Thursday, June 29, 2017

2643. Climate change: Three years to safeguard our Climate

By  Christiana Figueres, Hans Joachim Schellnhuber, Gail Whiteman, Johan Rockström, Anthony Hobley & Stefan Rahmstorf, Nature, June 28, 2017
Fort Lupton solar farm, United States. 

In the past three years, global emissions of carbon dioxide from the burning of fossil fuels have leveled after rising for decades. This is a sign that policies and investments in climate mitigation are starting to pay off. The United States, China and other nations are replacing coal with natural gas and boosting renewable energy sources. There is almost unanimous international agreement that the risks of abandoning the planet to climate change are too great to ignore.

The technology-driven transition to low-carbon energy is well under way, a trend that made the 2015 Paris climate agreement possible. But there is still a long way to go to decarbonize the world economy. The political winds are blustery. President Donald Trump has announced that the United States will withdraw from the Paris agreement when it is legally able to do so, in November 2020.

The year 2020 is crucially important for another reason, one that has more to do with physics than politics. When it comes to climate, timing is everything. According to an April report1(prepared by Carbon Tracker in London, the Climate Action Tracker consortium, the Potsdam Institute for Climate Impact Research in Germany and Yale University in New Haven, Connecticut), should emissions continue to rise beyond 2020, or even remain level, the temperature goals set in Paris become almost unattainable. The UN Sustainable Development Goals that were agreed in 2015 would also be at grave risk.

That’s why we launched Mission 2020 — a collaborative campaign to raise ambition and action across key sectors to bend the greenhouse-gas emissions curve downwards by 2020 (www.mission2020.global).

As 20 leaders of the world’s largest economies gather on 7–8 July at the G20 summit in Hamburg, Germany, we call on them to highlight the importance of the 2020 climate turning point for greenhouse-gas emissions, and to demonstrate what they and others are doing to meet this challenge. Lowering emissions globally is a monumental task, but research tells us that it is necessary, desirable and achievable.

After roughly 1°C of global warming driven by human activity, ice sheets in Greenland and Antarctica are already losing mass at an increasing rate. Summer sea ice is disappearing in the Arctic and coral reefs are dying from heat stress — entire ecosystems are starting to collapse. The social impacts of climate change from intensified heatwaves, droughts and sea-level rise are inexorable and affect the poorest and weakest first.

The magnitude of the challenge can be grasped by computing a budget for CO2 emissions — the maximum amount of the gas that can be released before the temperature limit is breached. After subtracting past emissions, humanity is left with a ‘carbon credit’ of between 150 and 1,050 gigatonnes (Gt; one Gt is 1 × 109 tonnes) of CO2 to meet the Paris target of 1.5 °C or well below 2 °C (see go.nature.com/2rytztf). The wide range reflects different ways of calculating the budgets using the most recent figures.

At the current emission rate of 41 Gt of CO2 per year, the lower limit of this range would be crossed in 4 years, and the midpoint of 600 Gt of CO2 would be passed in 15 years. If the current rate of annual emissions stays at this level, we would have to drop them almost immediately to zero once we exhaust the budget. Such a ‘jump to distress’ is in no one’s interest. A more gradual descent would allow the global economy time to adapt smoothly.

Harness momentum
The good news is that it is still possible to meet the Paris temperature goals if emissions begin to fall by 2020 (see ‘Carbon crunch’).

Greenhouse-gas emissions are already decoupling from production and consumption. For the past three years, worldwide CO2 emissions from fossil fuels have stayed flat, while the global economy and the gross domestic product (GDP) of major developed and developing nations have grown by at least 3.1% per year (see go.nature.com/2rthjje). This is only the fourth occasion in the past 40 years on which emission levels have stagnated or fallen. The previous three instances — in the early 1980s, 1992 and 2009 — were associated with global economic predicaments, but the current one is not2.

Emissions from the United States fell the most: by 3% last year, while its GDP grew by 1.6%. In China, CO2 emissions fell by 1% in 2016, and its economy expanded by 6.7% (ref. 2). Although it is too early to tell whether this plateau will presage a fall, the signs are encouraging.

In 2016, two-thirds of China’s 5.4% extra demand for electricity was supplied by carbon-free energy resources, mostly hydropower and wind2. In the European Union, wind and solar made up more than three-quarters of new energy capacity installed; coal demand was reduced by 10% (ref. 3). In the United States, almost two-thirds of the electricity-generating capacity installed by utility companies was based on renewables 
(see go.nature.com/2skv20g).

The International Energy Agency (IEA) has predicted that, by 2020, renewable sources could deliver 26–27% of the world’s electricity needs, compared with 23.7% of electric power at the end of 2015. But that underestimates the pace of change in energy systems.

Growth in electric vehicles alone could displace 2 million barrels of oil per day by 2025, according to a February report4. It suggests that, by 2050, this could reach 25 million barrels of oil per day — a stark contrast to expectations from the fossil-fuel industry that demand for oil will rise. And solar power alone could supply 29% of global electricity generation by 2050. This would remove the need for coal and leave natural gas with only a 1% market share. However, the oil firm ExxonMobil predicts that all renewables will supply just 11% of global power generation by 2040 (ref. 4).

Investors, meanwhile, are growing wary of carbon risks. BlackRock and Vanguard, the two largest fund managers, voted — along with many others — against ExxonMobil management at its annual general meeting on 31 May and instructed the company to report on the profit impact of global measures to keep climate change below 2 °C. Earlier this month, Norway’s US$960-billion sovereign-wealth fund declared that it will ask the banks in which it has invested to disclose how their lending contributes to global greenhouse-gas emissions.

Last year, the installed capacity of renewable energy set a new record of 161 gigawatts; in 2015, investment levels reached $286 billion worldwide, more than 6 times that in 2004. Over half of that investment, $156 billion, was for projects in developing and emerging economies5.

There is a strong headwind against the low-carbon transition in some places, which may impede progress. For example, the Financial CHOICE Act — a bill passed by the US House of Representatives on 8 June — would make it nearly impossible for investors to challenge companies on climate-risk disclosure through shareholder proposal processes, as at ExxonMobil. However, as the UN Secretary General, António Guterres, said in New York last month: “The sustainability train has left the station.” The fossil-free economy is already profitable6 and creating jobs (www.clean200.org). A report this year by the International Renewable Energy Agency and the IEA shows that efforts to stop climate change could boost the global economy by $19 trillion7. The IEA has also said that implementing the Paris agreement will unlock $13.5 trillion or more before 2050.

