À ows more densely through living organisms than through nonliving entities.
The complex structures of living organisms seem exquisitely designed to
maintain and handle these dense energy À ows. The energy À ows, in turn,
help living organisms maintain a high level of complexity.
Three distinctive “emergent” properties distinguish living organisms from
non-life and contribute to their complexity. (Some of these properties are
present in nonliving entities; it is the combination that really distinguishes
life from non-life.) These properties are metabolism, reproduction,
and adaptation.
x Metabolism means the ability to use and process energy from the
environment. All living things require a constant À ow of energy to
maintain themselves, and their metabolism consists of the chemical
reactions through which they extract energy. Humans extract energy
mainly by eating and breathing. Plants extract energy directly from
sunlight through photosynthesis. (In a sense, of course, even stars
and planets depend on energy À ows; the difference is the astonishing
variety of ways in which living organisms extract energy from
their environments.)x Reproduction is the ability of living organisms to make multiple
copies of themselves.x Adaptation is the ability of living species to change over time so as to
¿ nd new ways of extracting energy from their environments. What
is remarkable is that adaptation allows organisms to keep extracting
energy from their environments even if their environments change.
Adaptation is the ability, unique to living organisms, to change over
time so as to ¿ t better into their surroundings.Even this list of features does not de¿ ne the borderline between life and non-
life completely. Viruses are little more than bundles of genetic material with
no metabolism of their own. What they can do is hijack the metabolism of
other organisms in order to reproduce. And because they can reproduce, they
can also adapt. This is why viruses such as the AIDS virus can survive even
in the hostile environment humans create for them by using antiviral drugs.