No banana-eating snakes or flying donkeys are to be found here

Arguments about the existence of intelligent life elsewhere in the universe commonly take two forms. The argument from plenitude or abundance asserts that intelligent life is a virtual certainty given that the universe contains so many billions upon billions of star systems. Those of us old enough to remember the late stargazer, astronomist and champion of the Seti movement (the Search for Extraterrestrial Intelligence) Carl Sagan, will never forget his other-worldly voice as he repeated the phrase "billions and billions" in pushing the argument from abundance. Even children grasp its intuitive force.

Doubters call on the argument from contingency to rule out intelligent life elsewhere in the universe. This argument asserts that the evolutionary road leading to intelligent life has so many Whitmanesque forks in it that it is enormously unlikely that the precise route has been navigated more than once. Contingency "theorists" delight in the assertion that intelligent life might not have happened here on Earth had even one of our ape ancestors even once put a foot wrong. The doubters' champion is the late physicist Enrico Fermi, who when asked about extraterrestrials famously quipped "where are they?" Fermi's remark dogs the abundance camp because for at least 100 years we have had the radio technology for receiving signals from outer space and yet not a peep has been heard.

In Life's Solution: Inevitable Humans in a Lonely Universe , Simon Conway Morris offers a new perspective on the conventional arguments. Invoking the evolutionary principle of convergence, Conway Morris deftly swats away the argument from contingency. Convergent evolution occurs when two initially different organisms evolve the same adaptive solution to some problem. The wings of birds and bats are different solutions to the problem of flying and evolved independently in their respective groups. Should you ever get a chance to exchange glances with an octopus or squid, be aware that you are both peering through camera-like eyes designed independently by nature.

Conway Morris scours the evolutionary biological literature documenting numerous compelling instances of convergence in complex features of organisms such as metabolic pathways, responses to environmental demands, and even in the precise ways that DNA and proteins evolve.

The ubiquity of convergence challenges the argument from contingency. It shows that beginning from different starting points, nature is startlingly good at traversing the vast realm of possible organisms, repeatedly arriving at the same or similar solutions. A more subtle lesson is that the number of useful end points of evolution is severely constrained, with fewer different kinds of organism observed than are possible: no banana-eating snakes or flying donkeys. Conway Morris' argument is that these features of biological evolution make intelligent life more or less inevitable. Why? Because if nature has produced it once, it is in the realm of useful end points, and if it can be produced once, it is likely that nature would produce it again and again were the tape of life replayed.

Add Conway Morris' arguments to the argument from plenitude and the universe should be teeming not just with life but with intelligent life.

So, why are we alone, or at least apparently so? Conway Morris allows that we may not be alone, but avers that it is reasonable to believe that we are. A large number of precise initial conditions relating to the fundamental physics and chemistry of the universe must be met even for rudimentary life to arise. Happily, our universe meets these rather improbable conditions. Beyond the fundamental conditions, our solar system seems peculiarly well structured for life: Earth is the right distance from the Sun, our gravity is about right, we have water and an atmosphere.

In spite of all this, numerous laboratory and theoretical studies are not even close to understanding how to get self-replicating life spontaneously to arise out of the primordium. Forget those reports of experiments that direct electrical jolts at aquaria full of ooze and get life: they don't.

It appears that life itself is exceptional and hugely improbable. But if you can get it to evolve, wait around long enough and you will get rabbits, begonias, frogs, carrots, elephants, slugs and even large-brained intelligent beings: "inevitable humans in a lonely universe".

In one sense, Conway Morris has merely dressed up the argument from contingency in new clothes. But his contribution is to remove the notion that we are special, replacing it with the assertion that the real contingent and improbable event is life itself. So improbable in Conway Morris' view that in a surprising final chapter he raises the question of whether the universe has a purpose and flirts with a theology of evolution.

How else, Conway Morris seems to be saying, can we accommodate the utter strangeness of a universe seemingly ready-made for life here on Earth. Some will read this with dismay, seeing a careful scientific position abandoned just when the going gets tough. Others will see it as courageous.

Either way, this lively and well-researched book contains an impressive breadth of detail on subjects ranging from the nature of the early universe and the formation of stars and planets to the biological details of life.

Scientists and the scientifically interested will find its arguments intelligent and thought provoking.

Meanwhile, we all eagerly await word from the Martian rovers snooping around in the dust of that intriguing planet.

Mark Pagel is professor of evolutionary biology, Reading University, and editor-in-chief, Oxford Encyclopedia of Evolution .