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 behavior through out
history, has been pretty stupid, and not calculated to aid the
survival of the species.
Two questions I shall discuss are,
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:
In biology, these two parts are
called genes and metabolism. But it is worth
emphasizing 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
favorable 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 synthesizer 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 organization, 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 favorable 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 specialize, 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
colonize 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 colonized. 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 civilizations. 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
civilization, 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.
