In a new book titled Assembling Life: How Can Life Begin on Earth and Other Habitable Planets, David Deamer from the University of California in Santa Cruz argues that the critical chemical reactions that led to life’s appearance on Earth required fresh water rather than seawater. Furthermore, he says, alternate wetting and drying cycles would have had to occur to achieve the necessary organic complexity. Because neither fresh water nor these types of cycles are found in the deep ocean, he concludes that life must have arisen at hydrothermal fields on land rather than at smoking ocean vents, as most scientists currently believe.
In other words, Charles Darwin’s notion of a “little warm pond” as the cradle of life may be true after all.
Deamer touches on one of the most critical questions facing astrobiologists. I have long wondered exactly how nucleic acids, proteins, and membrane molecules came together; it must have occurred in a very small area, and over a relatively short time period. Even more intriguingly, how did these diverse building blocks become the first cell? Deamer’s book points out some of the processes and chemical compounds that must have been involved, and nicely illustrates the progress we’ve made toward solving the puzzle in recent decades.
Even if we can’t yet answer how life originated, it would help a lot to know what environmental conditions were necessary, because that would tell us where else to search in the solar system. If life begins more easily in deep oceans, we should explore the ice-covered oceans of Europa or Enceladus. If it originated in hydrothermal pools on land, as Deamer proposes, then continents are required, and life is more likely to have developed on Mars and perhaps Venus. If, in addition, strong tidal forces—which our own Moon must have exerted on our young Earth—are needed, then Earth may actually be the only planet in our Solar System where conditions were right for biology.
The search for life in our own Solar System lets us test different ideas about how, and how often, life might arise on other planets. It may occur not just in one type of environment, but in many different circumstances. For example, Titan has methane and ethane lakes on its surface and an atmosphere that resembles that of early Earth. Did the mix of organic molecules we know exists on Titan assemble to produce life? A “yes” answer would mean that life can originate under very different environmental conditions than we have on Earth, and that it might be common in the universe. A “no” answer could mean that we first need to understand exactly how life originated on Earth before we focus our search elsewhere.