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ARE you ever, when absent-mindedly cleaning the house, troubled by those deep
philosophical questions? You know the sort of thing. How did we get here? What
is the meaning of life? Why does a collection of carbon, hydrogen and oxygen
atoms like me feel the need to ask such difficult questions?

The answers probably won鈥檛 come to you while you鈥檙e still cleaning, so stop
dusting and pay attention to Sorin Solomon, a physicist at the Hebrew University
of Jerusalem. Solomon has discovered that adaptive, almost intelligent behaviour
can emerge from the interaction of just two very stupid kinds of entity. We鈥檒l
call them angels and mortals. From their simple dance comes an explanation for
our very existence.

These ideas are embodied in a game that Solomon developed last year. In a
paper submitted to the National Academy of Sciences, he shows that life gains
the upper hand in what ought to be disastrous circumstances. It has other happy
consequences, too. It shows that financial markets will survive even in the
hands of dunces. In the future it could even provide you with an army of robot
cleaners. So put your feet up, pour yourself a glass of something refreshing,
and drink a toast to Solomon鈥檚 angels.

The complexity of any system, such as life on Earth, must somehow arise from
the interaction of its simplest parts. If you can find and map those simple
interactions, whole areas of seemingly impenetrable complex phenomena should be
laid bare. Such 鈥渕icroscopic representations鈥� can be used to break down the
Universe into galaxies, and a nucleus into protons and neutrons. See how the
component parts work, then put them back together again, and you should have an
explanation of the most complex phenomena.

Solomon and his team work from the bottom up, with what they consider to be
the most basic of ingredients. First they scatter a race of 鈥渕ortals鈥� evenly
over a square grid. Life for these beings is bleak, as every hour a fraction of
the population dies. But there is also a ray of hope, in the form of eternal
agents, or 鈥渁ngels鈥�, scattered over the board. The mortals and angels hop around
randomly like soot particles in Brownian motion. There is only one rule: when
mortal and angel meet, the mortal multiplies. There, in the presence of
immortality, a life begins.

What fate awaits this world? Well, that depends on how you look at it. If you
stand far from the playing board, you see only a smeared-out cast and not the
individual players. Given the average population densities of angels and
mortals, you can work out an equation that predicts the average death rate and
birth rate. If the mortals die out faster than they are born, the race becomes
extinct. This way of looking at the world is called the 鈥渃ontinuum
补辫辫谤辞补肠丑鈥�.

But with Solomon鈥檚 microscopic representation, the outcome is starkly
different. Although the population slumps at first, it can recover. 鈥淚t
constitutes the difference between life and death,鈥� says Solomon. Whereas the
continuum approach predicts extinction, the direct simulation uncovers the
emergence of a thriving, developing system. 鈥淭he continuum is utterly
misleading,鈥� says Solomon.

Why should that be? When Solomon looked closely at his game, he found that
some groups of mortals, though completely ignorant of everything around them,
appear to follow the angels around. Thanks to the new births in each angel鈥檚
presence, there is an overall increase in the mortal population at these sites.
The new mortals move randomly away from their birthplace, but if the angel鈥檚
random hop is onto their turf, they multiply again.

The result is islands of life that move around the playing area, following
their angels. Islands can grow, join and split up again. Small islands are
unstable, but can become more stable when they merge to form larger islands.
Because of this apparently adaptive behaviour, the pockets of population survive
and proliferate. Ever the underdog, life simply blows a raspberry at the big bad
world.

