The Odds of Life

Source: The Odds of Life - THIS CHANGED MY MIND, Cool Worlds, 20:22, uploaded 2024-10-27, Watch Later position 708.

David Kipping has spent years doubting the claim that the universe must be full of microbial life. The usual case for optimism starts with Earth’s history. Life seems to appear soon after the planet becomes habitable, which suggests that abiogenesis, the spontaneous emergence of life, happens easily. New work on the earliest life and the future of the terrestrial biosphere changes his view. Kipping still keeps the claim narrow: the evidence now favours easy abiogenesis on a close rerun of Earth, while the number of such planets in the universe remains unknown.

The early fossil record

The clearest early evidence comes from microfossils that can be dated with radiometric methods. In 2006, William Schopf reported a collection of microfossils in the Apex chert deposits of Western Australia. The oldest were dated to about 3.42 billion years ago. Alan Nutman and colleagues later found fossilised microbial mats, or stromatolites, in southwest Greenland and dated them to 3.7 billion years ago. Their paper describes layered structures in 3,700-million-year-old metacarbonate rocks, whilst also noting that the interpretation of very early life remains contested.

The Earth formed a little over 4.5 billion years ago. A collision with a Mars-sized body, which formed the Moon, probably sterilised the young planet and sets a lower limit on the time available for life to begin. Earth then spent time as a molten world under heavy bombardment. Detrital zircons studied by Simon Wilde and colleagues provide evidence for oceans around 4.4 billion years ago, which gives a more useful starting point for a habitable surface. Life therefore appears within the first 700 million years after the oceans form.

Abigail Allwood described the early fossil evidence as showing that life is “not a fussy, reluctant, or unlikely thing”. Kipping has never accepted that conclusion on its own. The date of life’s appearance carries a selection effect because we observe it from a planet that produced us.

The time available for intelligence

Evolution appears to have taken at least 3.7 billion years to move from the earliest life to a technological civilisation. Kipping asks us to treat that duration as typical for a moment. If life began as soon as the oceans formed, a civilisation like ours could have emerged roughly 700 million years ago. A later origin would move the emergence of intelligence closer to the planet’s future limit.

That limit arrives before the Sun becomes a red giant. As the Sun grows brighter, Earth’s climate changes until the biosphere can no longer support life of the kind that interests this calculation. James Kasting and James Carter estimated that the deadline could arrive in about 900 million years. If evolution usually needs 3.7 billion years, life would have to begin no later than 2.8 billion years ago for a civilisation like ours to appear before that deadline.

This makes an early origin necessary for our existence. Countless Earth-like planets could take longer to produce life and still be perfectly ordinary. Their inhabitants would never observe that delay because those planets would produce no observers like us. The early date therefore carries little force as evidence that abiogenesis itself is easy when the time needed for evolution remains unknown.

Kipping calls this the weak anthropic principle. He treats the time for abiogenesis and the time for intelligence to evolve as covarying parts of the same problem. A Bayesian analysis has to infer both rather than fix one and draw a conclusion about the other. His 2020 paper uses an objective Bayesian framework, which aims to make the result less dependent on the choice of prior. The paper reports a minimum Bayes factor of roughly 2.8 in favour of a rapid abiogenesis scenario from the earliest microfossil evidence, rising to 8.7 when the more disputed carbon evidence in ancient zircons is included.

The video presents an updated calculation with an initial odds ratio of 3.8 in favour of easy life. Kipping says that researchers usually want a ratio above 10 before calling the evidence strong. The ancient zircon evidence raises his figure to 8.7, which encourages the easy-life hypothesis without crossing that threshold.

Two ways to move the odds

The first new paper that Kipping discusses changes little. Gavin Schmidt and Adam Frank’s Silurian hypothesis asks whether an earlier technological civilisation could have existed during the Carboniferous period, around 350 million years ago, and left geological traces that have disappeared. Kipping’s calculation already allows for a civilisation emerging late in Earth’s history. Moving the date back by a few hundred million years, or even to the emergence of complex multicellular life, leaves the odds nearly unchanged.

Edmund Moody and colleagues then move the early boundary in the other direction. Their study uses duplicated genes that existed before LUCA, the last universal common ancestor of all living cellular lineages. The universal genetic code, the shared machinery for building proteins, the common use of ATP, and the nearly universal set of twenty amino acids all point towards common ancestry. Moody’s team combines molecular-clock analysis with fossil and isotope calibrations and estimates that LUCA lived around 4.2 billion years ago, with a reported 95 per cent confidence interval from 4.09 to 4.33 billion years.

The result places a common ancestor close to the beginning of the habitable Earth. Plugging the new date into Kipping’s model raises the video’s odds ratio to 13 in favour of easy abiogenesis. For the first time, he says, the result passes the threshold he uses for strong evidence. The inference remains about the rate at which life starts on a planet with Earth’s history. It does not show that all planets with liquid water produce life.

