Life, for all its remarkable diversity, displays also a remarkable unity.
There is, of course, the way by which animals reproduce, as genetic information encoded in DNA is passed on to new generations in nearly all animals. The two strands of the huge molecule coil as if forming a spiral staircase, with each rung consisting of pairs of chemical groups (bases) which combine in very specific ways: adenine to thymine and guanine to cytosine. In the many ways by which life attempted to emerge on Earth billions of years ago, this was the system that worked best.
Given the complexity of the DNA molecule, it is reasonable to suppose that the first living creatures had simpler mechanisms for genetic reproduction. Clearly, in the long run, the one using DNA and RNA was the winner, surviving the test of time. Amazingly, we can trace all current life to a common ancestor, or LUCA (Last Universal Common Ancestor), dating to around 3.5 billion years ago. According to modern biology, and as Darwin had intuited in On the Origin of Species, the mother of all life forms was a simple bacterium.
When it comes to growth rates and shapes, three numbers characterizing an animal are important: their approximate linear size (L), their surface area (S), and their volumes (V). In isometric growth, as L increases, the area increases with the square of L (as L2) and the volume with the cube of L (as L3). So, as an animal grows, its volume grows faster than its area. This is usually a problem for animals, since as they grow their muscular cross-sections scale as the square and their mass as the cube of their sizes. A 60-foot tall giant would weight 1,000 times more. His bones would have to support 10 times more weight per square inch. They'd break down unless scaled differently, that is, unless they bulk up with size. The same with its respiratory organs. This type of non-isometric scaling is called allometric scaling.
Life obeys certain allometric scaling laws that seem to reveal a sort of overarching design principle at work. We don't know what this principle is, although it's probably related to optimization: What's the best shape for the least amount of energy consumption? A famous allometric law is known as Kleiber's Law, where the metabolic rate of an animal grows as its mass to the 3/4 power. (The metabolic rate can be measured in terms of the rate at which an animal consumes oxygen, for example.) Although there are small variations (due to motion, disease, aging), the relation holds over a wide range of masses. (There are disputes for very small animals without a circulatory system.)
Geoffrey West, Brian Enquist and James Brown proposed a model based on blood flow to explain this and a few other general allometric scaling laws with body weight observed in animals (for a review paper see this): Apart from Kleiber's Law mentioned above, life span scales as 1/4 power (so take two square roots of the mass), and heart rate as -1/4 power. Put together, these two laws explain why all species have a similar amount of heartbeats, 1.5 billion, over their life spans.
Pause for amazement.
The laws are not absolutely precise but do indicate a common trend across an enormous variety of living creatures. On Monday night, I was on a panel on Complexity with Geoffrey West at the New York Academy of Sciences. At some point, I asked West whether alien life, if it exists, would follow the same sort of unifying allometric laws. With a twinkle in his eye, West replied, a big smile on his face: "Well, I can only speculate here, but it seems plausible that this sort of design principle for life does have universal characteristics."
It would be amazing if life as we don't know it is, after all, life as we do know it.
Marcelo Gleiser is a theoretical physicist and cosmologist — and professor of natural philosophy, physics and astronomy at Dartmouth College. He is the co-founder of 13.7, a prolific author of papers and essays, and active promoter of science to the general public. His latest book is The Island of Knowledge: The Limits of Science and the Search for Meaning. You can keep up with Marcelo on Facebook and Twitter: @mgleiser
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