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In this talk, I would like to speculate a little, on the development of
life in the universe, and in particular, the development of intelligent
life. I shall take this to include the human race, even though much of
its behaviour through out history, has been pretty stupid, and not
calculated to aid the survival of the species. Two questions I shall
discuss are, 'What is the probability of life existing else where in the
universe?' and, 'How may life develop in the future?'
It is a matter of common experience, that things get more disordered and
chaotic with time. This observation can be elevated to the status of a
law, the so-called Second Law of Thermodynamics. This says that the
total amount of disorder, or entropy, in the universe, always increases
with time. However, the Law refers only to the total amount of disorder.
The order in one body can increase, provided that the amount of
disorder in its surroundings increases by a greater amount. This is what
happens in a living being. One can define Life to be an ordered system
that can sustain itself against the tendency to disorder, and can
reproduce itself. That is, it can make similar, but independent, ordered
systems. To do these things, the system must convert energy in some
ordered form, like food, sunlight, or electric power, into disordered
energy, in the form of heat. In this way, the system can satisfy the
requirement that the total amount of disorder increases, while, at the
same time, increasing the order in itself and its offspring. A living
being usually has two elements: a set of instructions that tell the
system how to sustain and reproduce itself, and a mechanism to carry out
the instructions. In biology, these two parts are called genes and
metabolism. But it is worth emphasising that there need be nothing
biological about them. For example, a computer virus is a program that
will make copies of itself in the memory of a computer, and will
transfer itself to other computers. Thus it fits the definition of a
living system, that I have given. Like a biological virus, it is a
rather degenerate form, because it contains only instructions or genes,
and doesn't have any metabolism of its own. Instead, it reprograms the
metabolism of the host computer, or cell. Some people have questioned
whether viruses should count as life, because they are parasites, and
can not exist independently of their hosts. But then most forms of life,
ourselves included, are parasites, in that they feed off and depend for
their survival on other forms of life. I think computer viruses should
count as life. Maybe it says something about human nature, that the only
form of life we have created so far is purely destructive. Talk about
creating life in our own image. I shall return to electronic forms of
life later on.
What we normally think of as 'life' is based on chains of carbon atoms,
with a few other atoms, such as nitrogen or phosphorous. One can
speculate that one might have life with some other chemical basis, such
as silicon, but carbon seems the most favourable case, because it has
the richest chemistry. That carbon atoms should exist at all, with the
properties that they have, requires a fine adjustment of physical
constants, such as the QCD scale, the electric charge, and even the
dimension of space-time. If these constants had significantly different
values, either the nucleus of the carbon atom would not be stable, or
the electrons would collapse in on the nucleus. At first sight, it seems
remarkable that the universe is so finely tuned. Maybe this is
evidence, that the universe was specially designed to produce the human
race. However, one has to be careful about such arguments, because of
what is known as the Anthropic Principle. This is based on the
self-evident truth, that if the universe had not been suitable for life,
we wouldn't be asking why it is so finely adjusted. One can apply the
Anthropic Principle, in either its Strong, or Weak, versions. For the
Strong Anthropic Principle, one supposes that there are many different
universes, each with different values of the physical constants. In a
small number, the values will allow the existence of objects like carbon
atoms, which can act as the building blocks of living systems. Since we
must live in one of these universes, we should not be surprised that
the physical constants are finely tuned. If they weren't, we wouldn't be
here. The strong form of the Anthropic Principle is not very
satisfactory. What operational meaning can one give to the existence of
all those other universes? And if they are separate from our own
universe, how can what happens in them, affect our universe. Instead, I
shall adopt what is known as the Weak Anthropic Principle. That is, I
shall take the values of the physical constants, as given. But I shall
see what conclusions can be drawn, from the fact that life exists on
this planet, at this stage in the history of the universe.
