“Is the Earth an organism?
The Gaia hypothesis states that our biosphere is evolving. Once sceptical, some prominent biologists are beginning to agree
W Ford Doolittle
is professor in biochemistry and molecular biology at Dalhousie University in Canada.
Many of us, scientists included, harbour contradictory intuitions about Mother Nature. We can see that ecosystems often have an inherent ability to self-stabilise, and know that we wouldn’t be here if the planet hadn’t maintained conditions suitable for life for the 4 billion years since its first appearance. One reaction is to claim that some Earth-wide equilibrium, though fragile, does exist, and reflects the fact that species have evolved to cooperate with one another. Another is to say that the first response is nonsense: organisms are ‘selfish’, and evolution isn’t cooperative but rather a brutish Darwinian competition that selects individual organisms based on their ability to survive and reproduce. The primordial balancing act performed by our biosphere, if it exists at all, is more or less a lucky accident.
The idea that the Earth itself is like a single evolving ‘organism’ was developed in the mid-1970s by the independent English scientist and inventor James Lovelock and the American biologist Lynn Margulis. They dubbed it the ‘Gaia hypothesis’, asserting that the biosphere is an ‘active adaptive control system able to maintain the Earth in homeostasis’. Sometimes they went pretty far with this line of reasoning: Lovelock even ventured that algal mats have evolved so as to control global temperature, while Australia’s Great Barrier Reef might be a ‘partly finished project for an evaporation lagoon’, whose purpose was to control oceanic salinity.
The notion that the Earth itself is a living system captured the imagination of New Age enthusiasts, who deified Gaia as the Earth Goddess. But it has received rough treatment at the hands of evolutionary biologists like me, and is generally scorned by most scientific Darwinists. Most of them are still negative about Gaia: viewing many Earthly features as biological products might well have been extraordinarily fruitful, generating much good science, but Earth is nothing like an evolved organism. Algal mats and coral reefs are just not ‘adaptations’ that enhance Earth’s ‘fitness’ in the same way that eyes and wings contribute to the fitness of birds. Darwinian natural selection doesn’t work that way.
I’ve got a confession though: I’ve warmed to Gaia over the years. I was an early and vociferous objector to Lovelock and Margulis’s theory, but these days I’ve begun to suspect that they might have had a point. So I’ve spent the past five years trying to ‘Darwinise Gaia’ – to see widespread cooperation as a result of competition occurring at some higher (even planetary) level. I can see a few paths by which a Darwinian might accept the idea that the planet as a whole could boast evolved, biosphere-level adaptations, selected by nature for their stability-promoting functions.
This is not exactly a recanting of views, but it’s certainly a marked departure from how I thought 40 years ago. Darwinising Gaia seems important not just to me personally, but because it would offer a satisfyingly deep theoretical basis for efforts to maintain a habitable planet – and a way to reflect on contemporary environmental crises beyond applying a simple label such as ‘Gaia’s revenge’, with its anthropocentric and theistic implications.
Back in 1979, when Lovelock’s first popular book, Gaia: A New Look at Life on Earth, came out, the wider field of evolutionary biology was becoming a very reductionist discipline. Richard Dawkins’s The Selfish Gene had been published three years earlier, and it promoted a hardcore gene-centrism insisting that we look at genes as the fundamental units of selection – that is, the thing upon which natural selection operates. His claim was that genes were the reproducing entities par excellence, because they are the only things that always replicate and produce enduring lineages. Replication here means making fairly exact one-to-one copies, as genes (and asexual organisms such as bacteria) do. Reproduction, though, is a more inclusive and forgiving term – it’s what we humans and other sexual species do, when we make offspring that resemble both parents, but each only imperfectly. Still, this sloppy process exhibits heritable variation in fitness, and so supports evolution by natural selection.
In recent decades, many theorists have come to understand that there can be reproducing or even replicating entities evolving by natural selection at several levels of the biological hierarchy – not just in the domains of replicating genes and bacteria, or even sexual creatures such as ourselves. They have come to embrace something called multilevel selection theory: the idea that life can be represented as a hierarchy of entities nested together in larger entities, like Russian dolls. As the philosopher of science Peter Godfrey-Smith puts it, ‘genes, cells, social groups and species can all, in principle, enter into change of this kind’.
But I want something more than this – a mechanism by which selection at the level of the biosphere would be likely to produce stability. Such a mechanism – a Darwinian way of making beneficial ‘accidents’ into the equivalent of heritable variations that could evolve via natural selection – will be possible, I think. The work is far from complete, and much needs to be aligned or contrasted with emerging work in evolutionary theory. But I’d hope that Darwin, were he alive today, wouldn’t balk at the non-traditional steps I’m about to take.
