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Biophysics and bioengineering

Biophysics and bioengineering

Yeah, but what about the crayfish?

01 Jul 2001

Geoffrey West has developed a theory that can explain scaling laws in animals and plants. He tells Edwin Cartlidge that more of his fellow physicists ought to consider devoting their talents to understanding the biological world.

Evolutionary

Geoffrey West has a mission: to put some “quantitative meat” into the principle of natural selection. He believes that physicists – whom he says possess “the most powerful way of thinking about the universe” – should divert some of their attention from the inanimate world and unravel the most fundamental problems: consciousness, and the origin and nature of life. He’s made a start, having come up with a theory to explain the “quarter power” scaling laws seen throughout nature. But he thinks there is plenty more to do and is working feverishly on trying to quantify as much as he can about the living world.

“Physics so far has concerned itself with the relatively uninteresting stuff in the universe,” he says. “My view is having done that you now return to the problem that started these inquiries in the first place, namely where did we come from and what is going on inside our heads?”

It has been known since the 1930s that there is a well defined relationship between the mass of a species and its rate of metabolism. The metabolic rate of a species is proportional to its mass raised to the power of three-quarters. This is just one of many scaling relationships that involve a quarter or three-quarter power and it holds true all the way from micro-organisms to blue whales. In fact, West has found that it even applies to the mitochondria inside cells.

West, who is based at the Los Alamos National Laboratory and the Santa Fe Institute in the US, has come up with a mathematical theory that can explain these remarkable empirical findings. Ultimately, West would like to see the whole of evolutionary theory quantified. He believes that up till now too much emphasis has been placed on genetic algorithms, and that in a literal sense the theory has no flesh and bones.

Making the move

West’s move into biology was part design and part luck. He’d been thinking in general terms about how to put the biological sciences on a more mathematical footing and was teaching biological examples of scaling laws to students struggling with maths. Then he got a phone call from a distinguished ecologist called Jim Brown from the University of New Mexico in Albuquerque. As a particle physicist, West had been working with scaling laws for many years and was recommended to Brown by a mutual acquaintance in Santa Fe. West got together with Brown and his research student Brian Enquist, and thus was born a highly productive interdisciplinary scientific team.

At first West looked upon his work on scaling in biology as little more than a hobby and did not really believe he could make any worthwhile contributions. But as he explored the literature it became clear that there was a lot of quantitative data that were very open to physics thinking, and that all the work that had been done on these data so far by biologists was, he says, mediocre.

The problem that really fascinated West was ageing. The lifetime of a species increases as mass to the one quarter and heart rate decreases as mass to the one quarter. Therefore the total number of heart beats, he realized, is the same across all species (within a particular group of species, such as mammals). “The scaling laws for mortality fit in with the scaling laws for living,” he says. “I realized that to come to grips with ageing and mortality you’d first better understand how living things are sustained.”

West, Brown and Enquist wrestled with the idea that the scaling laws may be related to the structure and hydrodynamics of the networks that supply nutrients to the cells in an animal’s body. After a year of intense activity, the trio discovered that scaling results from the fractal-like structure of the network. They came up with three fiendishly simple universal postulates, grounded in the principle of natural selection, from which the scaling laws can be deduced mathematically. The first of these was that the network fills the whole of an organism’s body. The second was that the diameter of the smallest branches in the network does not vary from one species to another since cell size is about the same in all species. And the third was that fluid flows throughout the network with minimum energy loss.

A different mind set

The work has drawn praise from many biologists, including the popular science writer and Oxford professor Richard Dawkins, who describes it as “a theory of enormous power, explaining a huge range of facts with great economy”.

West says that while many referees reviewing his work have also been highly supportive, some have taken the opposite view. “You get some referees’ reports back that say our work is fantastic, the greatest thing that they’ve ever read. Others say that it’s horse shit and that everything is derived from molecular biochemistry. And then there’s the classic, ‘yeah, but what about the crayfish’.”

“In general,” he says, “although this was not true of my collaborators, biology tends to be dominated by a certain type of person in the opposite way to physics. They are always looking at the particular, and everything is an exception.” He says he does not understand how such people can work in science if they do not believe there are such things as universal laws. “If you had biologists working, for example, in nuclear physics you would have someone working on deuterium and then someone else working on helium and they would not realize they were working in the same field.”

West is, however, also critical of physicists. “As I’ve branched out I’ve become aware about how conservative many physics departments are. There are very well defined groups and each group wants to maintain its own research strength and is often reluctant even to look to other groups in the same department.” Coupled with the pejorative and arrogant view that physicists sometimes have of other scientific disciplines, he thinks this will hold physics back. “Because physics deals with fundamental problems at all scales that are open to quantitative analysis, it should be reaching out to other subjects like biology where there are important basic problems to be solved. Some departments have moved gingerly in that direction but they are always concerned with the deadly question, ‘is it really physics?’.”

West moved out of particle physics when he realized that there was a widening gulf between theory and experiment. “Since the Large Hadron Collider at CERN will not come on line until 2006, there will have been a long, dry period of 10 years or more when there have been no major new results from accelerators. And this may continue if the scales relevant for unification are way beyond the scales at which you can do experiments. High-energy physics cannot survive like that. In fact, I’ve been surprised that more people from the field haven’t moved into other things.”

Marriage vows

West certainly has no regrets about his change of scientific direction. “There’s no question that the interface between physics and biology is going to be a major area of investigation,” he says. “I think that some of the big problems in biology will only be cracked once researchers start to nurture this interface more.”

Dawkins also believes that physicists can make – and have made – important contributions to biology. But he adds: “Not all of them appreciate that biologists too have something to contribute to biology. Geoffrey West does.”

“Interdisciplinary research is an awfully difficult process,” explains West. “It is extremely important that physicists do not just say, oh this looks interesting and work on their own thing. I think you have to be involved in a collaboration at an intense level in a committed way, which is like a marriage. But not a marriage of 2001, a marriage of 1901. You go in with the attitude that divorce is a very unlikely thing.”

There looks to be no divorce on the horizon as far as West and his collaborators are concerned. In fact, their relationship is in rude health, with several papers lined up for publication. Not content with single organisms, the group is extending its mathematical description of nature to complete ecosystems and it has been sharpening its knives for a major assault on natural selection with a thermodynamic description of evolution.

Ultimately, West hopes that by combining their theory with genetics and studies of the brain, it may be possible to integrate the neural system and genetic code with the body’s resource networks. West calls these his “night thoughts”, the ideas he turns over in his mind late at night when there’s nothing good on television. But he believes and hopes that the study of living things will one day be part of physics.

“You could imagine that physics departments of the very distant future might have a sub-department of life and consciousness,” he says. “50 years ago one might have thought that it was not possible to have a unified theory that explained the laws of the elementary particles and the evolution of the universe. So it’s not entirely crazy to think that there might not be a credible theory that explains life and consciousness.”

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