First published 2000. Copernicus hardback, 2000, pp 361 (pp 287 main text), c.90,000 words (main text).
I was planning to write a book about why humans have not yet encountered any evidence for intelligent life elsewhere in the universe. My thesis was to be that the conditions that led to life here on Earth are unlikely to be repeated elsewhere because of all the apparently random and unlikely events that may well have stimulated the emergence of such life here. Unfortunately for me, Ward and Brownlee got there first.
At first glance one might think that the universe should be teeming with intelligent aliens. After all there are something between one and four hundred billion stars in our galaxy ( https://asd.gsfc.nasa.gov/blueshift/index.php/2015/07/22/how-many-stars-in-the-milky-way/ ) and there are somewhere between two hundred billion and two trillion galaxies in the observable universe ( https://science.nasa.gov/missions/hubble/hubble-reveals-observable-universe-contains-10-times-more-galaxies-than-previously-thought/ ). Combine that with the evidence that a high proportion of stars have planets, then surely some must contain intelligent life, and as many of those stars and planets are much older than our Sun and the Earth, there must be many, much more advanced, aliens than us humans. So why, despite decades of trying, have we failed to detect any signs of intelligent life elsewhere: no radio signals, laser beams, or signs of star energy-conversion by intelligent life? Nothing.
Ward and Brownlee go about examining the factors that led to Earth being a suitable planet for life, and how that life evolved through a series of almost-catastrophes into us; beings who are capable of asking such questions and searching for answers. Both authors are (were) professors at the University of Washington, Seattle, United States of America. Ward is a geologist and authority on mass extinctions. Brownlee is an astronomer and astrobiologist. Consequently this book covers the science of our planet and life in some detail, perhaps even a bit more than the lay reader requires in a few places. The science is well backed by references (twenty-five pages of them) and a good index (seventeen pages). There are also summary pages on ‘Dead Zones of the Universe’ (pp. xxv-xxvi) and ‘Rare Earth Factors’ (pp. xxvii-xxviii), as well as a Preface and an Introduction.
As an example of the sometimes-confusing detail, at one point, in a discussion of the three domains of life on Earth (Eubacteria, Archaea and Eucarya), they state that Archaea ‘are united by exhibiting a cell type known as a prokaryotic’. A little further down the page, they state about the archaeans and the bacteria ‘we can hereafter refer to as prokaryotes’. [p85]. Well it confused this non-biologist.
The book makes a strong case for how rare Earth is, and how extremely unlikely it is that intelligent life has emerged anywhere nearby in our galaxy. There are just so many marginal factors, unlikely coincidences and lucky breaks in our planet’s history, that despite the perhaps trillions of planets in the universe, very, very few, if any will have followed a similar pattern and resulted in the same emergence of intelligent life. Not life itself, however. They think it highly likely that life is common in the universe, emerging almost everywhere liquid water is present for a sustained period of time. After all, life arose on Earth very early in its history, and the required building blocks are widespread in our galaxy. What took much longer, and required a whole host of uncommon factors, was the emergence first of multi-cellular life, then animal life, and finally intelligent life. This book makes an excellent job of clearly laying out a host of these factors; resulting in a convincing case for the rarity of intelligent life.
For long periods of Earth’s history, life barely evolved. Changes were more about refinement rather than radical. Life went nowhere. Then some catastrophe happened and evolution was pushed into fast-forward. Radical change was a consequence. As an example, take the most recent and perhaps most famous catastrophe, the strike by a massive asteroid that led to the extinction of the dinosaurs and the emergence of the mammals as the planet’s dominant life form. Consider that the dinosaurs had reigned for around a hundred and fifty million years and shown no signs of developing intelligence of a form that we would recognise. If the asteroid had been twice as big, it would most likely have destroyed all life on Earth. If it had arrived a few seconds earlier or later, or been on a slightly different trajectory it would have landed in deep ocean and almost certainly many dinosaurs would have survived and continued their dominance.
The authors make a strong case for plate-tectonics being a crucial factor in stabilising our planet’s atmosphere; vital they claim for the emergence of complex life. Neither Mars nor Venus have it. The planet’s size and formation history seem to be critical. Similarly, the role of the Moon seems essential in life’s story, and again it was a number of exceptionally unlikely events that led to the exact formation of the Earth-Moon system. Jupiter probably played an important role in that story.
There remains a nagging doubt for me about their hypothesis though. If catastrophes are vital to spur life on in to higher forms, then planets elsewhere are just as likely to have experienced their own versions, even if they were different from Earth’s. Perhaps intelligent life’s emergence is a bit more common than the authors’ express here. However, what they only touch on is the critical element of how long intelligent life, and in particular, technological civilisations last. We cannot say. We have only had telescopes for five hundred years and radio telescopes for less than a hundred. A fleeting moment in Earth’s, over four-billion-year, history. And we could easily wipe ourselves out: nuclear war, biological disaster, induced climate change, declining births, AI termination. Maybe technological civilisations only last for a very short period, in which case we almost certainly will never encounter an alien one. Earth may revert to simpler life forms for its remaining history – as someone once said, ‘evolution has no direction’; maybe simpler will become fitter. Over the longer term, if we don’t precipitate our own demise, the Moon is spinning away and will eventually no longer stabilise the Earth. The radioactivity that heats the interior of our planet and which drives plate tectonics will fade and the Sun will continue to expand, finally cooking the planet, before destroying it one way or another.
This is a very well written and comprehensive analysis of life on Earth, and makes an excellent read, even without any interest in the question of the likelihood of intelligent life emerging nearby. The dips into obscure science are few: most is very clearly explained. The science and data gathering have moved on in the period since this was written, but the conclusions are unlikely to be any different if this book was updated.
Wikipedia biography of Peter D Ward: https://en.wikipedia.org/wiki/Peter_Ward_(paleontologist)
Wikipedia biography of Donald Brownlee: https://en.wikipedia.org/wiki/Donald_E._Brownlee
Wikipedia summary of the book: https://en.wikipedia.org/wiki/Rare_Earth:_Why_Complex_Life_Is_Uncommon_in_the_Universe
Others’ reviews of the book: https://www.goodreads.com/book/show/88552.Rare_Earth?ref=nav_sb_ss_1_10
© William John Graham, February 2024