Recent geopolitical events, too, have galvanized activity in support of the Paris agreement. For example, the #WeAreStillIn campaign — involving more than 1,000 governors, mayors, businesses, investors and universities from across the United States — has declared that it will ensure the nation remains a leader in reducing carbon emissions.

Six milestones
To prioritize actions, we’ve identified milestones in six sectors. Developed with knowledge leaders, these were reviewed and refined in collaboration with analysts at Yale University, the Climate Action Tracker consortium, Carbon Tracker, the low-carbon coalition We Mean Business, the Partnership on Sustainable, Low Carbon Transport (SLoCaT), advisory firm SYSTEMIQ, the New Climate Economy project and Conservation International.

These goals may be idealistic at best, unrealistic at worst. However, we are in the age of exponential transformation and think that such a focus will unleash ingenuity. By 2020, here’s where the world needs to be:

Energy. Renewables make up at least 30% of the world’s electricity supply — up from 23.7% in 2015 (ref. 8). No coal-fired power plants are approved beyond 2020, and all existing ones are being retired.

Infrastructure. Cities and states have initiated action plans to fully decarbonize buildings and infrastructures by 2050, with funding of $300 billion annually. Cities are upgrading at least 3% of their building stock to zero- or near-zero emissions structures each year9.

Transport. Electric vehicles make up at least 15% of new car sales globally, a major increase from the almost 1% market share that battery-powered and plug-in hybrid vehicles now claim. Also required are commitments for a doubling of mass-transit utilization in cities, a 20% increase in fuel efficiencies for heavy-duty vehicles and a 20% decrease in greenhouse-gas emissions from aviation per kilometer traveled.

Land. Land-use policies are enacted that reduce forest destruction and shift to reforestation and afforestation efforts. Current net emissions from deforestation and land-use changes form about 12% of the global total. If these can be cut to zero next decade, and afforestation and reforestation can instead be used to create a carbon sink by 2030, it will help to push total net global emissions to zero, while supporting water supplies and other benefits. Sustainable agricultural practices can reduce emissions and increase CO2 sequestration in healthy, well-managed soils.

Industry. Heavy industry is developing and publishing plans for increasing efficiencies and cutting emissions, with a goal of halving emissions well before 2050. Carbon-intensive industries — such as iron and steel, cement, chemicals, and oil and gas — currently emit more than one-fifth of the world’s CO2, excluding their electricity and heat demands.

Finance. The financial sector has rethought how it deploys capital and is mobilizing at least $1 trillion a year for climate action. Most will come from the private sector. Governments, private banks and lenders such as the World Bank need to issue many more ‘green bonds’ to finance climate-mitigation efforts. This would create an annual market that, by 2020, processes more than 10 times the $81 billion of bonds issued in 2016.

Further, faster, together
If we delay, the conditions for human prosperity will be severely curtailed. There are three pressing and practical steps to avoid this.

First, use science to guide decisions and set targets. Policies and actions must be based on robust evidence. Uncensored and transparent communication of peer-reviewed science to global decision-makers is crucial. Academic journal articles are not easily read or digested by non-experts, so we need a new kind of communication in which Nature meets Harvard Business Review. Science associations should provide more media training to young scientists and hold communication boot camps on how to make climate science relevant to corporate boards and investors.

Those in power must also stand up for science. French President Emmanuel Macron’s Make Our Planet Great Again campaign is a compelling example. He has spoken out to a global audience in support of climate scientists and invited researchers to move to France to help accelerate action and deliver on the Paris agreement. To encourage others to speak, scientists should forge connections with leaders from policy, business, and civil society. The Arctic Basecamp at Davos in January, for instance, brought scientists into high-level discussions on global risk at the World Economic Forum’s annual meeting in Switzerland.

“The fossil-free economy is already profitable.”
Second, existing solutions must be scaled up rapidly. With no time to wait, all countries should adopt plans for achieving 100% renewable electricity production, while ensuring that markets can be designed to enable renewable-energy expansion.

Third, encourage optimism. Recent political events have thrown the future of our world into sharp focus. But as before Paris, we must remember that impossible is not a fact, it’s an attitude. It is crucial that success stories are shared. Demonstrating where countries and businesses have over-achieved on their targets will raise the bar for others. More-ambitious targets become easier to set.

The upcoming G20 meeting in Hamburg is the perfect moment for heads of state to integrate the six milestones into their discussions on how to ensure a resilient, prosperous, inclusive and interconnected global economy. This would pave the way for a year of raised ambition in 2018 when nations take stock of progress and revise national commitments under the Paris agreement.

The G20 is due to adopt the recommendations of the Financial Stability Board’s Task Force on Climate-related Financial Disclosures, on how the global finance system will manage the risk of climate change. It requires financial institutions to design, disclose and implement a transition strategy with a view to full decarbonization of operations, value chains and portfolios by 2050. National governments and financial regulators must enact these recommendations swiftly.

Cities and provincial governments must help to drive the ambition of national governments on climate change, particularly through smart infrastructure and transport policy. C40 Cities, a network of megacities committed to addressing climate change, has adopted a strategy called Deadline 2020 that aligns its emissions-reductions plans with the Paris agreement. Other cities now have an opportunity to follow suit, for example through the Global Covenant of Mayors for Climate and Energy.

Our co-signatory list, which includes eminent scientists, business leaders, economists, analysts, influencers and representatives of non-governmental organizations, is an example of the strength of radical collaboration across unusual partners, who all share a mission to seize this opportunity to improve people’s lives, the planet, and the global economy.
There will always be those who hide their heads in the sand and ignore the global risks of climate change. But there are many more of us committed to overcoming this inertia. Let us stay optimistic and act boldly together.