First, we’d need to accept differential persistence – mere survival – as a legitimate form or mechanism of natural selection.
Put another way, what selection really accomplishes is an increase in the ratio of selected entities to total entities in a population. And, actually, this can be achieved in two ways. First is differential reproduction discussed above, generally taken to be the be-all-and-end-all of evolution. Selected entities, by out-reproducing their competitors, ultimately become the only entities in a population (what biologists call achieving fixation). In effect, the top number in the ratio increases. But the phenomenon of differential persistence, in which selected entities achieve fixation through the death, extinction or disappearance of their competitors, could also work, and has been unfairly neglected.
Let’s transpose this argument to Gaia. Gaia (the biological part of it, at least) is nothing more than the single clade of all living things descended from life’s last universal common ancestor (LUCA)…
Beyond differential persistence, there’s a second way that we might Darwinise Gaia. One element in this approach is multilevel selection theory sketched above, now illustrated in the figure below. This figure shows the four levels at which natural selection is effective, plus two more. It embraces the idea that natural selection can operate at different levels, sometimes even several at once, as long as there is reproduction among entities at that level. Dawkins’s own thought-experiment in The Selfish Gene offers an appropriate anchoring example, in which he shows how genes can be individually selfish but still get along to add up to a unified, competitive organism, also ‘selfish’.
So to Darwinise Gaia we also need what’s called the replicator/interactor framework, developed by the philosopher David Hull. Hull characterised the actors in natural selection as follows:
replicator: an entity that passes on its structure directly in replication.
interactor: an entity that directly interacts as a cohesive whole with its environment in such a way that replication is differential …
selection: a process in which the differential extinction and proliferation of interactors cause the differential perpetuation of the replicators that produced them.
Taking this back to Gaia, what we’d need to do is sometimes substitute ‘reproducer’ for ‘replicator’, and also ‘persistence’ for ‘reproduction’ on occasion.
The replicator/interactor idea can in fact be used to explain a range of fascinating biological phenomena. Humans and their gut microbiota are now often said to be holobionts, multispecies entities that interact as ‘cohesive wholes’ with their environment. This interaction is now claimed to have nutritional, developmental, immunological and even psychiatric dimensions. So, to the extent that well-nourished, fully developed and healthy individual humans are likely to survive longer and leave more progeny, these human-bacteria holobionts will ‘go extinct’ less often and ‘proliferate’ (if only by recurrence) more prolifically. In so doing, they will serve to ‘perpetuate’ the lower-level reproducers and replicators (individual Homo sapiens and many millions of bacterial individuals of the thousands of species in a healthy gut) that make up a holobiont. Beneficial strains or species of bacteria are thus differentially perpetuated through the success of a human-microbial holobiont, interacting with its environment.
There’s a third and final step that I’d hope Charles Darwin might be willing to take, when assessing whether or not the Earth is an evolving entity: a theory known as ‘It’s the song, not the singers’ (ITSNTS), as recently elaborated with the philosopher Andrew Inkpen. Songs such as ‘Happy Birthday’ recur (are re-produced, with a hyphen) because people sing them. The singers aren’t the same, but the song arguably is (or at least it exhibits only incremental, ‘evolutionary’ change). It’s perpetuated (‘persists’) only through periodic performances. Meme theory encourages us to believe that songs that are more singable, and ‘mutations’ of existing songs that make them so, could evolve by natural selection.
For some dispersed metabolic processes, such as the global nitrogen cycle, these species need not be in the same place or function at the same time, or even be related to each other. The existence of these processes encourages the evolution of (‘recruits’) species that are capable of making a living by performing individual steps: because there’s a song, there are singers.
Songs don’t themselves reproduce, but they are re-produced and do evolve. The current nitrogen cycle is not that of the Archaean Earth, but it can be seen as its continuation, insofar as earlier cycles stimulated the evolution of species that then evolved to perform later versions.
A problem here might lie in the implication that processes or patterns of interaction, which are arguably not material things, can cause the evolution of species, which are.
Beyond the benefit to science, ‘Darwinising Gaia’ would also have some political benefits. It might encourage us to look at nature as a coherent whole, with an evolutionary trajectory that we can foster or deflect as we choose. After all, we are already doing that, whether we realise it or not. Certainly, it would be a relief to heal the rift between traditional Darwinian thinking and believers in the possibility of Gaia, though there’s still much work to do to cement and validate the theory. And we’ll never really know what Darwin might have accepted as ‘Darwinian’, had he lived another 138 years. I’m just hoping that he’d applaud these efforts to render Gaia acceptable within a selectionist framework, and that he wouldn’t think we’d stretched his grand theory past the breaking point.”