There was no carbon, when the universe began in the Big Bang, about 15
billion years ago. It was so hot, that all the matter would have been in
the form of particles, called protons and neutrons. There would
initially have been equal numbers of protons and neutrons. However, as
the universe expanded, it would have cooled. About a minute after the
Big Bang, the temperature would have fallen to about a billion degrees,
about a hundred times the temperature in the Sun. At this temperature,
the neutrons will start to decay into more protons. If this had been all
that happened, all the matter in the universe would have ended up as
the simplest element, hydrogen, whose nucleus consists of a single
proton. However, some of the neutrons collided with protons, and stuck
together to form the next simplest element, helium, whose nucleus
consists of two protons and two neutrons. But no heavier elements, like
carbon or oxygen, would have been formed in the early universe. It is
difficult to imagine that one could build a living system, out of just
hydrogen and helium, and anyway the early universe was still far too hot
for atoms to combine into molecules.
The universe would have continued to expand, and cool. But some regions
would have had slightly higher densities than others. The gravitational
attraction of the extra matter in those regions, would slow down their
expansion, and eventually stop it. Instead, they would collapse to form
galaxies and stars, starting from about two billion years after the Big
Bang. Some of the early stars would have been more massive than our Sun.
They would have been hotter than the Sun, and would have burnt the
original hydrogen and helium, into heavier elements, such as carbon,
oxygen, and iron. This could have taken only a few hundred million
years. After that, some of the stars would have exploded as supernovas,
and scattered the heavy elements back into space, to form the raw
material for later generations of stars.
Other stars are too far away, for us to be able to see directly, if they
have planets going round them. But certain stars, called pulsars, give
off regular pulses of radio waves. We observe a slight variation in the
rate of some pulsars, and this is interpreted as indicating that they
are being disturbed, by having Earth sized planets going round them.
Planets going round pulsars are unlikely to have life, because any
living beings would have been killed, in the supernova explosion that
led to the star becoming a pulsar. But, the fact that several pulsars
are observed to have planets suggests that a reasonable fraction of the
hundred billion stars in our galaxy may also have planets. The necessary
planetary conditions for our form of life may therefore have existed
from about four billion years after the Big Bang.
Our solar system was formed about four and a half billion years ago, or
about ten billion years after the Big Bang, from gas contaminated with
the remains of earlier stars. The Earth was formed largely out of the
heavier elements, including carbon and oxygen. Somehow, some of these
atoms came to be arranged in the form of molecules of DNA. This has the
famous double helix form, discovered by Crick and Watson, in a hut on
the New Museum site in Cambridge. Linking the two chains in the helix,
are pairs of nucleic acids. There are four types of nucleic acid,
adenine, cytosine, guanine, and thiamine. I'm afraid my speech
synthesiser is not very good, at pronouncing their names. Obviously, it
was not designed for molecular biologists. An adenine on one chain is
always matched with a thiamine on the other chain, and a guanine with a
cytosine. Thus the sequence of nucleic acids on one chain defines a
unique, complementary sequence, on the other chain. The two chains can
then separate and each act as templates to build further chains. Thus
DNA molecules can reproduce the genetic information, coded in their
sequences of nucleic acids. Sections of the sequence can also be used to
make proteins and other chemicals, which can carry out the
instructions, coded in the sequence, and assemble the raw material for
DNA to reproduce itself.
We do not know how DNA molecules first appeared. The chances against a
DNA molecule arising by random fluctuations are very small. Some people
have therefore suggested that life came to Earth from elsewhere, and
that there are seeds of life floating round in the galaxy. However, it
seems unlikely that DNA could survive for long in the radiation in
space. And even if it could, it would not really help explain the origin
of life, because the time available since the formation of carbon is
only just over double the age of the Earth.
One possibility is that the formation of something like DNA, which could
reproduce itself, is extremely unlikely. However, in a universe with a
very large, or infinite, number of stars, one would expect it to occur
in a few stellar systems, but they would be very widely separated. The
fact that life happened to occur on Earth, is not however surprising or
unlikely. It is just an application of the Weak Anthropic Principle: if
life had appeared instead on another planet, we would be asking why it
had occurred there.
