Rare Earth

One of the most important and interesting question that science can ask is whether or not life exists elsewhere in the universe. The recent book Rare Earth by Brownlee and Ward hypothesizes that multicellular life is quite rare in the universe, so rare in fact that it is likely that we are the only intelligent civilization in the galaxy. Other’s of course have alternative views.

In most respects, I found Rare Earth to be a well
organized and well thought out book. It reviews a great deal
of information about the formation of our solar system and the
history of climatic change. I doubt that anyone who has studied
these topics in any length will find much new, but for the amateur
who may not be well versed in these topics, it provides a helpful
review.

Brownlee and Ward believe that bacterial life (simple single celled
organisms) may in fact be quite common in the universe, but that the
conditions which lead to highly organized and specialized multi-cellular
organisms (metazoans) are remarkably rare. Some of the
factors which they list as essential include:

  • planet must be within a narrow galactic habitable zone
  • presence of a suitable gas giant in the solar system to act
    as a vacuum for potential collision fragments

  • a large natural satellite
  • plate tectonics

I didn’t really find Rare Earth to offer a compelling argument. It seemed to me as if Brownlee and Ward merely chose to take each feature of the Earth’s biosphere, and hypothesize that it must be present for any multicellular life to develop.

The fact is, we have no idea whether the earth is a good or poor enviornment for the formation of life. As near as we can tell, as soon as the earth cooled to the point where long lasting organic molecules could form, life began to develop on this planet. Yet from that time (3.8 billion years ago) to the Cambrian explosion (only 540 million years ago) this life was pretty much single celled organisms.
Can we really view the earth as an ideal environment for life when it took over three billion years
to develop multicellular organism?

There is an
interesting multi-part debate with quotes from Brownlee, Ward, Drake and others that is well
worth scanning.