Life's Engines

There’s a surprising twist to Paul Falkowski’s book, Life’s Engines. From its subtitle – How Microbes Made Earth Habitable – you’d expect discussion about microbes, their energy-harvesting nanomachines, and biological evolution. You do get all of this, in easy-to-read clear prose, but you also get the germ of a profound idea – that the origin of life and its subsequent evolution is about the global-scale movement of electrons across minerals, organisms, and the environment. It’s not a new idea. For example, this week my Origin of Life class is reading an American Scientist article from 2009 titled “The Origin of Life: A case is made for the descent of electrons.” But it’s an idea normally found in scientific articles rather than a book aimed at the general public.

 

The argument is compelling, in my opinion as a chemist who studies the origin of life, although Falkowski leaves out many of the esoteric details. He tries to give readers the big picture, and in that he is mostly successful. But the interesting pieces, for those interested in the origin-of-life, are in the trickier details. Grand sweeping views are always easier to explain in broad brushstrokes. But because no one is an expert on everything, the view provided is always limited, with the strongest examples coming from the expertise of the story-teller. The parts I was most interested in, a global view of electron-economics, had much less page time than some very interesting discussions about microbes and their biochemical machinery – the expertise of the author.

 

Chapter 7 is cleverly titled “Cell Mates”. Here’s the opening paragraph: “One of the strategies nature uses to insure that its intellectual property is resilient in the face of potential massive catastrophic events is to spread the risk across a wide range of microbes. The instructions for nanomachines are spread by means of horizontal gene transfer. Although horizontal gene transfer is the principal mode of evolution in microbes, the process is not totally haphazard. One of the major drivers is ecological – the symbiotic association of microbes to optimize the use of scarce nutrients. That driver has served the evolution of life well.”

 

The author argues for the importance of studying microbes in their more complicated ecological environment, and not just isolating and growing them as pure cultures (which has its benefits). The community of microbes has its own urban jungle and economy, much like the ones humans experience in some of the largest cities especially those in the developing world. The currency in this economy is electrons – in the form of chemical molecules. There are electron-rich molecules such as methane, there are electron-poor ones such as molecular oxygen, and the rich tapestry of chemical diversity provides a sprawling bartering marketplace like no other. While we learn in school that ATP is the universal currency of biochemistry, if you look a little deeper you’ll see a messier and more diverse set of chemical substances involved. And there are positive and negative feedbacks in this microscopic world, similar to things you might have heard about in our macroscopic world, global warming for instance.

 

One puzzle I learned about in this chapter had to do with endosymbiosis and the entrance of mitochondria to the global electron-energy game. I already knew the broad strokes: an archaea eats a bacteria, and the latter evolves into a symbiotic energy powerhouse. But I hadn’t considered the details. Turns out that the ancient bacteria that was engulfed is closely related to extant “purple nonsulfur photosynthetic bacteria” which carry out photosynthesis only under anaerobic (no oxygen) conditions. When oxygen is available however, an “electrical circuit is inhibited, and the cells lose their capacity to synthesize the pigments that absorb light. To survive they rewire their internal electronic circuits and allow oxygen to become an acceptor of hydrogen that comes from organic matter. The same bacterium that is a photosynthetic Dr. Jekyll during the day under anaerobic conditions can become a respiratory Mr. Hyde under aerobic conditions.” Falkowski goes on to explain why the evolution of the engulfed bacteria into today’s mitochondria requires a delicate balance – one that we don’t completely understand yet.

 

Last week I gave a talk to first-year students about the interdisciplinarity of origin-of-life research and why it is intriguing to me. I’m thoroughly enjoyed learning about different areas and have a newfound love for biology (which I thought was a slog back in secondary school). Falkowski reminds us readers of the close connection between the reduction-oxidation reactions of chemistry and the intricacies of evolution in biology, not to mention the physics of electron transfer or the minerals of geology that provide sources and sinks of electrons. Life’s Engines is a wonderful little book that I will re-read as I continue to ponder the origin of life.