Origins of a habitable planet

Tim Lenton University of Exeter

How has Earth remained habitable?

• Pure luck: Life on Earth has survived by chance alone

• Lucky Gaia: Earth regulates in a habitable state by chance

• Probable Gaia: The presence of life makes regulation in a habitable state a more probable outcome

Habitable extra-solar planets

Silicate weathering feedback

CO2Silicate

WeatheringRate

Global temperature

-

Walker, Hays and Kasting (1981)

+

+

Solarluminosity

+

How do you detect life on a planet?

Free energy

Matter Low internalentropy

Life

High entropywaste products

Environment

E. Schrodinger (1944) What is life?

How do you detect life on a planet?

Free energy

Matter Low internalentropy

Life

High entropywaste products

Thermodynamic Disequilibrium

Entropy/Free energyGradient

Environment

J. E. Lovelock (1965) Nature 207: 568-570E. Schrodinger (1944) What is life?

How do you detect life on a planet?

Free energy

Matter Low internalentropy

Life

High entropywaste products

Thermodynamic Disequilibrium

Entropy/Free energyGradient

Environment

Observation

J. E. Lovelock (1965) Nature 207: 568-570E. Schrodinger (1944) What is life?

Atmospheric compositions

10-1

10-2

10-3

10-4

10-5

10-6

10-7 0.0000001

0.000001

0.00001

0.0001

0.001

0.01

0.1

1

Earth Mars Venus

Mix

ing

Rat

io

HydrogenMethaneNitrogenNitrous OxideCarbon MonoxideCarbon DioxideOxygen

Fluxes of gases

0.01

0.1

1

10

100

1000

10000

Earth Without Life

Surfa

ce F

lux

(1012

mol

es y

r-1)

HydrogenMethaneIsopreneDimethyl SulphideAmmoniaNitrogenNitrous OxideCarbon MonoxideCarbon DioxideOxygen

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

-4.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5Time (Gyr from present)

Sola

r lu

min

osity

(nor

mal

ised

)The faint young Sun puzzle

A. I. Boothroyd (1992)Photo credit: ESA-NASA

Formula for luminosity:

S(t) = S0 (1 – 0.38t/τ0)-1

S0 = 1368 W m-2

Time t expressed in Gyr from the present- 4.55 < t < 4.77 Gyrτ0 = 4.55 Gyr

Environmental regulation?

Environmental regulation?

Organisms contribute to self-regulating feedback mechanisms that have kept the surface of the Earth habitable for life.“Symbiosis as seen from space”.

Environmental regulation?

Organisms contribute to self-regulating feedback mechanisms that have kept the surface of the Earth habitable for life.“Symbiosis as seen from space”.

The Earth is not a unit of selection! Why should the organisms that leave the most descendants be ones that contribute to regulating their planetary environment?

Origin of the Earth (4.6 Ga)

The Hadean (4.6-3.8 Ga)

The Archean (3.8-2.5 Ga)

Origin of life (3.8-3.3 Ga)

3.5 Ga stromatolite from S. Africa

Dividing cells (3.26 Ga)

