Science

Organisms as Complex Systems

By John Guckenheimer

Adaptability and “emergent” properties are two characteristics of complex systems, whether naturally occurring or engineered. Structurally, a complex system might be made up of a large number

of simpler components, or it might be formed from hierarchies of smaller numbers of interacting

subsystems. Organisms have all of these features. Animals are organisms that integrate different systems: the nervous system, the respiratory system, the immune system, the endocrine system, and the

musculo-skeletal system, among others. Each of these systems has many components, often organized

into subsystems. The human nervous system, for example, is estimated to have on the order of 1011

neurons; mediating their interactions are thousands of synapses connecting each neuron to others.

Moreover, the brain is organized into regions; some process sensory information of different kinds,

some regulate breathing and movement, and others contain hormone-releasing cells. All of this makes

the whole organism a complex system. This essay discusses a few insights that result from viewing

organisms from a systems perspective.

Two remarkable features of living creatures are (1) their reproduction and growth from a single

cell into elaborate shapes characteristic of each species and (2) their ability to repair injury, recover

from illness, and regulate their vital parameters within viable limits. Additionally, most animals are

mobile, moving through their environment to find mates, capture food, and avoid predators. Despite

spectacular advances in modern biology over the past sixty years, the organizing principles of these

capabilities remain a mystery. Though some biologists maintain that genomes are the secret of life,

the difference between dead and live animals is not a matter of genes: The important distinction is

that a dead animal no longer interacts with its internal or external environment. Moreover,...