Nature 546, 593–595 (29 June 2017) doi:10.1038/546593a

References
1. Mission 2020. 2020: The Climate Turning Point (Mission 2020, 2017); available at http://go.nature.com/2takuw3
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2. International Energy Agency. World Energy Outlook 2016 (International Energy Agency, 2016).
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3. WindEurope. Wind in Power: 2016 European Statistics (WindEurope, 2017).
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4. Carbon Tracker. Expect the Unexpected (Carbon Tracker, 2017).
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5. Frankfurt School–UNEP Centre/BNEF. Global Trends in Renewable Energy Investment 2016(Frankfurt School, 2016).
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6. IRENA. Renewable Energy and Jobs: Annual Review 2017 (IRENA, 2017).
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7. IEA/IRENA. Perspectives for the Energy Transition (IEA/IRENA, 2017).
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8. REN21. Renewables 2016: Global Status Report (REN21, 2016).
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9. Climate Action Tracker. 10 Steps (Climate Action Tracker, 2016); available at http://go.nature.com/2ryh56j
Show context
Related stories and links
From nature.com
• Let Trump claim a better deal on climate
14 June 2017

• The G20 must govern the shift to low-carbon energy
07 June 2017

• Don't link carbon markets
21 March 2017

• Renewables need a grand-challenge strategy
05 October 2016

• Economics: Support low-carbon investment
04 March 2015

• Climate science: Vast costs of Arctic change
24 July 2013

From elsewhere
• Mission 2020
• G20 Germany 2017
Author information
Affiliations
. Christiana Figueres is vice-chair of the Global Covenant of Mayors for Climate and Energy, and Convener of Mission 2020.
. Hans Joachim Schellnhuber is director of the Potsdam Institute for Climate Impact Research, Germany.
. Gail Whiteman is director of the Pentland Centre for Sustainability in Business, Lancaster University, UK.
. Johan Rockström is executive director of the Stockholm Resilience Centre, Stockholm University, Sweden.
. Anthony Hobley is chief executive of Carbon Tracker, London, UK.
. Stefan Rahmstorf is head of Earth system analysis at the Potsdam Institute for Climate Impact Research, Germany.
Corresponding author
• Correspondence to: Christiana Figueres

Supplementary information
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Wednesday, June 28, 2017

2642. Carbon in Atmosphere Is Rising, Even as Emissions Stabilize

By Justin Gillis, The New York Times, June 26, 2017

CAPE GRIM, Tasmania — On the best days, the wind howling across this rugged promontory has not touched land for thousands of miles, and the arriving air seems as if it should be the cleanest in the world.

But on a cliff above the sea, inside a low-slung government building, a bank of sophisticated machines sniffs that air day and night, revealing telltale indicators of the way human activity is altering the planet on a major scale.

For more than two years, the monitoring station here, along with its counterparts across the world, has been flashing a warning: The excess carbon dioxide scorching the planet rose at the highest rate on record in 2015 and 2016. A slightly slower but still unusual rate of increase has continued into 2017.

Scientists are concerned about the cause of the rapid rises because, in one of the most hopeful signs since the global climate crisis became widely understood in the 1980s, the amount of carbon dioxide that people are pumping into the air seems to have stabilized in recent years, at least judging from the data that countries compile on their own emissions.
That raises a conundrum: If the amount of the gas that people are putting out has stopped rising, how can the amount that stays in the air be going up faster than ever? Does it mean the natural sponges that have been absorbing carbon dioxide are now changing?

“To me, it’s a warning,” said Josep G. Canadell, an Australian climate scientist who runs the Global Carbon Project, a collaboration among several countries to monitor emissions trends.
Scientists have spent decades measuring what was happening to all of the carbon dioxide that was produced when people burned coal, oil and natural gas. They established that less than half of the gas was remaining in the atmosphere and warming the planet. The rest was being absorbed by the ocean and the land surface, in roughly equal amounts.

In essence, these natural sponges were doing humanity a huge service by disposing of much of its gaseous waste. But as emissions have risen higher and higher, it has been unclear how much longer the natural sponges will be able to keep up.Should they weaken, the result would be something akin to garbage workers going on strike, but on a grand scale: The amount of carbon dioxide in the atmosphere would rise faster, speeding global warming even beyond its present rate. It is already fast enough to destabilize the weather, cause the seas to rise and threaten the polar ice sheets.The record increases of airborne carbon dioxide in 2015 and 2016 thus raise the question of whether this has now come to pass. Scientists are worried, but they are not ready to draw that conclusion, saying more time is needed to get a clear picture.

Many of them suspect an El Niño climate pattern that spanned those two years, one of the strongest on record, may have caused the faster-than-usual rise in carbon dioxide, by drying out large parts of the tropics. The drying contributed to huge fires in Indonesia in late 2015 that sent a pulse of carbon dioxide into the atmosphere. Past El Niños have also produced rapid increases in the gas, though not as large as the recent ones.

Yet scientists are not entirely certain that the El Niño was the main culprit; the idea cannot explain why a high rate of increase in carbon dioxide has continued into 2017, even though the El Niño ended early last year.

Scientists say their inability to know for certain is a reflection not just of the scientific difficulty of the problem, but also of society’s failure to invest in an adequate monitoring system to keep up with the profound changes humans are wreaking on the planet.
“It’s really bare bones, our network, contrary to common misperceptions about the government wasting money,” said Pieter Tans, chief of a unit that monitors greenhouse gases at the National Oceanic and Atmospheric Administration.

While the recent events have made the scientific need for an improved network clear, the situation may be about to get worse, not better. President Trump’s administration has targeted American science agencies for cutbacks, with NOAA, the lead agency for tracking greenhouse gases, being one of those on the chopping block.

Australia also had a recent fight over proposed cutbacks in climate science, but so far that country’s conservative government has promised continued funds for the Cape Grim science program, Australia’s most important contribution to global climate monitoring. The atmospheric observatory here, which receives some money from NASA, is one of the most advanced among scores of facilities around the world where greenhouse gases and other pollutants are monitored.

The network is complete enough to give a clear picture of the overall global trends in industrial gases in the air, scientists say. But it is too sparse to give definitive information about which parts of the planet are absorbing or releasing greenhouse gases at a given moment. Lacking such data, scientists have trouble resolving some important questions, like the reasons for the rapid increase of carbon dioxide over the past three years.
“It’s really important that people get that there’s an awful lot that’s just not known yet,” Sam Cleland, the manager of the Cape Grim station, said.

Human activity is estimated to be pumping almost 40 billion tons of carbon dioxide into the air every year, an amount that Dr. Canadell of the Global Carbon Project called “staggering.” The atmospheric concentration of the gas has risen by about 43 percent since the Industrial Revolution.

That, in turn, has warmed the Earth by around 2 degrees Fahrenheit, a large number for the surface of an entire planet.

With a better monitoring network, scientists say they might be able to specify in greater detail what is causing variations in the amount of carbon dioxide staying in the air — and, perhaps, to give a timely warning if they detect a permanent shift in the ability of the natural sponges to absorb more.

Dr. Tans of NOAA would like to put sensors on perhaps a hundred commercial airplanes to get a clearer picture of what is happening just above land in the United States. The effort would cost some $20 million a year, but the government has not financed the project.
The uncertainty stemming from the recent increases in carbon dioxide is all the more acute given that global emissions from human activity seem to have stabilized over the past three years. That is primarily because of changes in China, the largest polluter, where an economic slowdown has coincided with a conscious effort to cut emissions.