But Solomon鈥檚 mortals are totally unaware of their environment, and have no
life goals. 鈥淲e start with very stupid microscopic components. The islands are
made up of individuals who don鈥檛 have the slightest clue of where they are
going,鈥� says Solomon. 鈥淭he microscopic agents are nonadaptive, but the
collective object has a behaviour which can be called adaptive.鈥�

This is not a conclusion that can be drawn from other simulations, says
Solomon. He points to 鈥渁daptive agents鈥�, generated by John Holland, a
simulations expert at Michigan State University. According to Solomon, Holland鈥檚
agents have complexity already built in. 鈥淭hey have strategies, efficiency
criteria, and make choices,鈥� he says. 鈥淪ince you are putting it in, you can鈥檛
claim that you are studying the emergence of adaptability.鈥�

Holland takes the opposite view. He says he would hesitate to describe the
behaviour of Solomon鈥檚 system as being adaptive. He likens it more to a kind of
self-regulation maintained by feedback. For instance, when the body鈥檚
sensors register a high temperature, they trigger the sweating mechanism. 鈥淥f
course there are no sharp lines here, so the distinctions are almost a matter of
肠辞苍惫别苍颈别苍肠别.鈥�

Solomon and his colleagues insist their model is genuinely adaptive. 鈥淭he
islands are not just self-regulating, they are self-serving. They move in a way
that prolongs their life,鈥� he says.

If adaptability really does emerge at this basic level, the implications are
far-reaching. For instance, the angels could represent the necessities of life,
such as edible animals for a population of carnivores. Most researchers into
population dynamics would treat the animals as a resource that is spread evenly
across the whole area. With just a few animals, the situation would look grim:
there just wouldn鈥檛 be enough meat in a given area to allow the carnivores to
survive. But Solomon鈥檚 microscopic view reveals that a few animals are bound to
be in just the right place, allowing a few bands of carnivores to become
established.

Jeff Kirkwood, a population dynamics researcher at Imperial College, London,
says this close look is particularly valuable when predicting population growth
in a diverse environment. 鈥淚f you looked `on average鈥�, the conditions are just
hopeless and no one has any right to survive,鈥� he says. But if there are patches
where it is possible to survive, some faster-growing species like pest
plants and bacteria can hang in there for ages. 鈥淎s soon as the conditions get
good in one little area, up they come,鈥� says Kirkwood.

The number of dimensions available on the playing board turns out to be
crucial. If Solomon lets his angels and mortals move in three dimensions instead
of two, the players tend to cross each other鈥檚 paths too rarely for life to
survive. But with just two dimensions, life always wins. Even with a high death
rate, a single angel enables life to flourish on Solomon鈥檚 two-dimensional
board.

鈥淭his may explain the fact that most ecological systems are two-dimensional,鈥�
says Solomon. Even creatures that can move in three dimensions, like birds, fish
and microbes, tend to stick with one particular level, limiting themselves to
largely two-dimensional movement because their particular angels鈥攂e they
light, oxygen or food鈥攖end to be found within a small vertical range.

According to John Beringer, an expert on microbial biology at the University
of Bristol: 鈥淢icrobes that need oxygen will be found close to the surface of
soil, and microbes that are very fastidious about oxygen concentration will be
found in bands at the appropriate oxygen concentration.鈥� Microbes concentrating
on a two-dimensional resource may have been more successful than their cousins
who tried exploiting a three-dimensional feast.

Set up the game in a slightly different way, says Solomon, and it can explain
why there is no such thing as a duck-billed hippopotamus. Instead of a place in
real space, like a stretch of savannah, the playing area could represent all
possible ways in which genes can be arranged. Biologists call this sort of
abstract space a fitness landscape.

Now think of Solomon鈥檚 angels as the perfect genomes for the habitats and
niches available, and the mortals as species wandering through the fitness
landscape. Far away from the perfect genome, a species will probably fade out of
existence. But around the angels, islands of similar species will develop.

鈥淭he space of species is very sparsely populated鈥攖here is nothing in
the `space鈥� between giraffes and elephants, or between lizards and snails,鈥� says
Solomon. The finite number of environments that exist on Earth significantly
reduces the number of genomes that can survive.

And what about the chemicals that needed to co-exist in order to create life?
The angels and mortals in Solomon鈥檚 game have such simple properties that they
could be single molecules. Their playing space could be something like the
Earth鈥檚 prebiotic oceans, where all that existed were a few relatively simple
compounds. As these primordial chemicals floated through the waters, one kind of
mortal molecule might have encountered its long-lived angelic catalyst, sparking
a self-sustaining chemical reaction.