Thanks to Andy Knoll (Harvard) for this photograph

1s1h1yr103yr106yr109yrTimescale

Spatialscale

1mm1m103m10

6 m109m101

7 m102

0 m102

5 m

1s1h1yr103yr106yr109yrTimescale

Spatialscale

1mm1m103m10

6 m109m101

7 m102

0 m102

5 m

Oceancirculation

Rockweathering

Glaciation Platetectonics

Soilerosion

Abiotic processes

1s1h1yr103yr106yr109yrTimescale

Spatialscale

1mm1m103m10

6 m109m101

7 m102

0 m102

5 m

Oceancirculation

Rockweathering

Glaciation PlatetectonicsMarineNcycle

Individualhomeostasis

Aridlandpatterning

Soilerosion

Desertification

CO2 /CaSiOcycle

Feedbacks

1s1h1yr103yr106yr109yrTimescale

Spatialscale

1mm1m103m10

6 m109m101

7 m102

0 m102

5 m

Individualselection

Oceancirculation

Rockweathering

Glaciation PlatetectonicsMarineNcycle

Individualhomeostasis

Aridlandpatterning

Soilerosion

Desertification

CO2 /CaSiOcycle

Selection mechanisms

Emergence of nutrient recyclingThe ‘Flask’ model

Williams & Lenton (2007) Oikos 116: 1087-1105

Emergence of nutrient recyclingThe ‘Flask’ model

Nutrient input

Nutrient output

Williams & Lenton (2007) Oikos 116: 1087-1105

Emergence of nutrient recyclingThe ‘Flask’ model

Nutrient input

Nutrient output

Abiotic variables

Williams & Lenton (2007) Oikos 116: 1087-1105

Emergence of nutrient recyclingThe ‘Flask’ model

Nutrient input

Nutrient output

Seeded with clonal population of microbes

Abiotic variables

Williams & Lenton (2007) Oikos 116: 1087-1105

Emergence of nutrient recyclingThe ‘Flask’ model

Nutrient input

Nutrient output

Population diversifies

Abiotic variables

Rec

yclin

g R

atio

Time

Williams & Lenton (2007) Oikos 116: 1087-1105

Emergence of nutrient recyclingThe ‘Flask’ model

Nutrient input

Nutrient output

Recycling population expands

Abiotic variables

Rec

yclin

g R

atio

Po

pula

tion

Time

Williams & Lenton (2007) Oikos 116: 1087-1105

1s1h1yr103yr106yr109yrTimescale

Spatialscale

1mm1m103m10

6 m109m101

7 m102

0 m102

5 m

Individualselection

Groupselection

Oceancirculation

Rockweathering

Glaciation PlatetectonicsMarineNcycle

Individualhomeostasis

Aridlandpatterning

Soilerosion

Desertification

CO2 /CaSiOcycle

Selection mechanisms

1s1h1yr103yr106yr109yrTimescale

Spatialscale

1mm1m103m10

6 m109m101

7 m102

0 m102

5 m

Individualselection

Groupselection

Oceancirculation

Rockweathering

Glaciation PlatetectonicsMarineNcycle

Individualhomeostasis

Aridlandpatterning

Soilerosion

Desertification

CO2 /CaSiOcycle

Nicheconstruction

Ecosystemengineering

Selection mechanisms

Communityassembly

Emergence of environmental regulation

Spatial system of ‘flasks’ connected in a ring

Measure the ‘Error ’ = Mismatch between the state of the abiotic environment and the organisms’ preference

Williams & Lenton (2008) PNAS 105(30), 10432-10437

Emergence of environmental regulation

Spatial system of ‘flasks’ connected in a ring

Measure the ‘Error ’ = Mismatch between the state of the abiotic environment and the organisms’ preference

Envi

ronm

enta

l Err

or

Time

Williams & Lenton (2008) PNAS 105(30), 10432-10437

Emergence of environmental regulation

Spatial system of ‘flasks’ connected in a ring

Measure the ‘Error ’ = Mismatch between the state of the abiotic environment and the organisms’ preference

Envi

ronm

enta

l Err

or

Time

Mea

n Er

ror

Mixing rate (log scale)

Extin

ctio

ns

Williams & Lenton (2008) PNAS 105(30), 10432-10437

Mechanism of regulation

Williams & Lenton (2008) PNAS 105(30), 10432-10437

Mechanism of regulation

Net transferof organisms

Environment-improvingecosystem

Environment-degrading ecosystem

Large population Small population

Williams & Lenton (2008) PNAS 105(30), 10432-10437

Spatial structure and time delays• Mechanism only works for heterogeneous environmental variables

• Some key variables are well mixed e.g. O2, CO2, therefore the world is a single ‘flask’ and a different mechanism of regulation is needed

• Time delays due to long residence times of geochemical reservoirs can disable negative feedback and promote instability

Regulated variable Timescale Mechanism

Marine Nutrients 104 yr ‘Biotic plunder’ (R* Tilman 1982, Tyrell 2004)

Temperature 106 yr Silicate weathering with biotic enhancement(local competition for nutrients)

Atmospheric Oxygen

107 yr Biota overproduces, fire/toxicity upper limit

Could life wipe itself out?• Detrimental effects of life on the environment should

become self-limiting before they render the planet uninhabitable

• But in ‘Flaskworld’ sometimes a new life form drives everyone extinct (an ‘over-virulent parasite’).

• On Earth, if a geophysical positive feedback regime is entered, this could conceivably cause disaster:– e.g. ‘Snowball Earth’

Ice-albedo feedback

Absorptionof sunlight

Ice and snow cover

Global temperature

-

Budyko (1968), Sellers (1969)

+Solar

luminosity

+

-

Gain = 0.12 (0.03-0.21) about present state

but Gain → 1 when ice-line reaches ~30° latitude

5 4022.5Local temperature (C)

Gro

wth

rate

Effect of environment on life (temperature on daisy growth)

5

40

22.5

Tem

pera

ture

(C)

Areal coverage

Effect of life on environment (black daisies on temperature)

5 4022.5

Temperature (C)

Life

(are

al c

over

age)

Life

Temp.

+

+

Life

Temp.