“I’d estimate that we are about at the emissions peak, or if there are further rises, they won’t be much,” said Wang Yi, a professor at the Chinese Academy of Sciences in Beijing, who also belongs to the national legislature and advises the government on climate policy.

Emissions in the United States, the second-largest polluter after China, have also been relatively flat, but Mr. Trump has started tearing up President Barack Obama’s climate policies, raising the possibility that greenhouse gases could rise in coming years.
Dr. Tans said that if global emissions flattened out at today’s high level, the world would still be in grave trouble.

 “If emissions were to stay flat for the next two decades, which could be called an achievement in some sense, it’s terrible for the climate problem,” he said.
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2641. California Adds Glyphosate to Cancer Watchlist


By Jake Johnson, Common Dreams, June 27, 2017

In a move celebrated by scientists and activists, California on Monday announced it would add glyphosate—the active ingredient in the Monsanto-produced weed killer Roundup—to its list of chemicals known to cause cancer.

The decision, made by California's Office of Environmental Health Hazard Assessment (OEHHA), was reportedly precipitated by the World Health Organization's classification of glyphosate as a "probable carcinogen" in May of 2015.

"California is required under the Safe Drinking Water and Toxic Enforcement Act of 1986, better known as Proposition 65, to publish a regularly updated list of chemicals thought to cause cancer or birth defects," Newsweek reported.

Monsanto has been quick to respond to the move; as USA Today's Emily Bohatch noted, Monsanto is appealing a ruling on a case it brought against California last year, when the OEHHA first attempted to add glyphosate to its list of cancer-causing agents. 

In response to the agrochemical giant's legal maneuvering, activists and scientists have insisted that Monsanto's motive is profit alone—not scientific accuracy or the health of the public—and hailed California's decision as a step in the direction of justice.

The Center for Biological Diversity (CBD) noted in a press release on Monday that a recent analysis "found more than half of the glyphosate sprayed in California is applied in the state's eight most impoverished counties."

"The analysis also found that the populations in these counties are predominantly Hispanic or Latino," CBD continued, "indicating that glyphosate use in California is distributed unequally along both socioeconomic and racial lines."

Nathan Donley, a former cancer researcher and a senior scientist at the CBD, called California's move "remarkable" and congratulated the state for standing up to "special-interest politics [that] hamstring our federal government from taking action to protect people from this dangerous pesticide."

"California's decision makes it the national leader in protecting people from cancer-causing pesticides," Donley concluded. "The U.S. EPA now needs to step up and acknowledge that the world's most transparent and science-based assessment has linked glyphosate to cancer."
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Tuesday, June 27, 2017

2640. The Energy Expansions of Evolution



By Olivia P. Judson, Nature, April 28, 2017

Abstract

The history of the life–Earth system can be divided into five ‘energetic’ epochs, each featuring the evolution of life forms that can exploit a new source of energy. These sources are: geochemical energy, sunlight, oxygen, flesh and fire. The first two were present at the start, but oxygen, flesh and fire are all consequences of evolutionary events. Since no category of energy source has disappeared, this has, over time, resulted in an expanding realm of the sources of energy available to living organisms and a concomitant increase in the diversity and complexity of ecosystems. These energy expansions have also mediated the transformation of key aspects of the planetary environment, which have in turn mediated the future course of evolutionary change. Using energy as a lens thus illuminates patterns in the entwined histories of life and Earth, and may also provide a framework for considering the potential trajectories of life–planet systems elsewhere.

Free energy is a universal requirement for life. It drives mechanical motion and chemical reactions—which in biology can change a cell or an organism1,2. Over the course of Earth history, the harnessing of free energy by organisms has had a dramatic impact on the planetary environment3, 4, 5, 6, 7. Yet the variety of free-energy sources available to living organisms has expanded over time. These expansions are consequences of events in the evolution of life, and they have mediated the transformation of the planet from an anoxic world that could support only microbial life, to one that boasts the rich geology and diversity of life present today. Here, I review these energy expansions, discuss how they map onto the biological and geological development of Earth, and consider what this could mean for the trajectories of life–planet systems elsewhere.


In the beginning

From the time Earth formed, around 4.56 billion years ago (Ga), two sources of energy were potentially available to living organisms: geochemical energy and sunlight. Sunlight is a consequence of the planet's position in the Solar System, whereas geochemical energy is an intrinsic property of the Earth. Geochemical energy arises when water reacts with basalts and other rocks8, 9, 10. These water–rock reactions—which continue today11—generate reduced compounds such as hydrogen, hydrogen sulfide, and methane8, 9, 10. Oxidation of these compounds releases energy, which organisms can capture and store in the form of chemical bonds. Although sources of geochemical energy can be at or near Earth's surface, they need not be: many are deep within the planet, out of reach of sunlight.
Assuming that life did not parachute in, fully formed, from elsewhere, a number of authors12, 13, 14, 15 have argued that the transition from non-life to life took place in the context of geochemical energy, with the ability to harness sunlight evolving later (Fig. 1). Consistent with this, both phylogenetic16 and biochemical13,17 evidence suggest that the earliest life forms were chemoautotrophs, perhaps living by reacting hydrogen with carbon dioxide and giving off acetate, methane and water13,16. Mounting evidence18, 19, 20, 21, 22 suggests that the transition from non-life to life may have taken place before 3.7 Ga—a time from which few rocks remain23.