Small changes in the surrounding conditions would then produce slightly
different molecular structures. These shadowy reactions, which Nobel
prizewinning biochemist Christian de Duve called 鈥減rotometabolism鈥�, might
eventually have produced RNA, one of the ancient building blocks of life. These
must have been robust and repeatable chemical reactions, not some one-off chance
combination of circumstances, said de Duve.

According to Solomon, the angels and mortals game demonstrates that even a
low concentration of the right chemicals could produce a robust self-sustaining
reaction, eventually leading to the proliferation of life. What might seem
unlikely, given the scarcity of chemicals, needs only the smallest of chances in
order to take root on Solomon鈥檚 playing board.

There have been other attempts to explain how the complexity of chemical life
arose. Stuart Kauffman of the Santa Fe Institute in New Mexico has produced
simulations that allow a variety of chemicals to react together, in which he has
seen complex chemistry emerge. But Kauffman achieves complexity from a multitude
of substances and interactions, whereas Solomon believes his angels and mortals
simulation starts with the most basic components. 鈥淲e have very simple
reactions鈥擜 catalysing B鈥攁nd we get a lot of complexity.鈥� Having
shown that the chemical adaptability can come for free, Solomon plans to put in
more 鈥渟ubstances鈥� to see how different islands would learn to exploit different
compositions and compete with each other.

Another area that could benefit from the angels and mortals simulation is
immunology. Here, the emergence of population islands is not such good news.
Solomon has been working with Israeli immunologists to explain how HIV can
survive in what should be impossible circumstances. Here the simulation is
inverted: antibodies attach themselves to virus particles, which allows immune
cells to mop them up. An antibody has to have just the right sequence to grab
hold of a particular strain of the virus, so the immune system generates
antibodies at random until one fits. Then a flood of similar antibodies are
produced, obliterating that viral strain.

But HIV mutates rapidly. You can imagine strains of virus wandering around in
an abstract genetic space as they mutate. Every strain will eventually encounter
a deadly antibody, and then the game鈥檚 up for that strain. But Solomon鈥檚
simulation shows that, if the rate of mutation is fast enough, islands of virus
proliferate. 鈥淲e find using this model that the immune system wins in every
confrontation with any particular HIV strain,鈥� says Solomon, 鈥渂ut as the mutant
strains become more numerous, the immune system eventually collapses under their
collective pressure.鈥�

But never mind the origins of life or the tenacity of death. What about more
important questions, like how to make pots of money? Think of the game board as
an array of investment opportunities, with the angels representing the
profitable ones. Dollar bills flock around these sites, and when they meet the
angels they give birth to baby dollars. In the gaps between the profitable
investments, money lies dead and decaying. Solomon鈥檚 simulation shows that
financial markets don鈥檛 need intelligent investors to work. Money can survive
and even proliferate simply by being multiplied in good investments and reduced
in bad ones.

Solomon鈥檚 ignorant agents can teach us something about robotics too. Chris
Melhuish of the University of the West of England in Bristol says he has seen
unconscious adaptation occurring in very simple robot systems. In some cases, he
says, complex behaviour can be a manifestation of simple rules.

Melhuish thinks this kind of characteristic could help roboticians create
swarms of cheap, small 鈥渄umb鈥� robots that move through and act on their
environment. Ideally, he would have them perform their small tasks without being
encumbered with senses, computing power or communication devices.

These little robots might herd around more complex 鈥渁ngelic鈥� control units
with more senses and intelligence, which give them new life by performing
repairs and providing power. Solomon鈥檚 simulation shows that these higher beings
could be few and far between, and the dumb mortals could be very dumb indeed. So
it won鈥檛 cost a fortune to assemble an army of robotic cleaners that will clean
your car and dust your house, self-sufficient and supervised by the foreman from
heaven. Then you鈥檒l have to find some other mindless activity to pursue while
musing on the meaning of life.