The hidden ways of trees
Forest growth may look chaotic, but University of Arizona professor Brian Enquist is finding that trees, plants and, ultimately, forests develop according to patterns of branching.

Mammals get 1 billion heart beats per lifetime, give or take, regardless of their size or normal lifespan.

Organisms use the same amount of energy, proportionally, regardless of size.

Ancient algae, your lungs, leaves and the branches of trees share similar patterns. The repeating patterns called fractals are ubiquitous in nature.

Every living cell has similar mechanisms, like the citric acid cycle that uses oxygen to convert fuel into energy and carbon dioxide, or — spun backwards — does the reverse.

There are rules and principles that govern evolution, said Brian Enquist, the University of Arizona professor of Ecology and Evolutionary Biology, who gave Monday's lecture in the UA's "Life in the Universe" series.

Enquist proposed an either/or choice to the audience in his title: "Life on Earth: By Chance or by Law?"

Did the path of evolution arise from "multiple events that are essentially unpredictable" or "is life organized by a set of general rules and principles?" 

It's pretty much both, he said. Evolution followed the rules, interrupted by chance historical happenings —those five mass extinctions, for example.

Enquist, the third speaker in a seven-week series that seeks to answer whether life exists elsewhere in the universe and what form it might take, said he had no answer to the first question.

If it does exist, we might be able to recognize it by looking for the patterns that emerge from those physical and chemical rules through which life on Earth evolved.

Contact reporter Tom Beal at or 573-4158.