A longer future for the biosphere

The second new paper changes the other end of the timeline. The Sun grows more luminous over time. Extra energy increases evaporation and precipitation, and rain removes carbon dioxide from the air as weathering turns minerals into solid carbonates. A low-carbon atmosphere eventually prevents photosynthesis. Carter and Kasting estimated that atmospheric carbon dioxide could fall below 10 parts per million in about 900 million years, bringing most of the biosphere to an end.

R. J. Graham, Itay Halevy, and Dorian Abbot revisit that estimate. Their model weakens the assumed dependence between weathering and temperature and gives plants a lower carbon-dioxide limit of about 2.9 parts per million. It also raises the upper temperature limit. Carter and Kasting used about 50 degrees Celsius, while some plants survive at that temperature and cyanobacteria can photosynthesise at temperatures up to 74 degrees. Graham and colleagues use 63 degrees Celsius as a more viable upper limit for the end of plant life.

Those changes extend the estimated lifespan of Earth’s terrestrial biosphere to between 1.6 and 1.86 billion years from now. The video rounds this to 1.8 billion years, twice the older estimate. Almost all of the history familiar to us, including animals, the Cambrian explosion, dinosaurs, and humans, occupies the last half-billion years of that longer span.

The longer future gives evolution more time to produce intelligence after a late origin of life. With Graham’s estimate, Kipping’s odds ratio rises to 15 in favour of easy abiogenesis. He then tests the result against a range of assumed habitable lifetimes, including an extreme case in which the world ends tomorrow. The minimum odds factor in this exercise remains above 11.3, with the latest possible emergence of a civilisation like ours moving to roughly 200 million years from now. Even pushing an earlier technological civilisation back to the Carboniferous period, or to the Cambrian explosion, keeps the ratio above the threshold for strong evidence in the video’s calculation.

The claim stays tied to Earth

Kipping treats the new result as robust to changes in several parameters, provided that the 4.2-billion-year estimate for LUCA survives independent scrutiny. He still resists the wider claim that life must be common throughout the universe. The analysis reruns Earth’s history. A planet with the same mass, radius, and orbit as Earth can have different chemistry, internal structure, and water content. The rare Earth hypothesis describes a stronger possibility: Earth may be an unusually good environment for life, even among planets that look similar from a distance.

That question cannot be settled with Earth’s history alone. Telescopes will have to examine other worlds and search for signs of life under conditions that can be compared with our own. Kipping ends with a position that holds both parts together. The new evidence gives him reason to think that simple life may be common, whilst the conditions that make an Earth-like evolutionary history possible remain open. The evidence calls for excitement about the search and deliberate agnosticism about how far the conclusion travels.

Limits

Kipping’s presentation and the complete English original caption track supply the account for this 20-minute video. The source’s chapters list music changes rather than argumentative sections, so they add no argument here. Sponsor material, music credits, acknowledgements, and the closing request to like, subscribe, and support the Cool Worlds Lab are excluded.

The odds ratios of 3.8, 8.7, 13, 15, and the minimum of 11.3 belong to the video’s updated calculation. Kipping’s 2020 paper reports a different minimum figure in its abstract, so these numbers should be treated as the presentation’s reported results rather than silently merged with the earlier publication. The 3.7-billion-year stromatolite evidence and the 4.2-billion-year LUCA estimate depend on geological and molecular reconstructions with stated disputes and confidence intervals. The LUCA date does not directly date abiogenesis, since a common ancestor can be separated from the first life by an unknown interval.

The source uses “Earth-like” as a model assumption and then asks how far that assumption can travel. The conclusion stays at the level supported by the argument: the early start of life looks increasingly likely to be an easy process on a close rerun of Earth. The evidence concerns one planet and says little about the frequency of Earth-like conditions elsewhere.

Further reading / references

• David Kipping, “An objective Bayesian analysis of life’s early start and our late arrival”, Proceedings of the National Academy of Sciences, 117 (2020), 11995–12003.
• R. J. Graham, Itay Halevy, and Dorian Abbot, “Substantial extension of the lifetime of the terrestrial biosphere”, accepted for publication in the Planetary Science Journal.
• Edmund R. R. Moody et al., “The nature of the last universal common ancestor and its impact on the early Earth system”, Nature Ecology & Evolution, 8 (2024), 1654–1666.
• Allen P. Nutman et al., “Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structures”, Nature, 537 (2016), 535–538.
• Gavin A. Schmidt and Adam Frank, “The Silurian Hypothesis: Would it be possible to detect an industrial civilization in the geological record?”, International Journal of Astrobiology, 18 (2019), 142–150.
• J. William Schopf, “Fossil evidence of Archaean life”, Philosophical Transactions of the Royal Society B, 361 (2006), 1470.
• Bruce P. Wilde et al., “Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago”, Nature, 409 (2001), 175–178.
• Abigail C. Allwood, “Evidence of life in Earth’s oldest rocks”, Nature, 537 (2016), 500–501.
• Elizabeth A. Bell et al., “Potentially biogenic carbon preserved in a 4.1 billion-year-old zircon”, Proceedings of the National Academy of Sciences, 112 (2015), 14518–14521.