-+

Positive feedbackregime

Negative feedbackregime

1s1h1yr103yr106yr109yrTimescale

Spatialscale

1mm1m103m10

6 m109m101

7 m102

0 m102

5 m

Sequentialselection

Individualselection

Groupselection

Oceancirculation

Rockweathering

Glaciation PlatetectonicsMarineNcycle

Individualhomeostasis

Aridlandpatterning

Soilerosion

Desertification

CO2 /CaSiOcycle

Nicheconstruction

Ecosystemengineering

Selection mechanisms

Communityassembly

Sequential selection

• “I imagine that “learning” through repetitions over time alone in a sufficiently complex system has to be shown able to replace the currently understood (and I am sure much more powerful) “learning” through repetitions over both time and space that is natural selection as we know it”

W. D. Hamilton (letter to J. Lovelock 19/1/1997)

Appeared in Gaia Circular (2007)

and as Hadley Centre Technical Note 77 (2008)http://www.metoffice.gov.uk/media/pdf/9/l/HCTN_77.pdf

Sequential selection for the Earth

Effect on environment

Stability?

Evolutionary innovation

Startlife

Persistence

Yes

Environment

Life

E→L L→E

Sequential selection for the Earth

Eliminate destabilising

effects

ResetEvolutionary innovation

Effect on environment

Stability?

Startlife

No

Approach bounds of habitability

e.g. Snowball Earth

Persistence

Yes

Environment

Life

E→L L→E

W. Ross Ashby’s Ultrastability

W. R. Ashby (1952) Design for a Brain

The Homeostat(1948)

Evolutionary regime shifts

Williams & Lenton (2010) Oikos

Population

Nutrients

Recycling

Environment

Earth history

Hadean Archean Proterozoic Phan.

4 3 2 1 0Gyr ago

Earth history• There have been a series of habitable states

Hadean Archean Proterozoic Phan.

4 3 2 1 0Gyr ago

Life

Environment

Originof life

Oxygenicphotosynthesis

AnimalsEukaryotes Us

No O2 Low O2 Mid O2 High O2

Earth history• There have been a series of habitable states• Separated by extreme environmental changes

Hadean Archean Proterozoic Phan.

4 3 2 1 0Gyr ago

Life

Environment

Originof life

Greatoxidation

Oxygenicphotosynthesis

SnowballEarth

AnimalsEukaryotes Us

???No O2 Low O2 Mid O2 High O2

Origin ofrecycling

Earth history• There have been a series of habitable states• Separated by extreme environmental changes• Driven by co-evolution of life and the planet

Hadean Archean Proterozoic Phan.

4 3 2 1 0Gyr ago

Life

Environment

Originof life

Greatoxidation

Oxygenicphotosynthesis

SnowballEarth

AnimalsEukaryotes Us

???No O2 Low O2 Mid O2 High O2

Origin ofrecycling

The remaining puzzle? Progress!

• Biosphere free energy capture increases in steps– New forms of photosynthesis, land colonisation– Explicable in terms of natural selection?

• Biological complexity progressively increases– Just a random walk bounded by zero on one side???

• Earth system ‘master variables’ have a trend over time and appear to get more tightly regulated (?)– e.g. atmospheric oxygen (O2) and CO2…

Oxygen over Earth history

PAL = Present Atmospheric Level

Lenton (2016) Earth System Science: A Very Short Introduction (OUP)

1s1h1yr103yr106yr109yrTimescale

Spatialscale

1mm1m103m10

6 m109m101

7 m102

0 m102

5 m

Observerself-selection

Sequential selection

Individualselection

Groupselection

Oceancirculation

Rockweathering

Glaciation PlatetectonicsMarineNcycle

Individualhomeostasis

Aridlandpatterning

Soilerosion

Desertification

CO2 /CaSiOcycle

Nicheconstruction

Ecosystemengineering

Selection mechanisms

Communityassembly

Observer self-selection• The explanation of last resort • The history of Earth that we

see has to be consistent with our existence as conscious observers– Rise in oxygen to ~1 PAL– Increasing biological

complexity without elimination (e.g. stem group animals in Snowball Earth)

– Tight regulation of O2, CO2, T...

• Invoke observer self-selection to explain apparent progress

0

0.05

0.1

0.15

0.2

0.25

0.3

1 2 3 4 5 6 7 8 9 10

Biospherestableenoughforcomplexlife

Numberofstabilisingfeedbacks(n)Probability(n)

Observer self-selection for the tail of a distribution?

Watson (1999) GSL Special PublicationWatson (2008) Astrobiology 8: 175-185

Welcome to Teleological Gaia!

Lenton ‘Earth system science: A very short introduction’ (OUP, 2016)

Thank you

Stuart Daines

Peter Cox

Rich Boyle Colin Goldblatt Andy Watson

Jim Lovelock Noam Bergman

Ben Mills