Figure 1: Key events during the energy expansions of evolution.
Figure 1
(i) Life emerges; epoch of geochemistry begins. (ii) Anoxygenic photosynthesis: start of energy epoch 2, sunlight. (iii) Emergence of cyanobacteria. (iv) Great Oxidation Event: energy epoch 3, oxygen. (v) Probable eukaryotic fossils appear. (vi) Fossils of red algae appear. (vii) Start of energy epoch 4, flesh. (viii) Vascular plants colonize land; fire appears on Earth. Finally, the burning logs indicate the start of energy epoch 5, fire. The dates of (i)–(iii) are highly uncertain. For (i) I have taken the earliest date for which there is evidence consistent with life20. For (ii) I have taken the earliest date for which there is evidence consistent with photosynthesis18,19,21. For (iii), I have marked the date currently supported by fossil evidence for the presence of cyanobacteria (see main text, ‘Cyanobacteria and the oxygenation of the air’). Tick marks represent intervals of 25 million years. Figure drawn by F. Zsolnai.
Full size image

Energy epoch one: geochemical energy

Analysis of biochemical pathways suggests that, under favourable environmental conditions, early autotrophs could readily have adopted a heterotrophic lifestyle, feeding on the contents of dead cells24. At this time in Earth history, oxygen was at trace levels25, so the first ecosystems would have been anaerobic.
Early ecosystems may have quickly diversified to take the form of a microbial mat, where the waste products of one group of life forms feed the metabolism of another26,27. Such an arrangement generates layered communities of organisms, each layer having a different metabolic speciality28,29. In anaerobic ecosystems of this type, mobile predation is essentially nonexistent: growth rates are so low that hunting and consuming other organisms doesn't yield enough energy30. Viruses, however, are likely to have been an important force from early in the history of life31. They act as agents of death—and by lysing cells, they would have provided additional sources of organic carbon to heterotrophs. Viruses also transport genes from one host to another, and thus may have enabled the spread of evolutionary innovations. Many of the coevolutionary selection pressures of the modern biosphere would have been minimal (for example, predation and the opportunity to live inside other organisms) or absent (for example, sexual selection).
The niches available would have been those near sources of geochemical energy, suggesting a patchy, local distribution of life. Consistent with this, geochemical models32, 33, 34 suggest that the productivity of the biosphere before it was powered by the sun would have been at least a thousand times less than it is today, and may have been one million times less.
Owing to the scarcity of rocks from Earth's remote past, the impact of early life on the planetary environment is also hard to assess. Life inevitably creates a suite of changes in its environment (Box 1), and the establishment of life would have initiated biogeochemical cycling, but owing to the low productivity of the biosphere, the initial effects are likely to have been small32, 33, 34.


Energy epoch two: sunlight

At some point early in the history of the Earth—perhaps by 3.7 Ga18,19,21(Fig. 1)—organisms evolved to harness the energy in sunlight to drive chemical reactions. Today, several groups of bacteria engage in photosynthesis, using a variety of different pathways35. One pathway, oxygenic photosynthesis, gives off oxygen as a by-product; the others, all forms of anoxygenic photosynthesis, do not. Genetic35, fossil36, and biochemical37 evidence all suggest that of the two, anoxygenic photosynthesis evolved first.
Because sunlight is abundant across the planet's surface, the ability to use it made far more of the planet available to life. Consistent with this, models of the early Earth suggest that the advent of anoxygenic photosynthesis greatly increased the productivity of early ecosystems32,33. At the same time, microbial ecosystems were able to become more diverse. Forms of photoheterotrophy38 may also have begun to evolve. This lifestyle does not involve fixing carbon—organisms still require a source of organic carbon—but does involve transducing sunshine into ATP, which reduces energy needs from other sources.
During this epoch, the impact of life on the planetary environment expanded too. Structures such as stromatolites39 and banded iron formations19 began to appear, and methane may have started to build up in the atmosphere40. Indeed, the climate of the early Earth appears to have been temperate41 despite the fact that, back then, the sun had only about 70% of its current brightness42. Methane, along with ethane, which can be produced from methane by photochemical reactions in the atmosphere43, are greenhouse gases: thus, methane production on the part of living organisms may have helped to keep the early Earth from freezing25,43.
But the crucial event of this period—the one that would go on to have by far the most biological and geological impact—was the evolution of oxygenic photosynthesis, an innovation that appeared in just one phylum, the cyanobacteria.
Cyanobacteria and the oxygenation of the air. In the absence of a biotic source of oxygen, trace quantities of the gas can be generated abiotically: water molecules can be split by sunlight44 or radioactive decay45. However, these abiotic processes are much less efficient than their biotic equivalent34,44. Had cyanobacteria, or something like them, never evolved, oxygen would never have built up in the atmosphere of the Earth.
But build up it did. Between 2.45 and 2.32 Ga (ref. 46), significant quantities of oxygen began to accumulate in the air, an episode known as the Great Oxidation Event. Before the Great Oxidation, atmospheric oxygen levels were less than 10−5 of the present atmospheric level of ∼21%. By ∼2 Ga, they had risen to perhaps 0.1–1% of the present atmospheric level25. Although the subsequent history of oxygen is complex and many details are uncertain47,48, Earth's atmosphere has contained an appreciable level of the gas ever since. (Full oxygenation of the oceans, however, would not happen until around 1.8 billion years after the Great Oxidation47.)
Of all the events in the early history of the Earth, the Great Oxidation is the least controversial. It marks a line across the history of the planet, with a suite of geological markers showing a shift in the prevailing chemistry44,49. In contrast, there is enormous uncertainty about when cyanobacteria first evolved, with estimates spanning a period of one billion years35,47. However, genetic50, fossil51, and geochemical47,52evidence all suggest that cyanobacteria evolved at least 300 million years before the Great Oxidation Event.
But if cyanobacteria evolved hundreds of millions of years before the Great Oxidation, why did oxygen take so long to accumulate? This question has been studied extensively, and various hypotheses have been put forward (for a review see refs 25,53). In essence, though, it's a matter of planetary chemistry. Both the atmosphere and ocean of the early Earth were full of molecules such as hydrogen, methane and ferrous iron that oxygen reacts with; oxygen may thus have been removed as fast as it was produced25,54. Until sources of oxygen began to exceed the sinks, the gas would have been unable to accumulate25.
Even before the Great Oxidation, the emergence of cyanobacteria would have increased both the productivity and complexity of microbial ecosystems. As well as a variety of heterotrophs, modern microbial mats and stromatolites often contain photosynthetic organisms of several different types55. Moreover, in evolving to extract electrons from the hydrogen in water, rather than from substances such as ferrous iron or hydrogen sulfide, cyanobacteria would have been far less constrained in the habitats they could occupy. Cyanobacteria may even have been among the first organisms to colonize land surfaces56, increasing the weathering of rocks, and thus the flow of nutrients into the oceans57. But these impacts are dwarfed by those that resulted from the accumulation of oxygen in the air.
Oxygen and the planetary environment. The Great Oxidation Event had a dramatic impact on the planetary environment. First, the transition to an oxygen-rich atmosphere took place in tandem with the establishment of the ozone layer54,58,59, thus changing the physical context in which organisms, especially those on land, evolve. Second, the diversity of minerals at the Earth's surface began to increase60, eventually more than doubling61.
Third, the appearance of atmospheric oxygen created a variety of new abiotic niches. As well as the anoxic and micro-oxic niches that had existed from the outset, the oxygenation of the atmosphere created an abundance of oxygen-rich niches, too. Today, aerobic prokaryotes show an enhanced ability to tolerate extremes of salinity and pH compared to their anaerobic counterparts62, suggesting that the availability of oxygen might also have allowed for the colonization of other, previously inaccessible, abiotic niches. At the same time, the availability of oxygen would have increased the availabilities of oxidants such as nitrate and sulfate—and thus would also have increased the productivity of chemotrophic life forms.
Fourth, the Great Oxidation seems to have coincided with a series of extreme ice ages63. The reasons for this are unresolved63, but some authors25,64,65 have suggested it could have been due to a decline in the flux of biogenic methane reaching the atmosphere, and a corresponding decline in the contribution of methane and its byproducts to keeping the climate warm.
But the most significant environmental impact of the Great Oxidation was a change in the prevailing chemistry, and the ready availability of oxygen gas as a source of energy for living organisms.