If the appearance of life on a given planet was very unlikely, one might
have expected it to take a long time. More precisely, one might have
expected life to appear just in time for the subsequent evolution to
intelligent beings, like us, to have occurred before the cut off,
provided by the life time of the Sun. This is about ten billion years,
after which the Sun will swell up and engulf the Earth. An intelligent
form of life, might have mastered space travel, and be able to escape to
another star. But otherwise, life on Earth would be doomed.
There is fossil evidence, that there was some form of life on Earth,
about three and a half billion years ago. This may have been only 500
million years after the Earth became stable and cool enough, for life to
develop. But life could have taken 7 billion years to develop, and
still have left time to evolve to beings like us, who could ask about
the origin of life. If the probability of life developing on a given
planet, is very small, why did it happen on Earth, in about one 14th of
the time available.
The early appearance of life on Earth suggests that there's a good
chance of the spontaneous generation of life, in suitable conditions.
Maybe there was some simpler form of organisation, which built up DNA.
Once DNA appeared, it would have been so successful, that it might have
completely replaced the earlier forms. We don't know what these earlier
forms would have been. One possibility is RNA. This is like DNA, but
rather simpler, and without the double helix structure. Short lengths of
RNA, could reproduce themselves like DNA, and might eventually build up
to DNA. One can not make nucleic acids in the laboratory, from
non-living material, let alone RNA. But given 500 million years, and
oceans covering most of the Earth, there might be a reasonable
probability of RNA, being made by chance.
As DNA reproduced itself, there would have been random errors. Many of
these errors would have been harmful, and would have died out. Some
would have been neutral. That is they would not have affected the
function of the gene. Such errors would contribute to a gradual genetic
drift, which seems to occur in all populations. And a few errors would
have been favourable to the survival of the species. These would have
been chosen by Darwinian natural selection.
The process of biological evolution was very slow at first. It took two
and a half billion years, to evolve from the earliest cells to
multi-cell animals, and another billion years to evolve through fish and
reptiles, to mammals. But then evolution seemed to have speeded up. It
only took about a hundred million years, to develop from the early
mammals to us. The reason is, fish contain most of the important human
organs, and mammals, essentially all of them. All that was required to
evolve from early mammals, like lemurs, to humans, was a bit of
fine-tuning.
But with the human race, evolution reached a critical stage, comparable
in importance with the development of DNA. This was the development of
language, and particularly written language. It meant that information
can be passed on, from generation to generation, other than genetically,
through DNA. There has been no detectable change in human DNA, brought
about by biological evolution, in the ten thousand years of recorded
history. But the amount of knowledge handed on from generation to
generation has grown enormously. The DNA in human beings contains about
three billion nucleic acids. However, much of the information coded in
this sequence, is redundant, or is inactive. So the total amount of
useful information in our genes, is probably something like a hundred
million bits. One bit of information is the answer to a yes no question.
By contrast, a paper back novel might contain two million bits of
information. So a human is equivalent to 50 Mills and Boon romances. A
major national library can contain about five million books, or about
ten trillion bits. So the amount of information handed down in books, is
a hundred thousand times as much as in DNA.
Even more important, is the fact that the information in books, can be
changed, and updated, much more rapidly. It has taken us several million
years to evolve from the apes. During that time, the useful information
in our DNA, has probably changed by only a few million bits. So the
rate of biological evolution in humans, is about a bit a year. By
contrast, there are about 50,000 new books published in the English
language each year, containing of the order of a hundred billion bits of
information. Of course, the great majority of this information is
garbage, and no use to any form of life. But, even so, the rate at which
useful information can be added is millions, if not billions, higher
than with DNA.
This has meant that we have entered a new phase of evolution. At first,
evolution proceeded by natural selection, from random mutations. This
Darwinian phase, lasted about three and a half billion years, and
produced us, beings who developed language, to exchange information. But
in the last ten thousand years or so, we have been in what might be
called, an external transmission phase. In this, the internal record of
information, handed down to succeeding generations in DNA, has not
changed significantly. But the external record, in books, and other long
lasting forms of storage, has grown enormously. Some people would use
the term, evolution, only for the internally transmitted genetic
material, and would object to it being applied to information handed
down externally. But I think that is too narrow a view. We are more than
just our genes. We may be no stronger, or inherently more intelligent,
than our cave man ancestors. But what distinguishes us from them, is the
knowledge that we have accumulated over the last ten thousand years,
and particularly, over the last three hundred. I think it is legitimate
to take a broader view, and include externally transmitted information,
as well as DNA, in the evolution of the human race.
