In The Equations of Life: How Physics Shapes Evolution, Charles S. Cockell, University of Edinburgh astrobiology professor, uses physics to explain the pervasiveness of convergent evolution and to challenge open-ended expectations of what extraterrestrial life might look like. The book is oriented around a perceived dichotomy between simple or predictable physics and complex or unpredictable biology, with Cockell arguing that the universal laws of physics place remarkable constraints on the options for life – “physics trumps individuality.” “Evolutionary convergence” is simply “similarity caused by the laws of physics.” Life is “endless in detail, restricted in form.”
Cockell tours life at several fascinating scales, delving into the physics of a ladybug to remark on the ubiquitous physical principles it must cope with to open its wings or hang onto walls or diffuse oxygen into its body. We see, among other examples, how a mole’s narrowing snout is urged on by P=F/A.
The basic concept of a cell seems to be a requirement for life, to keep one’s innards from diffusing into its environment. Cockell speculates on the cell’s origin by describing molecules with hydrophilic heads and hydrophobic tails that spontaneously form spherical membranes in water before noting that cell size is constrained by the laws of physics, most notably the surface area to volume ratio, which affects the transfer of resources into the cell.
On the many limitations of life: “Between absolute zero and the temperature of a star, say, the Sun, life occupies only 0.007 percent of this temperature range” due to the laws of physics on the “chemical compounds we call life.” Too hot – molecules can’t hold together; too cold – they can’t move at all. But temperature is not the only narrow property. Honey’s resistance to contamination “shows us that some places that contain liquid water are uninhabitable” (the water activity is too low for chemical reactions; it resists osmosis). Or how about the level of salt in the water? Too much is just too much. “We need not visit alien worlds to find where life has reached its physical limits, where no amount of chance or evolution will push it beyond the barrier of salt,” where “over three and a half billion years” of evolutionary experiments have failed.
In conclusion, “the biosphere is like a zoo, surrounded by a wall,” with extraordinary diversity within, but limited by “the insuperable laws of physics.” “The physical space that life occupies at the planetary scale, and the physical and chemical conditions it can adapt to, within the vast range of conditions found across the known universe, are petite.”
Expounding on themes in Simon Conway Morris’s Life’s Solution, Cockell remarks on the remarkable efficiency of the genetic code mapping as well as other peaks. When considering the 20 amino acids used for most of life, out of a much larger possible set, those 20 have an “uncanny” maximum of “even, wide distribution” of possible properties (size, charge, hydrophilic/phobic, etc), like a good set of wrenches, making an extremely flexible tool kit for life. The routes involved in glycolysis and gluconeogenesis (the breaking and making of glucose) have been found to “produce the highest flux of compounds” of thousands of tested alternatives. Cockell attributes these pinnacles to the relentless selective constraints of the laws of physics.
It’s clear that Cockell thinks extraterrestrial likely looks a lot like us, from the choices of lower-scale molecules to the higher-scale body plan dealings with gravity and air pressure. But he doesn’t dogmatically insist on it, and he sympathetically considers alternate suggestions, only to repeatedly conclude that the things we see on Earth are likely the best – and maybe even the only – solutions the periodic table has to offer.
Water is not just an incredible solvent; it also facilitates molecular reactions in the ways it binds to molecules inside cells. Proposed alternatives (like liquid methane) just aren’t as versatile, and come with drawbacks from their temperature range. Carbon is superior to silicon due to its electron number, which makes it just the right size for binding to molecules (the binding electrons are not too close to the nucleus to resist reactions, nor too far to make compounds too unstable). Cockell concedes that silicon has some known potential – and is careful to admit that we don’t understand it as well as carbon – but it mostly seems to just be good for building boring rocks.
Other larger elements have their interesting niches, but hydrogen, oxygen, carbon – indeed all the elements in the CHNOPS acronym – are likely to be dominant wherever life may exist, in Cockell’s view. Not only do they have the most favorable range of stability due to their size, but they also seem to form the most abundant molecules in the universe. The spectroscopic study of diffuse interstellar clouds and giant molecular clouds have found them to contain loads of carbon-based molecules. So have the meteors and comets we’ve studied closer to home. If we Copernically assume that other solar systems have the same basic elements that ours does (though he notes that “our solar system’s architecture” of rocky inner and gassy outer planets is surprisingly “not typical”), then “amino acids, sugars, nucleobases, and fatty acids are raining down on planets” throughout our galaxy and the universe. What else could life use?
Even different levels of gravity would not necessarily have drastically different effects on animals on other planets. While land animals would require massively thicker legs in higher gravity, there would be a negligible influence on animals the size of insects, where molecular forces dominate (as in the ladybug sticking to the wall), and it would apparently be completely cancelled out regarding water buoyancy. Cockell is careful to hedge his arguments with the gaps in our knowledge and imagination, but he offers strong reason to be skeptical of the free-wheeling optimists who claim that life in the universe might be so different from our own – sentient gas clouds and the like – that we might not even recognize it. The period table is the same across the universe, and so are the physical constraints that derive from it. (So, no, the Fermi paradox cannot be dismissed so cavalierly.)
In the end, Cockell sees no demarcation between physics and biology. When considering a rapturous bird flight, “the sight is mesmerizing, a show of such unpredictability and beauty that anyone would be forgiven for thinking this was some gift of life on Earth, something that stands above physics, something rooted in a higher order of organization.” Cockell sees biology nested comfortably within physics (though I think the laws of physics, and the order that emerges from them, can be seen as gifts in and of themselves…)