Energy epoch three: oxygen

Oxygen is a rich source of energy: the use of oxygen as an electron acceptor releases more energy per electron transfer than that of any other element except for chlorine and fluorine66. (Neither chlorine nor fluorine is cosmically abundant, however, and both are so reactive as to be an unlikely foundation for any kind of biology66.) The diversification of the biosphere that would ultimately take place was, to a large extent, enabled by the growing abundance of oxygen.
The emergence of the ability of living organisms to use oxygen as an energy source is shrouded in at least as much mystery as the emergence of cyanobacteria. At issue is whether early life forms could have evolved to use trace oxygen or hydrogen peroxide produced through abiotic processes67—and thus whether aerobic respiration originated before the advent of cyanobacteria, or whether it evolved in conjunction with them. Whatever the case, long before the Great Oxidation Event, aerobic organisms, if they existed, could have prospered in oxygen-rich oases generated by cyanobacteria68,69.
As well as being a source of energy, oxygen is both a biological problem and an opportunity. Problem: the presence of oxygen inactivates some enzymes, and oxygen derivatives such as hydrogen peroxide and the superoxide ion are reactive compounds that damage both DNA and proteins70,71. To survive in the presence of oxygen, organisms need a superstructure of protective enzymes. Opportunity: the availability of oxygen permits the construction of new molecules, such as collagen72.
During this epoch, two momentous events took place: the emergence of eukaryotes and the emergence of the lineage that would eventually produce land plants. Both events represent fusions between two previously independent lineages, an archaeon and an alphaproteobacterium in the case of eukaryotes73,74, and a eukaryote and a cyanobacterium in the case of the plant lineage75; the alphaproteobacterium evolved to become the mitochondrion, the cyanobacterium, the chloroplast. Both events thus also represent important shifts in the capacity for organisms to transduce energy. Fossils of red algae show that both events had taken place by ∼1.2 Ga (ref. 76), and microfossils that are probably eukaryotic in origin date to 1.8 Ga (ref. 77).
In extant eukaryotes, organelles of mitochondrial origin take several different, but related, forms78. Notably, only one—the ‘standard’ mitochondrion found, for example, in humans—requires oxygen. Three others are involved in forms of anaerobic metabolism; of these, two produce hydrogen. These observations fit with the hypothesis, advanced by Martin and colleagues74,79, that the ancestral eukaryote resulted from a prior symbiotic association between a hydrogen-dependent archaeon and a metabolically flexible alphaproteobacterium that, in the absence of oxygen, lived anaerobically producing hydrogen, and in the presence of oxygen, lived aerobically. If this hypothesis is correct, the ancestral eukaryote could have been a facultative anaerobe, able to live in both oxic and anoxic environments. Such a scenario not only accounts for the different types of mitochondria seen in extant eukaryotes78, but also for the fact that, today, species with mitochondria that produce ATP through anaerobic pathways are sprinkled across the eukaryotic tree while exhibiting a similar underlying biochemistry78,80.
Eukaryotes differ from prokaryotes in many respects, from meiosis and syngamy to the presence of a cell nucleus, as well as a suite of other features. In addition, complex multicellularity and large size has evolved only in eukaryotes—which Lane and Martin81 have attributed to an enhanced capacity to generate energy owing to the possession of mitochondria (Box 2). From the point of view of the biosphere, the emergence and diversification of eukaryotes provided a new set of niches for prokaryotes to occupy—which in turn allowed eukaryotes to occupy a far wider variety of niches. Today, most, perhaps all, eukaryotes have symbiotic dependencies on consortia of prokaryotes—microbiomes—that give them access to a greater variety of energy sources and metabolic capabilities82.



For the purposes of this Perspective, however, one feature of eukaryotes is particularly important. This is the ability to engage in phagocytosis—the engulfment of particles and, sometimes, other life forms. The wholesale engulfment of other beings appears to be a eukaryotic invention83, and it whets the appetite for:

Energy epoch four: flesh

Around 575 million years ago (Ma), during the Ediacaran Period, a new form of life began to become abundant: animals84. And with animals would soon come a powerful new force of nature: the acquisition of energy through the active hunting and eating of other life forms, especially, other animals. This would produce a radical shift that, within a mere 40 million years, transformed the Earth. Before this epoch, ecosystems were microbial. The advent of widespread flesh-eating launched the Phanerozoic, triggering an enormous increase in organism size85, a new tempo of macroevolutionary change86,87, new kinds of ecosystems86, 87, 88, and an increased impact of life on the fabric of the planet87.
As in the case of oxygen, however, flesh-eating has a prehistory. Predation by single-celled eukaryotes may have caused the evolution of the first armoured algae, around 770 Ma89, 90, 91, as well as a major increase in eukaryotic diversity92. Moreover, animals represent one of several transitions to complex multicellular life93—transitions that Stanley86 suggested might, in part, have resulted from single-celled eukaryotes engulfing and consuming each other. Indeed, molecular clocks show that the first animals also evolved around this time94,95(Box 3), leading Knoll and Lahr92 to propose that tiny animals might have helped drive the diversification of eukaryotic protists.



