Literature

"Is there anybody out there?" is a question that drives media interest in astrobiology and probably all science fiction. Enthusiasts routinely rehearse their arguments that persuade them that the universe is teeming with life, some of which is intelligent. However, these arguments are not very robust. The answer to the question, according to a recent paper is "Probably not".

"Structurally complex and intelligent life evolved late on Earth and it has already been suggested that this process might be governed by a small number of very difficult evolutionary steps.
Prof Watson, from the School of Environmental Sciences, takes this idea further by looking at the probability of each of these critical steps occurring in relation to the life span of Earth, giving an improved mathematical model for the evolution of intelligent life."

A wishful wave of friendship to some passing aliens

There are constraints on the timescales for the evolution of life, according to the author. The sun has been the primary source of heat and light for the Earth for 4.5 Ga and models of solar activity give it another billion years.

"The Earth's biosphere is now in its old age and this has implications for our understanding of the likelihood of complex life and intelligence arising on any given planet," said Prof Watson. At present, Earth is the only example we have of a planet with life. If we learned the planet would be habitable for a set period and that we had evolved early in this period, then even with a sample of one, we'd suspect that evolution from simple to complex and intelligent life was quite likely to occur. By contrast, we now believe that we evolved late in the habitable period, and this suggests that our evolution is rather unlikely. In fact, the timing of events is consistent with it being very rare indeed."

Starting with some classic work by Maynard Smith and Szathmary, Watson lists 7 major transitions and uses a probability density function to allocate probabilities to each transition.

"These steps included the emergence of single celled life about half a billion years after the Earth was formed, multicellular life about a billion and a half years later, specialized cells allowing complex life forms with functional organs a billion years after that, and human language a billion years later still."

Crucial to the model are the probabilities assigned. Watson uses very generous figures, constrained by the thought that evolution will occur, given enough time. If any aspect of the transitions involve design inputs, the probabilities will inevitably be much, much lower. The paper gives an optimistic scenario:

"In his model, the probability of each evolutionary step occurring in any given epoch is 10 percent or less, so the total probability that intelligent life will emerge is quite low (less than 0.01 percent over 4 billion years). Even if intelligent life eventually emerges, the model suggests its persistence will be relatively short by comparison to the lifespan of the planet on which it developed."

Seth Shostak, Senior Astronomer at the SETI Institute, summarised Watson's argument thus: "intelligent life will be dismayingly rare". Watson, himself, sees his work as lending "some theoretical support to the Rare Earth hypothesis of Ward and Brownlee." It also lends support to the design-orientated arguments of Gonzalez & Richards in The Privileged Planet. To those who see an evolutionary path to intelligent life as an inevitability, Watson has this to say:

"From the perspective adopted here, this appearance of evolution as a monotonic "progress" toward ourselves results from "anthropic self-selection bias". In this case, there is no need to postulate any directionality to evolution; and, in general, the kind of outcome seen on Earth may be vanishingly unlikely."

Implications of an Anthropic Model of Evolution for Emergence of Complex Life and Intelligence
Andrew J. Watson
Astrobiology. February 1, 2008, 8(1): 175-185 | doi:10.1089/ast.2006.0115

Abstract: Structurally complex life and intelligence evolved late on Earth; models for the evolution of global temperature suggest that, due to the increasing solar luminosity, the future life span of the (eukaryote) biosphere will be "only" about another billion years, a short time compared to the ~ 4 Ga since life began. A simple stochastic model (Carter, 1983) suggests that this timing might be governed by the necessity to pass a small number, n, of very difficult evolutionary steps, with n less than 10 and a best guess of n equals 4, in order for intelligent observers like ourselves to evolve. Here I extend the model analysis to derive probability distributions for each step. Past steps should tend to be evenly spaced through Earth's history, and this is consistent with identification of the steps with some of the major transitions in the evolution of life on Earth. A complementary approach, identifying the critical steps with major reorganizations in Earth's biogeochemical cycles, suggests that the Archean-Proterozoic and Proterozoic-Phanerozoic transitions might be identified with critical steps. The success of the model lends support to a "Rare Earth" hypothesis (Ward and Brownlee, 2000): structurally complex life is separated from prokaryotes by several very unlikely steps and, hence, will be much less common than prokaryotes. Intelligence is one further unlikely step, so it is much less common still.

See also:

Is there anybody out there? University of East Anglia Communications Office, April 2008

Ruley, J.D. Intelligence: A Rare Cosmic Commodity, Astrobiology Magazine, 14 April 2008.

Bostrom, N., Where Are They? Why I hope the search for extraterrestrial life finds nothing. Technology Review, May/June 2008