Re: Life

Brian D Harper (
Mon, 10 May 1999 18:44:41 -0700

At 02:58 PM 5/10/99 -0600, you wrote:
>David Campbell wrote:
>"There are actually two separate issues. First is the question of
>the above criteria are adequate to recognize life, or whether additional
>criteria must be met. The second is what is the actual course taken in
>the formation of life on earth."
>I was not addressing the second; that is the more interesting question of
>course. What I was stating is that those three criteria seem quite
>adequate (to me) as a reasonable definition of life. I still hold that
>position. But I'm interested in how others view it. If some criteria is
>added, what would it be? And why?

As a reminder, the three proposed ingredients were:

(1) reproduction, (2) metabolism, and (3) response to external stimuli.

Interestingly, this list does not contain the ingredient which
Manfred Eigen emphasizes as the most important ingredient:

==========begin quote=======================================
"As a chemist I am often asked: what is the difference between
a coupled chemical system albeit arbitrarily complex, and a
living system in which we again find nothing other than an
abundance of chemical reactions. The answer is that all reactions
in a living system follow a controlled program operated from
an information center. The aim of this reaction program is the
self-reproduction of all components of the system, including the
duplication of the program itself, or more precisely of its
material carrier. Each reproduction may be coupled with a minor
modification of the program. The competitive growth of all
modified systems enables a selective evaluation of their
efficiency: 'To be or not to be, that is the question.'
There are three essential characteristics in this behaviour
which are found in all living systems yet known:

1 Self-reproduction--without which the information would be
lost after each generation
2 Mutation--without which the information is 'unchangeable'
and hence cannot even arise.
3 Metabolism--without which the system would regress to equilibrium,
from which no further change is possible (as Erwin Schrodinger
already rightly diagnosed in 1944).
--Manfred Eigen, "What will endure 20th century biology,"
in <What is Life, The Next Fifty Years> M.P. Murphy and
L.A.J. O'Neal eds. Cambridge University Press, 1995.
====================end quote============================

So, I would say that a potential ingredient not found explicitly
in your list of three is that whatever the dynamic features of
life are, that dynamics is organized by information which is
heritable and mutable.

Brian Harper
Associate Professor
Applied Mechanics
The Ohio State University

"All kinds of private metaphysics and theology have
grown like weeds in the garden of thermodynamics"
-- E. H. Hiebert