Today, animals influence diversity at all levels of an ecosystem, with grazers such as slugs96 or zooplankton97 maintaining the diversity of plants or phytoplankton, and carnivores such as wolves98 maintaining the diversity of plants through their predation on herbivores. This kind of ecology—complex food webs with many types of eaters—was absent from Earth until around 550 Ma, when the first animals that eat animals evolved. Their appearance seems to have triggered the rapid diversification of animal life sometimes referred to as the Cambrian Explosion.
In addition to their effects on the structure of ecosystems, the flourishing of flesh-eating animals heralded a step-change in both biomass and biodiversity87. In the oceans today, for example, Butterfield87 has estimated that animals may comprise as much as 80% of the biomass in the pelagic zone. Furthermore, with the evolution of animals, new coevolutionary selection pressures—in particular, arms races between the eaters and the eaten—appeared, accelerating the pace of macroevolution99. At the same time, animal guts and external surfaces provided new niches for other life forms, both symbiotic and hostile.
On the geological side, the flourishing of animals had at least four major impacts. First, the evolution of predation rapidly led to the evolution of armour—shells, scales, spikes and carapaces built from materials such as calcite and silica100. Although, as noted above, the first protective coverings (on algae) date back to around 770 Ma (ref. 90), it's not until the evolution of flesh-eating animals that shells and other forms of protection became widespread. This development would eventually result in vast deposits of materials such as radiolarite101, limestone102, coquina103 and chalk104 and would also produce changes in ocean chemistry, as organisms removed dissolved materials such as silica and calcium and used it for themselves105,106.
Second, animals produce faeces, which have important effects on the way that nutrients are distributed around the globe. For example, in the ocean, zooplankton faecal pellets sink more rapidly than individual algal or bacterial cells, and thus transport organic matter from the surface to the seabed107. Today, the faeces of sperm whales bring iron from the deep sea to the ocean surface108; the faeces of birds like cormorants transport nutrients from the ocean onto land, sometimes in fantastic quantities109.
A third geological impact of animals is caused by their ability to burrow. Simple, horizontal burrows appear in the fossil record around 555 Ma (ref. 110); by the early Cambrian, the abundance, size, depth and complexity of burrows had increased considerably110. Widespread burrowing creates a mixing of sediments known as bioturbation. As Darwin111 observed with respect to earthworms, burrowing is analogous to ploughing: it redistributes nutrients as well as sifting, irrigating, and aerating sediments and soils.
Finally, from bioturbation, faeces, and the evolution of armour, a fourth major impact of flesh-eating life forms emerges: a reorganization of Earth's biogeochemical cycles105,112, 113, 114.

Energy epoch five: fire

Of all the planets and moons in the Solar System, Earth is the only one to have fire. This is because, to have fire, all of three conditions must be met. (1) Fire needs a source of ignition—such as lightning strikes. Throughout Earth history, these have been abundant; today, there are more than 1.4 billion lightning strikes per year (ref. 115), of which an appreciable number ignite wildfires116. Lightning occurs on other planets117, but none of these meets the other two conditions. (2) Fire needs oxygen. Assuming current atmospheric pressure, Earth's air must contain at least 16% of the gas118,119. For most of Earth's history, oxygen levels have been lower than this threshold. (3) Fire needs fuel. So it is not until the evolution of vascular plants on land, around 420 Ma, that all three conditions were met120.
From the start, fire has had both geological and biological impacts. Fire regimes drive the evolution of plant traits121; fires affect soils and air quality; and although, each year, a significant amount of biomass goes up in smoke, fire can promote biodiversity122. Fire may even have driven the initial spread of flowering plants123—an event that led to radiations of many other groups, including ants124, bees125 and mammals126. Furthermore, fire contributes new material to the Earth—charcoal, ash and soot—and may also act as a control on planetary oxygen levels127. But as an energy source, per se? That's a more recent development, and has come in two phases.
The first phase began with the evolution of a fire creature. This creature—a member of the genus Homo128—began to control the use of fire, deliberately setting fires alight and using fire for cooking. Exactly when cooking began remains controversial, with possible dates ranging from 1.5 Ma to 0.4 Ma (ref. 129). The important point, though, is that cooking is a kind of predigestion: cooked food, be it meat130, vegetable130 or lipid131, delivers more energy than the same food eaten raw. In using fire to cook food, hominins thus developed a way to extract more energy from their diets, and to eat a wider variety of food.
The second phase of fire as an energy source is even more recent—but the onset is nonetheless difficult to pinpoint. Does it start with the use of fire to manufacture labour-saving tools? With the smelting of iron, something otherwise energetically impossible? With the burning of fossil fuels such as coal to generate heat and light? With the invention of the internal combustion engine? Or with the discovery of the Haber–Bosch process for fixing nitrogen—which, in 1925, Alfred Lotka132described as the start of “a new cosmic epoch”? Perhaps these last three are the most important contenders, as together, they have transformed the planet7. In particular, the human input of energy to manufacture and deliver an otherwise limiting nutrient has produced far higher crop yields, enormously larger human populations, and gigantic populations of human-associated animals such as pigs, cows, horses and chickens133. Erisman and colleagues134 estimate that between 1908 and 2008, industrially produced nitrogen fertilizer supported an additional four billion people and that by 2008, nitrogen fertilizers were responsible for feeding 48% of the human population. Meanwhile, Pimm and colleagues135 judge that extinction rates are now 1,000 times greater than the typical background rate. In sum, in this epoch of fire, total biomass has remained high, but biodiversity has begun to fall.
The geological impacts of the age of fire are also poised to be dramatic, with rising levels of carbon dioxide and other greenhouse gases in the air, rising sea levels, increasing levels of nitrogen and plastic pollution, a remaking of the landscape with mines, tunnels, dams and cities, the introduction of new chemical compounds, and massive shifts in several biogeochemical cycles. However, the full geological effects of this epoch are, as yet, unknown.

