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The road not taken

Comment on Humans – The Species that Changed Earth

Biology has remade the Earth many times through its history. Humans may represent the latest of these biological events, engineering a ‘used planet’, as cogently presented in Erle Ellis’ essay ‘Humans: the species that changed Earth’. But, can human societies engineer a world better for both humans and all other species that live on this planet? A reading of the immense record of biological evolution suggests that this is possible.

Humans are part of an evolutionary process that has been unfolding for about four billion years, since the prebiotic phase of the biosphere spawned the first cells as remarkable media for transmitting genetic inheritance across generations, and as a perfect structure in which to store and utilise energy. The first ecosystems were predicated on autotrophic microbes using localised sources of chemical energy. Even in the earliest stages of the biosphere, these microbes would have modified the chemical and physical environment around them, not least in providing a primary food source for the first heterotrophs: they would therefore have been ecosystem engineers. As time progressed, the biosphere evolved oxygenic photosynthesis using water, carbon dioxide, and sunlight. In this way the biosphere was freed to become global, no longer predicated on local sources of chemical energy. As a by-product of this biological innovation, oxygen was liberated to the atmosphere, a toxic gas for most of the pre-existing biosphere, which had evolved under the anaerobic conditions of the early Earth. Thus, a biological innovation engineered a fundamental change to the Earth System. There are many other examples in the geological record of where this has occurred, including the evolution of animals themselves – such as the trilobites more than 500 million years ago, with their guts, heads, and propensity for directional movement. And ultimately this process of animal evolution led to human sentience and ultrasociality. Such biological innovations have generally re-engineered the biosphere around them, subsuming, displacing, or sometimes making completely obsolete the pre-existing organisms and ecosystems.

What is also apparent, though, is that innovations in the biosphere often engineer environments in which other organisms can flourish. The development of marine reef systems is a good example. Evolving many times over the past half billion years, marine reefs harbour huge biodiversity. Modern coral reefs, for example, are estimated to contain some 25% or more of ocean species diversity, whilst representing much less than 1% of the ocean surface area. The evolution of animal burrowing, also more than half a billion years ago, fundamentally changed the seabed, developing new ecologies for other organisms to colonise and evolve into: this is sometimes called the ‘Cambrian agronomic revolution’ by palaeontologists, and it echoes the impact that humans have had on the terrestrial landscape half a billion years later. We can argue too, that humans have engineered entirely new ecosystems. This began about 50,000 years ago with the evolution of culturally modern humans, and is perhaps first manifested by our impact on large terrestrial mammals – precipitating extinctions of the megafauna, followed by clearing of forests, the spread of agriculture (the human agronomic revolution), urbanisation and industrialization. Each phase of human influence has potentially freed new space for organisms to evolve into, evidenced, for example, by the hundreds of species of bacteria found in the New York Subway1. However, in human engineered spaces it is often technology that thrives and diversifies, not biology. A typical office might house a pot plant and its soil microbes – an unsustainable ecosystem without its human component providing food and water, and there are the microbes lurking in the computer keyboard – that also rely on the human for sustenance. This is hardly a diverse, human-engineered ecology for biology to thrive in. By contrast, technology has evolved rapidly to fill that office (eco)space: computer, telephone, lights, blinds, books, coffee cups, table, chairs, paper, calculator, stapler, pens, pencils, shelves, and so on, and so on, and proliferating. The office of a palaeontologist (mine) has a microscope too, and glass jars imprisoning the fossil components of past biosphere stages.

Humans have changed the Earth fundamentally. Homo sapiens have modified three-quarters of the ice-free terrestrial landscape. Within these modified landscapes, the composition and formation of pre-human natural flora and fauna is markedly changed. Sometimes local biodiversity is enhanced by the introduction of complementary neobiotic species. Sometimes the biosphere is reduced to lines of trees and their visiting birds and insects garlanding major streets in cities. In all such modified landscapes the ‘natural’ is subsumed into the human, into a new ‘Anthropocene biosphere’. This resembles the response of the biosphere to some past major innovations, for example the way that anaerobic microbes may have been displaced from shallow marine seas, more than 2 billion years ago, by newly evolving aerobic forms.

Humans are different from microbes and trilobites though, and as Erle Ellis shows in his thoughtful essay, the sociality of modern humans is exceptional within the context of four billion years of biosphere evolution. We alone are an ultrasocial species, and encapsulated in that capacity is our ability to influence the direction of biological and planetary evolution, something that no other organism on Earth can do deliberately. Ellis argues cogently that we need a new approach to encapsulate this human influence, his new theory of ‘sociocultural niche construction’, which examines how natural selection acts simultaneously on individuals, social groups, and societies to cause long-term behavioural changes across human generational time. This is an important idea. It helps to frame why human technological evolution is sometimes so rapid that it appears to leave biology behind. It may help us to formulate new ways of understanding the interactions we have with the rest of biology and with the burgeoning world of technological evolution. As Ellis shows, humans have different potential future trajectories and here I am reminded of Robert Frost’s poem ‘The Road Not Taken’. One of our roads leads to planetary stewardship, in which a fundamentally changed biosphere becomes sustainable over geological timeframes, evolving in effect, a new state – as it did when aerobic microbes evolved more than 2.5 billion years ago. The other road may be one that leads to calamitous human over-consumption, so that humans precipitate a collapse in the biosphere equivalent to that of past mass extinctions. But humans are not a meteorite hitting the planet some 66 million years ago and annihilating the non-avian dinosaurs. They are not the dumb actors of the Permian-Triassic boundary extinction event that unfolded about 252 million years ago, when 95% of marine species diversity was extinguished by hostile environmental factors. Humans can use their sociocultural niche construction to engineer a path towards sustainability. We can build cities that are not only fruitful environments for technological biodiversity, but also provide a space for biology.  And so, to paraphrase Frost, sometime ages and ages hence, I hope that my daughter and her children will not be telling this [human] story with a sigh.


  1. Afshinnekoo, E., et al. (2015), Geospatial resolution of human and bacterial diversity with city-scale metagenomics, CELS 1, 1-15,



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