The time scale for evolution, in the external transmission period, is
the time scale for accumulation of information. This used to be
hundreds, or even thousands, of years. But now this time scale has
shrunk to about 50 years, or less. On the other hand, the brains with
which we process this information have evolved only on the Darwinian
time scale, of hundreds of thousands of years. This is beginning to
cause problems. In the 18th century, there was said to be a man who had
read every book written. But nowadays, if you read one book a day, it
would take you about 15,000 years to read through the books in a
national Library. By which time, many more books would have been
written.
This has meant that no one person can be the master of more than a small
corner of human knowledge. People have to specialise, in narrower and
narrower fields. This is likely to be a major limitation in the future.
We certainly can not continue, for long, with the exponential rate of
growth of knowledge that we have had in the last three hundred years. An
even greater limitation and danger for future generations, is that we
still have the instincts, and in particular, the aggressive impulses,
that we had in cave man days. Aggression, in the form of subjugating or
killing other men, and taking their women and food, has had definite
survival advantage, up to the present time. But now it could destroy the
entire human race, and much of the rest of life on Earth. A nuclear
war, is still the most immediate danger, but there are others, such as
the release of a genetically engineered virus. Or the green house effect
becoming unstable.
There is no time, to wait for Darwinian evolution, to make us more
intelligent, and better natured. But we are now entering a new phase, of
what might be called, self designed evolution, in which we will be able
to change and improve our DNA. There is a project now on, to map the
entire sequence of human DNA. It will cost a few billion dollars, but
that is chicken feed, for a project of this importance. Once we have
read the book of life, we will start writing in corrections. At first,
these changes will be confined to the repair of genetic defects, like
cystic fibrosis, and muscular dystrophy. These are controlled by single
genes, and so are fairly easy to identify, and correct. Other qualities,
such as intelligence, are probably controlled by a large number of
genes. It will be much more difficult to find them, and work out the
relations between them. Nevertheless, I am sure that during the next
century, people will discover how to modify both intelligence, and
instincts like aggression.
Laws will be passed, against genetic engineering with humans. But some
people won't be able to resist the temptation, to improve human
characteristics, such as size of memory, resistance to disease, and
length of life. Once such super humans appear, there are going to be
major political problems, with the unimproved humans, who won't be able
to compete. Presumably, they will die out, or become unimportant.
Instead, there will be a race of self-designing beings, who are
improving themselves at an ever-increasing rate.
If this race manages to redesign itself, to reduce or eliminate the risk
of self-destruction, it will probably spread out, and colonise other
planets and stars. However, long distance space travel, will be
difficult for chemically based life forms, like DNA. The natural
lifetime for such beings is short, compared to the travel time.
According to the theory of relativity, nothing can travel faster than
light. So the round trip to the nearest star would take at least 8
years, and to the centre of the galaxy, about a hundred thousand years.
In science fiction, they overcome this difficulty, by space warps, or
travel through extra dimensions. But I don't think these will ever be
possible, no matter how intelligent life becomes. In the theory of
relativity, if one can travel faster than light, one can also travel
back in time. This would lead to problems with people going back, and
changing the past. One would also expect to have seen large numbers of
tourists from the future, curious to look at our quaint, old-fashioned
ways.
It might be possible to use genetic engineering, to make DNA based life
survive indefinitely, or at least for a hundred thousand years. But an
easier way, which is almost within our capabilities already, would be to
send machines. These could be designed to last long enough for
interstellar travel. When they arrived at a new star, they could land on
a suitable planet, and mine material to produce more machines, which
could be sent on to yet more stars. These machines would be a new form
of life, based on mechanical and electronic components, rather than
macromolecules. They could eventually replace DNA based life, just as
DNA may have replaced an earlier form of life.