Implications

Different schemata for considering the history of life allow different types of insights. For example, de Duve136 identified a series of (mostly) biochemical events that happened just once, and discussed to what extent they would be likely to happen again were the tape of life to be replayed. Knoll and Bambach137 put forward six ‘megatrajectories’ in the history of life, where each megatrajectory corresponds to the ecological diversification of a new type of life form (prokaryotes, unicellular eukaryotes, land plants, etc), thus linking evolutionary change with ecological complexity. And famously, Maynard Smith and Szathmáry138,139 proposed a framework based on transitions between different replicating units (genes, chromosomes, individuals, and so on); this has been profoundly helpful in generating a deeper understanding of the levels at which natural selection operates140.
In recent work, Lenton and colleagues7 developed a schema for thinking about ‘revolutions’ in the history of life and Earth. As in the Perspective presented here, their focus is energy. But rather than considering expansions in the types of energy underpinning the biosphere, the authors examined a series of changes in free energy inputs and how these have altered global material cycles. On the basis of their analyses, they conclude that human sustainability will not only require a shift from fossil fuel to solar power, but also a far more active effort to recycle materials such as metals.
Here, I have taken a more bottom-up approach. In considering expansions in the types of energy underpinning the biosphere, I have sought to describe the step-wise construction of a life–planet system. Using energy expansions as the lens reveals a fundamental, recursive interplay between events in the evolution of life and the development of the planetary environment. From this viewpoint, a number of insights emerge.
First, increasing the types of energy sources available to life has led to a far more complex biosphere. Although only geochemical energy and sunlight can power the de novo transformation of inorganic carbon into living tissue, the complexity of the current biosphere rests on multiple levels of energy use. Cyanobacteria, for instance, often require the presence of non-light-using consort organisms in order to grow well141, 142, 143. Conversely, owing to the metabolic capacities of their prokaryotic symbionts and endosymbionts, eukaryotes are able to live in a far wider range of environments than they could otherwise access82. The step-wise diversification of the biosphere has, in turn, led to an expansion of possible niches, from more complex microbial mats to old shells and abandoned burrows. At the same time, the capacity of life to impact the planetary environment—and thereby the environment in which future life will evolve—has expanded dramatically with each epoch.
Because the construction of the biosphere has depended on these energy expansions, the vanishing of an energy source, even temporarily, could cause a corresponding contraction in the biosphere. In the context of the Phanerozoic, some authors have attributed large-scale patterns of both biospheric expansion and contraction to corresponding fluctuations in oxygen availability, with expanding ocean anoxia corresponding to mass extinction events (end-Permian144,145; end Triassic146). Likewise, Krin147 has suggested that one factor in the mass extinction at the end of the Cretaceous may have been dust ejected by the Chicxulub asteroid impact, which may have blocked out the sun long enough to cause a global collapse in photosynthesis. Quantifying this pattern further would be an interesting line for future research.
A related avenue for future research would be an examination of macroevolutionary trends of energy use. For example, Vermeij148 argued that the Phanerozoic has been characterized by the repeated replacement of low-energy life forms by those able to harness larger amounts of energy. Among the trends he identified were endotherms tending to replace ectotherms, and angiosperms tending to replace gymnosperms. (The lower-energy form does not always become extinct; sometimes its range is just restricted to a low-energy environment.) Investigating this trend for earlier epochs—or even applying it to human societies149—might be enlightening.
A second insight that emerges from this Perspective is that the two clear inflection points in the history of Earth—the Great Oxidation Event and the emergence of mobile animals—also coincide with expansions in the kinds of energy sources available to, and consumed by, living beings. The Great Oxidation shifted the prevailing chemistry of the atmosphere and upper ocean and made oxygen gas abundant. The emergence of life forms that eat one another transformed the nature of ecosystems, and introduced a powerful new set of evolutionary interactions, thus accelerating the pace of macroevolutionary change. From this point of view, the familiar observation that Earthly life is powered by the sun takes on a more nuanced aspect: the modern biosphere is powered not merely by sunshine but by the oxygen that results from using sunshine in a particular way.
This Perspective further suggests that, through the harnessing of fire as a source of energy, Earth has now arrived at a new inflection point. Considering life–Earth history through the lens of energy expansions supports the view that the Anthropocene is a genuinely novel phase of the planet's geological and biological development—a conclusion independently reached by Lenton and colleagues7. The technology of fire may also, perhaps, mark an inflection point for the Solar System and beyond. Spacecraft from Earth may, intentionally or not, take Earthly life to other celestial objects (though whether any Earthly life forms can thrive elsewhere remains unknown).
As this is the only life–planet system we currently know of, it is impossible to know how representative it is of life–planet systems in general. But if the development of other life–planet systems requires a similar series of energy expansions, the framework presented here suggests a way to anticipate the paths that such systems might take. For instance, if a planet has only geochemical energy—perhaps because it is far from its star, or because it is a nomad150,151 and has no star at all—any life present may have “a limited future in terms of the heights it could achieve”152. Or suppose a planet is unable to accumulate oxygen. This could happen if living organisms never evolve a way of splitting water to produce the gas in the first place6,153; but even if they do, the planet itself may have characteristics that prevent oxygen from ever building up6,66. Without oxygen, the geological, ecological and evolutionary potential of a life–planet system is likely to be constrained, even if life forms analogous to eukaryotes in their energy-harnessing power (Box 2) were to evolve. Conversely, some planets might be able to accumulate new forms of energy, and life forms able to take advantage of them, much faster than Earth has66.
In short, this Perspective of energy expansions suggests that the likely development of a life–planet system will depend on the interplay between the planet's cosmic situation, its intrinsic properties, and the paths that evolving life can potentially take. The example of this life–planet system suggests that the development of a flourishing, complex biosphere depends on a virtuous circle between evolving life forms and transformations of their planetary home.

Additional information

How to cite this article: Judson, O. P. The energy expansions of evolution. Nat. Ecol. Evol. 1, 0138 (2017).
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Acknowledgements

Many thanks to G. Carr, T. Carvalho, D. C. Catling, D. Haydon, T. Goldberg, P. Jarne, A. H. Knoll, E. Kroll, N. Judson, N. Lane, T. Lenormand, G. Lichfield, B. C. T. Mason, O. Morton, J. Rolff, J. Swire, and especially A. Courtiol for helpful discussions and for comments on an earlier draft of the manuscript. Many thanks to W. F. Martin and T. M. Lenton for insightful reviews that improved the manuscript. Figure 1was drawn by graphic designer F. Zsolnai, many thanks.


Figure 1: Key events during the energy expansions of evolution.
Figure 1
(i) Life emerges; epoch of geochemistry begins. (ii) Anoxygenic photosynthesis: start of energy epoch 2, sunlight. (iii) Emergence of cyanobacteria. (iv) Great Oxidation Event: energy epoch 3, oxygen. (v) Probable eukaryotic fossils appear. (vi) Fossils of red algae appear. (vii) Start of energy epoch 4, flesh. (viii) Vascular plants colonize land; fire appears on Earth. Finally, the burning logs indicate the start of energy epoch 5, fire. The dates of (i)–(iii) are highly uncertain. For (i) I have taken the earliest date for which there is evidence consistent with life20. For (ii) I have taken the earliest date for which there is evidence consistent with photosynthesis18,19,21. For (iii), I have marked the date currently supported by fossil evidence for the presence of cyanobacteria (see main text, ‘Cyanobacteria and the oxygenation of the air’). Tick marks represent intervals of 25 million years. Figure drawn by F. Zsolnai.
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