Dartmouth College theoretical physicist Marcelo Gleiser has an interesting essay this week which deals with the possibility of life around the universe and, more to the point, what such life might be like. It was spurred by the recent discovery of one of the most promising possible Earth-like worlds yet, orbiting in the “Goldilocks zone” of its parent star, where water could exist in liquid form. As more and more of these planets are identified, scientists will be focusing their search for possible forms of intelligent life in those regions of the galaxy.

But if life exists, Gleiser wonders, would it necessarily have advanced to a state of technological intelligence the same way it happened here on Earth? Dr. Gleiser thinks we might not want to get our hopes up too far. Many of his colleagues assume that the Universe is “just right for life” because it happened here, but we may be the exception to the rule.

The assumption here is that if physics and chemistry are the same then biology will develop. When we think alien life we are thinking in terms of Darwinian evolution via natural selection, which is a very good bet. Of course, we will only know for sure once we discover a sample of alien life, study its genetics, etc. But it’s hard to think that the very general principles set forth by Darwin won’t apply to other forms of life. If there are multiple life forms and limited resources, the rest will follow.

Of course, that says nothing of the particulars of possible alien life. A very clear distinction must be made between simple, unicellular life and more complex life forms. It’s hard to doubt that Earth is the only planet where life took hold. After all, we have seen how resilient it is here, with extremophiles defying our previously held assumptions of where life can thrive. However, there is a huge difference between simple life and complex life. Contrary to what many believe, evolution doesn’t lead to complex life forms: evolution leads to well-adapted life forms.

I’ve heard this argument made before and, as depressing as it may be, it carries a lot of weight. Some very well respected biologists have claimed that life on Earth only made the jump from what essentially amounted to little more than pond scum to more complex, multicellular forms through a rather remarkable and stressful series of events.

The theory, in short form, is that simple, unicellular life which thrives in a given climate has absolutely no reason to make the jump to something more complex and unlikely if the prevailing conditions are allowing it to succeed just fine as it is. A drastic change in environmental factors is required to challenge the organism and create the opportunity for something new and improved to adapt. But the catch is, if the environmental change is too drastic and harsh, the life form simply dies off and the process has to begin all over again. (Or have life disappear entirely.)

Should we develop the technology to get a really good look at any of these Goldilocks worlds, will we find ET hard at work building a rocket? Or even something as advanced as a cow? Or is it far more likely, as Dr. Gleiser seems to suspect, that we’ll find worlds covered in green slime which have dominated their environment and never found a need to advance further?