Messenger - Vol. 1, No. 2, Page 12
Winter 1992
Digital Darwinism; Synthetic organisms evolve spontaneously into
cheaters, freeloaders, good neighbors
     Using a computer as his petri dish, evolutionary biologist Tom
Ray has created an artificial world in which synthetic creatures
reproduce and spontaneously evolve, and, in the process, sometimes
behave like freeloaders, cooperative neighbors or sticky-fingered
     The inhabitants of Ray's world, known as Tierra, are
self-replicating digital organisms that compete for computer time
(energy) and memory (space). The primordial soup in which the
creatures evolve is a memory block of 60,000 instructions, and Tierran
life seeks to replicate itself to fill all available memory while
resisting other denizens that grow and struggle for the same space.
     Using an IBM supercomputer and his own machine language, Ray has
created a silicon world whose residents appear to demonstrate
evolutionary behavior "not designed in or preconceived by the
creator." Besides introducing technological innovations based on the
biology of natural organisms to the creation of artificial life, he
has developed a novel laboratory for the study of ecology and
evolution by biologists.
     In April, Ray won second place in a national IBM Supercomputing
Competition and a prize of $15,000 for a paper detailing his
experiments in synthesizing artificial life. Since then, his findings
have been reported in Science News, The New York Times, Nature, the
Chronicle of Higher Education, the New Scientist and numerous popular
European publications.
     His technological accomplishments are all the more remarkable
because Ray, who studies the evolution and ecology of rain forest
organisms, has never had any formal computer science training.
     Tierra (Spanish for Earth) and its inhabitants provide the first
method for experimental studies of large-scale evolution. Ray can make
multiple universes in minutes and observe emergent behavior in hours
or days, whereas until now, scientists could only speculate about the
process because natural systems evolved over millions of years. And he
can also "tweak" the model to focus on special areas of ecological
interest or pull out particularly interesting inhabitants to populate
yet another universe.
     "We have never been able to study evolutionary systems in action
in the natural world," Ray says. "Researchers have bred generations of
fruit flies for selected characteristics and conducted other animal
and plant breeding programs over a few generations, but we never have
been able to observe change at the species level or keep track of the
changing genome of any species. Evolution in my world is so much
faster that it allows us to study the process on a macroscale while
keeping track of the genetic code of individuals."
     An added advantage, he says, is that the experiment can be
duplicated exactly to retrieve any information missed in the first
     To prevent his digital organisms from running rampant in the real
world as computer viruses or worms, Ray sets up his artificial-life
scenarios within a software program that acts as a virtual computer.
"A virtual computer is a computer that could exist but doesn't," Ray
says. Outside the program, the computer creatures and their
environment are simply innocuous streams of data that are no more
risky than a text file on a word processor.
     No other attempts to create artificial life have been this
successful, according to Ray. "A number of people have simulated
evolution but none really created it," he says. "In simulations,
everything is predefined. You can define a gene for offense and one
for defense and you end up with a little evolutionary arms race. But,
if you set up preconditions ahead of time, you only get back what you
set up. Evolution is more freewheeling. Nature starts out with
molecules that stumble onto innovations, and they get ahead when they
make these innovations."
     By synthesizing artificial life rather than simulating it, Ray
says he "gave up control of the system to evolving entities so they
could invent their own games and strategies."
     Ray's only requirements were that the creatures could reproduce
and were capable of open-ended evolution. "Nothing else was predefined
except for the physics and chemistry of the universe they live in, the
architecture of the computer itself," he says.
     Several necessary mechanisms were built into the program. A "time
slicer" doles out computer time uniformly and chunks of memory are
always available. A "reaper" removes either the oldest creatures or
the ones that were most defective, that is, those who generated the
most errors in executing their procedures. "Without the reaper, the
creatures would live forever and there wouldn't be any room for anyone
else to be born," Ray says.
     A "gene banker" was also created to save to disk interesting
creatures of varying sizes as they evolved. These digital organisms
could be inserted in subsequent experiments.
     Spontaneous diversification occurs rapidly, Ray says. The
ancestor of Tierra was made up of 80 instructions, but creatures soon
emerged that were capable of reproducing in less time-that is, with
fewer instructions. The smallest creature to arise required only 22
instructions and could reproduce almost six times faster than the
ancestor. Some larger sizes also appeared and went extinct quickly.
The largest was over 23,000 instructions in size.
     Evolution occurred in Tierra in a completely natural way. "In the
Tierran world, creatures behave in ways that aren't programmed because
they interact and influence each other," Ray says.
     "Innovation was the surprising thing," Ray says. "I had no idea
that Tierra would get all these different types-parasites that cause
their hosts no harm; hosts that are immune to parasites;
hyper-parasites that steal energy; and social interaction where a
cluster of hyper-parasite neighbors pass an energy source (the
pointer) back and forth to help each other reproduce."
     Parasites and hyper-parasites, Ray says, behave like a virus that
invades a cell and uses the metabolic machinery in the cell to
replicate its own genome. "A parasite uses information from the host,
but doesn't take anything away and doesn't include the information in
its genome. It's like a person reading a newspaper from over your
shoulder," Ray says.
     "But a hyper-parasite steals energy, taking computer time from
another creature, and that is harmful. It's the equivalent of a
neighbor stealing your electric lines and rewiring them to the house
next door. You can't turn on the lights or run your television set,
but you are still paying for the electricity."
     An even faster, sneakier competitor- known as the hyper-hyper
parasite-ultimately appeared in Tierra and took advantage of the
social hyper-parasites by cheating. Once this creature was passed the
pointer by the cooperating group, it refused to relinquish the energy
     "They kept coming up with ways to exploit each other, and this is
just the way it works in the biological world," Ray says.
     Ray, who holds master's and doctoral degrees from Harvard
University, joined the University's School of Life and Health Sciences
in 1981. He says he believes his biological training helped him
successfully create "evolution in a bottle."
     Rays says he succeeded because of two innovations in computer
language. First he made his instruction set small, based on the size
of the human genetic code. The other feature borrowed from molecular
biology is the addressing mode. The creatures search for a matching
numerical pattern, just as a biological molecule would. For example, a
code with the order 0001 will search for the complimentary pattern
     Tierra illustrates the evolutionary theory that adaptation to
other organisms is the primary force driving the diversification of
organisms, Ray says. The artificial world also makes it possible to
study other ecological processes, including competitive exclusion and
coexistence, punctuated equilibrium (a period of stasis changing to
rapid evolution), the keystone predator effect (a predator increases
the diversity of plants and animals in a community) and population
     The first creatures he designed were not very clever and his
first instructions not very powerful, Ray says, yet many different
types of artificial life formed and evolved.
     "It appears that it's rather easy to create life," he says.
"Virtual life is out there waiting for us to provide environments for
it so it can evolve."