Q-Bio
The past year was a great one for science news, with two coming-of-age stories, decades in the making that are going to capture the headlines in the years to come. The first item was not in any newspaper, but if it had been, the splash headline would read something like this:
“A Mathematician With A Model Organism! Really?”
You know what a mathematician is, but what about the term “model organism”? It refers to how researchers attempt to uncover biological knowledge, by deep study, manipulation and control of a carefully-chosen organism. An example would be the fruit fly Drosophila melanogaster, which is probably the most widely-used laboratory organism for studying multicellular eukaryotes, because generations of researchers have discovered ingenious ways to manipulate its genetics and watch the cells as they grow. It’s almost unthinkable to associate a mathematician with a model organism! So what’s the story? Here’s a vignette that makes a serious point.
A few weeks ago, I was having lunch with a mathematician colleague of mine. My colleague is an expert in differential equations and dynamical systems. She writes papers with titles such as “Non-holonomic constraints and their impact on discretizations of Klein-Gordon lattice dynamical models.” During the course of our lunch, she blurted out that her favorite model organism was Daphnia. Daphnia are millimeter-sized planktonic organisms that live in ponds and rivers. They are transparent enough that one can easily see inside them and watch what happens when they eat or drink … alcohol, for example (their heart rate goes up). My colleague is part of a community that over the last few decades has developed ways to use mathematics to study ecology. They study populations, infectious diseases, ecosystem stability and the competition for resources. Their work makes real predictions, and is sufficiently detailed and explicit that my colleague (and others) co-author papers with card-carrying ecologists.
A mathematician with a model organism marks the coming-of-age for the discipline “Q-Bio,” short for “quantitative biology.” In the past, generations of biologists entered that subject, in part to escape the horrors of calculus and other advanced mathematics. Yesterday’s biology was a descriptive science. Today, biology is in the birth pangs of becoming a quantitative and predictive discipline. One remarkable example of the passing of the baton from descriptive to quantitative science is that the public domain Human Genome Project was spearheaded most notably by Eric Lander, the founding director of the MIT-Harvard Broad Institute, and a pure mathematician by training (PhD from Oxford as a Rhodes Scholar in the field of combinatorics and coding theory).
Applied mathematicians and theoretical physicists are rushing to develop new sophisticated tools that can capture the other, non-genomic challenges posed in trying to quantify biology. One of these challenges is that the number of individuals in a community may be large, but not as large as there are molecules of gas in your lungs, for example. So the traditional tools of physics based on statistical modeling have to be upgraded to deal with the large fluctuations encountered, such as in the number of proteins in a cell or individuals in an ecosystem. Another fundamental challenge is that living systems need an energy source.
They are inherently out of thermodynamic equilibrium, and so cannot be described by the century-old tools of statistical thermodynamics developed by Einstein, Boltzmann and Gibbs. Stanislaw Ulam, a mathematician who helped originate the basic principle behind the hydrogen bomb, once quipped, “Ask not what physics can do for biology. Ask what biology can do for physics.” Today, the answer is clear: biology is forcing physicists to develop new experimental and theoretical tools to explore living cells in action.
For physicists, the most fundamental biological question relates to the basic physical principles behind life. How do the laws of physics, far from thermal equilibrium, lead to the spontaneous formation of matter that can self-organize and evolve into ever more complex structures? To answer this, we will need to abstract the organizing principles behind living systems from the platform of chemistry that underlies the biology we study as chemical processes. Understanding this question could show that life on Earth is not a miraculous chance event, but an inevitable consequence of the laws of physics. Understanding why life occurs at all would enable us to predict confidently that life exists elsewhere, and perhaps even how it could be detected.
This is important because of another new discovery that just appeared online in the scientific journal Icarus with the title “Enceladus’s measured physical libration requires a global subsurface ocean,” authored by P.C. Thomas et al. This story is also a coming-of-age story, and a triumph of human ingenuity. NASA sends a spacecraft to Saturn and for seven years it observes with exquisite accuracy the rotation of the moon Enceladus. Enceladus wobbles as it rotates. You probably know that if you are given two eggs, one hard-boiled, the other not, you can tell which is which by spinning them, and seeing what happens when you suddenly stop them spinning (try it!).
The big news is that Enceladus is like the raw egg. It wobbles as if it were liquid, not solid. And so today we know that there is a world-wide ocean of liquid water under its surface of solid ice, presumably kept above freezing by tidal friction and geothermal activity. Enceladus is therefore the one place in the solar system where we know there is a large body of warm water and geothermal activity, potentially capable of supporting life as we know it.
Ten years ago, this same wonderful spacecraft photographed fountains of water spurting out the south pole of the moon, and has even flown through them to see what molecules are present. In the coming years, future missions to the Fountains of Enceladus will look specifically for life. I hope that Q-Bio will be there too, at least in spirit, predicting what to look for given the geochemistry of this moon. And perhaps even predicting that we should confidently expect life everywhere we look.