This mechanical life could also be self-designing. Thus it seems that
the external transmission period of evolution, will have been just a
very short interlude, between the Darwinian phase, and a biological, or
mechanical, self design phase. This is shown on this next diagram, which
is not to scale, because there's no way one can show a period of ten
thousand years, on the same scale as billions of years. How long the
self-design phase will last is open to question. It may be unstable, and
life may destroy itself, or get into a dead end. If it does not, it
should be able to survive the death of the Sun, in about 5 billion
years, by moving to planets around other stars. Most stars will have
burnt out in another 15 billion years or so, and the universe will be
approaching a state of complete disorder, according to the Second Law of
Thermodynamics. But Freeman Dyson has shown that, despite this, life
could adapt to the ever-decreasing supply of ordered energy, and
therefore could, in principle, continue forever.
What are the chances that we will encounter some alien form of life, as
we explore the galaxy. If the argument about the time scale for the
appearance of life on Earth is correct, there ought to be many other
stars, whose planets have life on them. Some of these stellar systems
could have formed 5 billion years before the Earth. So why is the galaxy
not crawling with self designing mechanical or biological life forms?
Why hasn't the Earth been visited, and even colonised. I discount
suggestions that UFO's contain beings from outer space. I think any
visits by aliens, would be much more obvious, and probably also, much
more unpleasant.
What is the explanation of why we have not been visited? One possibility
is that the argument, about the appearance of life on Earth, is wrong.
Maybe the probability of life spontaneously appearing is so low, that
Earth is the only planet in the galaxy, or in the observable universe,
in which it happened. Another possibility is that there was a reasonable
probability of forming self reproducing systems, like cells, but that
most of these forms of life did not evolve intelligence. We are used to
thinking of intelligent life, as an inevitable consequence of evolution.
But the Anthropic Principle should warn us to be wary of such
arguments. It is more likely that evolution is a random process, with
intelligence as only one of a large number of possible outcomes. It is
not clear that intelligence has any long-term survival value. Bacteria,
and other single cell organisms, will live on, if all other life on
Earth is wiped out by our actions. There is support for the view that
intelligence, was an unlikely development for life on Earth, from the
chronology of evolution. It took a very long time, two and a half
billion years, to go from single cells to multi-cell beings, which are a
necessary precursor to intelligence. This is a good fraction of the
total time available, before the Sun blows up. So it would be consistent
with the hypothesis, that the probability for life to develop
intelligence, is low. In this case, we might expect to find many other
life forms in the galaxy, but we are unlikely to find intelligent life.
Another way, in which life could fail to develop to an intelligent
stage, would be if an asteroid or comet were to collide with the planet.
We have just observed the collision of a comet, Schumacher-Levi, with
Jupiter. It produced a series of enormous fireballs. It is thought the
collision of a rather smaller body with the Earth, about 70 million
years ago, was responsible for the extinction of the dinosaurs. A few
small early mammals survived, but anything as large as a human, would
have almost certainly been wiped out. It is difficult to say how often
such collisions occur, but a reasonable guess might be every twenty
million years, on average. If this figure is correct, it would mean that
intelligent life on Earth has developed only because of the lucky
chance that there have been no major collisions in the last 70 million
years. Other planets in the galaxy, on which life has developed, may not
have had a long enough collision free period to evolve intelligent
beings.
A third possibility is that there is a reasonable probability for life
to form, and to evolve to intelligent beings, in the external
transmission phase. But at that point, the system becomes unstable, and
the intelligent life destroys itself. This would be a very pessimistic
conclusion. I very much hope it isn't true. I prefer a fourth
possibility: there are other forms of intelligent life out there, but
that we have been overlooked. There used to be a project called SETI,
the search for extra-terrestrial intelligence. It involved scanning the
radio frequencies, to see if we could pick up signals from alien
civilisations. I thought this project was worth supporting, though it
was cancelled due to a lack of funds. But we should have been wary of
answering back, until we have develop a bit further. Meeting a more
advanced civilisation, at our present stage, might be a bit like the
original inhabitants of America meeting Columbus. I don't think they
were better off for it.
That is all I have to say. Thank you for listening
