Life's Evolution May Depend on Galaxy

Intelligent life beyond Earth might not be as dim a hope as many scientists think, according to a new study challenging a widely held anti-ET argument.

Many skeptics tout an idea called the anthropic argument that claims extraterrestrial intelligence must be very rare because the time it takes for intelligent life to evolve is, on the average, much longer than the portion of a star's existence that is conducive to such life.

But now astrobiologist Milan M. Cirkovic and colleagues say they've found a flaw in that reasoning.

The anthropic argument, proposed by astrophysicist Brandon Carter in 1983, following on his pioneering work on anthropic principles in 1970s, is built on the assumption that the two timescales - the lifecycle of a star and the time required for evolution of living and intelligent creatures - are completely independent. If this is true, Carter argued, it's extremely unlikely that these two windows of possibility would last roughly the same amount of time, and would occur at the same time.

But that mode of thinking is outdated, Cirkovic claims. In fact, he says the relevant timescales are not independent; they are deeply entwined. "There are many different ways in which planets in our solar system are not isolated," Cirkovic said. "We must not regard habitable planets as closed boxes. If you abandon that assumption of independence, then you have a whole new background in which you can set up various models of astrobiological development."

Cirkovic points to gamma ray bursts, nearby supernovae, and perturbations of comet clouds as possible events in the astrophysical environment of the star that can influence the biological environment on a planet. For example, when a star travels through one of the dense spiral arms of the Milky Way, both its own development and that of its planets could be disrupted by higher levels of interstellar electromagnetic radiation and cosmic rays, due to the higher frequency of star-forming regions and supernova explosions.

All these connections conspire to rule out the independence suggested by Carter and connect the life of a star and the evolution of life on a planet, Cirkovic argues.

Lucky Earth

In the case of the Earth, the two timescales have lined up fortuitously to enable life. Our Sun is about 4.6 billion years old, and Earth is just slightly younger, at 4.5 billion years old. The first, most basic cells are thought to have formed on our planet about 3.8 billion years ago, although the homo genus, to which humans belong, did not appear until about 2.5 million years ago. And modern humans are only about 200,000 years old.

For more than 80 percent of the Sun's existence, life has existed in some form on Earth. It seems the timescales of biology and astrophysics have favorably aligned in our case. According to the anthropic argument, this coincidence means that Earth, and its life, are unique. But Cirkovic thinks the two timescales may not have overlapped by chance. Instead, they may be part of a complex history, involving interdependence of the Earth system with the rest of the Milky Way.

Clocking Catastrophes

Cosmic events like gamma ray bursts or nearby supernovae could reset the astrobiological clock to give a planet and star a second chance to sync up and try again to produce life. Gamma ray bursts are mysterious explosions that release huge amounts of energy, occurring either as the dying explosions of super-massive stars (like Eta Carinae) or collisions of neutron stars in close binary systems. If a gamma ray burst occurred in a large region near a planetary system, it might cause a flash of radiation and possibly cosmic-ray jets that could disrupt life on planets. Supernova explosions, though not quite as energetic as gamma ray bursts (but much more frequent overall), pack quite a wallop as well, and could send a shock of energy to any nearby planets.

"A gamma ray burst won't affect whether life will begin at some particular point in time, but it would affect how quickly life develops or takes hold by causing changes in atmospheric chemistry on the planet," Cirkovic said. "This can be interpreted as resetting astrobiological clocks which tick on each habitable planet in the Milky Way."

This idea leads to a new way of thinking about the origin of life. Instead of a long, gradual evolution, a catastrophic event could spur development of a complex biosphere and intelligent beings, much like the evolutionary theory of punctuated equilibrium predicts that species will undergo long periods of slow evolution punctuated by brief bouts of drastic change. 

For instance, paleontologists say that human beings evolved to our present state only thanks to an asteroid impact 65 million years ago that wiped out the planet's primary predator — the dinosaur. Earth has over the course of its history experienced many mass extinctions that had various causes.  While extinctions wipe out life, they are also a "reset" button that alters the environment and allows other types of life to emerge. Overall, this is part of a complex set of astrobiological histories that Cirkovic and colleagues dub the "astrobiological landscape" of our Galaxy.

"The speed of evolution is very variable," Cirkovic said. "There is no reason to think that life on Earth has only one single origin. It is quite possible that there were several beginnings of life on Earth."

Cirkovic also notes that the evolution of intelligent life could occur slower or faster in different settings, and need not follow the astrobiological history of the Milky Way.

"Large-scale correlations might cause more such SETI targets to be contemporary with us than would be expected on the basis of planetary age distribution only," Cirkovic said.

Cirkovic and team outline their argument in the June 2009 issue of the journal Astrobiology.