Lectures a brief history of mine
Life in the Universe
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.
