetary system, more robust, says ecologist at Princeton University. First, robust systems have some degree of redundancy, so the loss of any particular component 鈥 the extinction of a species, say 鈥 doesn鈥檛 critically compromise the whole. Second, they have diversity, which increases the odds that at least some species will be able to cope with unexpected changes. Third, they have modularity, so that a failure of part of the system doesn鈥檛 bring down the whole thing.
The longer a system evolves, the more likely it is to show all three of these features, says Levin. In other words, robustness increases over time. Earth鈥檚 biosphere has been around since the first life evolved more than 3.9 billion years ago, so it has had plenty of time to become more robust.
Planetary reboot
Although mathematical models can show that selection by persistence is possible, they cannot demonstrate that it actually happened on Earth. However, Lenton sees several ways to test his hypothesis. 鈥淚sn鈥檛 Earth history a test case of these ideas?鈥 he asks. In particular, we know the planet has gone through several catastrophic transitions that swept away the old biosphere and forced the few surviving life forms to create a new one.
Prime among these transitions would have been the so-called great oxygenation event about 2.4 billion years ago, when oxygen levels in the atmosphere rose dramatically from near zero after the evolution of photosynthesis. The appearance of highly reactive oxygen would have upset all the existing biogeochemical cycles and the metabolisms of organisms dependent on them. If Lenton is right, this would have triggered a period of planetary instability followed by the gradual emergence of a new, increasingly stable Earth system as the biosphere accumulated fresh metabolic pathways to regulate its novel regime.
There are no organisms from that long-ago time, but by working back down the tree of life, biologists are starting to figure out when particular metabolic innovations emerged. 鈥淚f we鈥檙e lucky, we鈥檒l be able to piece together how the origin of new metabolisms disrupted the status quo,鈥 says Lenton. 鈥淲e鈥檒l be able to test whether, when we get a more stable version of something like the nitrogen cycle, it displaces an earlier, less stable version.鈥
Looking at our own planet鈥檚 history isn鈥檛 the only way to assess the idea. The strongest test could come from life on other planets. How often does extraterrestrial life appear, and how long does it last? Are there millions of planets where life appeared but failed to self-regulate, suggesting a 鈥淕aian bottleneck鈥 that Earth was lucky to squeeze through? Or is selection by persistence ubiquitous, inevitably driving planetary processes towards stability wherever life arises?
鈥淭he strongest test of this idea could come from life on other planets鈥
Given the lack of success in our search for life elsewhere in the universe, answers to such questions are a long way off. But perhaps we needn鈥檛 go to such lengths. A change of perspective could be all that鈥檚 required, argues at the University of Montreal, a philosopher of science who researches the theoretical basis of evolution. He believes our habitual focus on reproduction as the way to measure fitness has blinded us to other possibilities. 鈥淚t鈥檚 as if you had only seen mammals,鈥 he says. 鈥淵ou鈥檇 infer that all species need legs. Then I show you a fern and you say it鈥檚 not an organism because it doesn鈥檛 have legs.鈥
From Bouchard鈥檚 perspective, natural selection favours traits that enhance persistence, with reproductive success being one way to do that, but not the only one. Consider, for example, one of the world鈥檚 oldest and largest organisms, a grove of quaking aspen trees in Utah that all sprout from a single interconnected root system. Although individual trunks come and go, the grove has been a consistent genetic entity for an estimated 80,000 years. It may reproduce sexually now and then, but it makes more sense to think of its success in terms of persistence, says Bouchard.
In fact, selection by persistence may be just one of a variety of selection mechanisms involved in evolution (see 鈥Things can only get better鈥). And, since persistence really comes into its own in bigger systems and on longer timescales, it may be important for much more than just Gaia.
Consider ecosystems, such as a coral reef or the collections of microbes in your digestive tract. Ecosystems may compete against each other, but they do not reproduce as a whole. Yet they do, arguably, have traits that call for an evolutionary explanation. Like Gaia, they are often self-regulating, maintaining relatively consistent functions, even as their component species come and go. For example, a study of revealed that the microbial ecosystems living on any two algae had, on average, just 15 per cent of species in common. Nevertheless, they shared 70 per cent of their ecological functions such as attachment, defence and biofilm formation. This consistency is hard to explain by selection acting on individual species, because no microbe performs all roles. But it fits the notion that selection by persistence has shaped the ecosystem. 鈥淓ven if you鈥檙e selecting only for persistence, you can get better and better,鈥 says evolutionary biologist at Dalhousie University in Canada, who was one of the first to consider the role of persistence in evolution. Such examples are common, he adds.
Lenton believes his unorthodox form of evolution might be the best way to understand social and cultural evolution, too. After all, cultural knowledge doesn鈥檛 exactly reproduce. Instead, concepts gain in influence by spreading, a dynamic that may be better described using selection by persistence than by conventional evolution via natural selection. Lenton is now exploring how this might work and whether it can give us fresh insights into cultural evolution.
Lenton鈥檚 novel evolutionary mechanism might even shed light on life鈥檚 biggest mystery. Back before the first living cell evolved, the primordial soup probably consisted of various molecules that collectively managed to create more molecules somewhat, but not exactly, like themselves. In such a system, success would have constituted the continued survival of a loose collection of molecules. In other words, says Bouchard, persistence rather than reproduction was the driving force behind the origins of life.
Selection by persistence could explain a lot about the world. The question is whether the idea itself will persist. Like Ashby鈥檚 Homeostat, it may require a reboot. But, in a pleasing circularity, if Lenton鈥檚 theory proves plausible and reaches a stable equilibrium, then it should accumulate further changes that will make it even more persistent.
